Lighting guide 11 : surface reflectance and colour : its specification and measurement

Lighting Guide 11 : Surface reflectance and colour Its specification and measurement for designers

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The Society of Light and Lighting is part of the c,mi Chartered Institution of Building Services Engineers

National Physical Laboratory

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This document is based on the best knowledge available at the time of publication. However, no responsibility of any kind for any injury, death, loss, damage or delay however caused resulting from the use of these recommendations can be accepted by the Chartered Institution of Building Services Engineers, the Society of Light and Lighting, the National Physical Laboratory, the authors or others involved in its publication. In adopting these recommendations for use each adopter by doing so agrees to accept full responsibility for any personal injury, death, loss, damage or delay arising out of or in connection with their use by or on behalf of such adopter irrespective of the cause or reason therefore and agrees to defend, indemnify and hold harmless the Chartered Insti- tution of Building Services Engineers, the Society of Light and Lighting, the National Physical Laboratory, the authors and others involved in their publication from any and all liability arising out of or in connection with such use as aforesaid and irrespective of any negligence on the part of those indemnified.

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O 2001 The Society of Light and Lighting and the National Physical Laboratory

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ISBN 1 903287 14 6

Typeset by the Society of Light and Lighting.

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Lighting Guide: Surface reflectance and colour 5

Contents

1 Introduction

2 Light reflecting properties o f surfaces

3 Light reflecting properties o f bui lding materials 3.1 Reflectance of building materials 3.2 Colour of building materials 3.3 Surface finish of building materials 3.4 Surface deterioration and maintenance 3.5 Depreciation of luminaire performance

4 Surface colour specification 4.1 Munsell system 4.2 Natural Colour System 4.3 DIN system

16 4.4 BS 5252: 1976: Framework for colour co-ordination

for building purposes 4.5 RAL design system

5 Measurement o f reflectance and colour 5.1 Measurement using a spectrophotometer 5.2 Measurement using a calorimeter 5.3 Measurement using luminance and Illuminance meters 5.4 Field measurements using the reflectance sample card 5.5 Visual assessment

6 Reflectance and colour standards

7 Surface appearance

8 Explanation o f terms

References

Reflectance sample card description

Reflectance sample card Inside back cover

Liohtino Guide: Surface reflectance and colour 7

1 Introduction

All lit environments, whether daylit or electrically lit, will contain an element of reflected light which will contribute to individual illuminance and luminance values within a space. The amount and nature of the light reflected from a surface will depend on the reflectance properties of the material, which include its total reflectance, its reflectance factor, the colour of the surface and the surface finish. These surface properties, in addition to the nature of the light illuminating it, will determine, in a physical sense, its appearance including its luminance, luminance factor and chromaticity. But the appearance of an element within a scene will also, to some extent, he determined by the lit environment of which it is part. This is through brightness and colour adaptation and constancy, as well as the contrast, both brightness and colour, between adjacent surface elements.

The guide will explain the various properties of reflecting surfaces and how they can be assessed and measured. It will also provide guidance on the reflectance of typical building materials and a range of colour samples. These can be used to assess, by com- parison, the reflectance value of materials.

It is expected that the guide will provide a valuable reference source as well as day to day information for all designers, but particularly lighting designers, architects and interior designers.

2 Light-reflecting properties

When light is incident on a surface, some may he absorbed, some transmitted and some reflected. The reflected light may be contained within a very small range of angles - termed specular reflection- or spread over a range of angles - termed diffuse reflection. For a perfectly specular surface, such as a mirror or a polished metal surface, all the incident light is reflected in the same plane as the incident light, and at an equal and opposite angle to the surface normal as the incident light (see Figure 1). There is no diffuse reflection and no losses. For a perfect diffuser (white blotting paper and matt emulsion paint are good approximations), the light is reflected so that the surface appears equally bright in all directions and there is no specular reflection (see Figure 2).

Incident liaht Reflected liclht Intensity of the reflected light varies as cos O reoardless of - > -

Incident liqht incide

Specular surface Diffusing surface

Figure 1. Reflection from a perfectly specular Figure 2. Reflection from a perfectly diffuse surface. The reflected beam is at the same surface. The luminance of the surface is angle to the surface as the incident beam and independent of the angle of vieq i.e. the is also of the same intensity luminance is a constant and the intensity of the

reflected light at an angle 0 to the perpendicular to the surface varies as cos B

8 Society of Light and Lighting and National Physical Laboratory

Reflected light Figure 3. Most types of surface exhibit a Peaks around combination of diffuse and specular reflection. the specular

angle The proportion of specular reflection determines how 'glossy' or 'shiny' the surface appears.

'Glossy' surface

For most surfaces the reflection is neither perfectly specular or perfectly diffuse - these surfaces may be termed 'glossy' (significant specular reflection), 'semi-matt' (little specular reflection) etc. (see Figure 3).

The reflectance properties of a material are determined by its structure. Specularly reflecting materials have a smooth, homogeneous surface which introduces no distortion into the path of the reflected light. Diffusely reflecting materials have a 'rough', inhomo- geneous structure which scatters the reflected light in all directions. In most cases light will also penetrate below the surface and, for any non-homogeneous material, scattering will occur. If the scattering particles exhibit selective absorption in the visible region, a colour is imparted to the reflected light and the material appears coloured. Consider the case of a coloured gloss paint layer, for example, in which coloured pigments are sus- pended in a clear gloss lacquer. Specular reflections occur from the surface of the lacquer but a reasonably high proportion of the light penetrates to the particle layer below, giving rise to coloured, diffuse, reflections. The overall effect is a coloured, high gloss finish.

If we plot the way in which the proportion of light reflected varies with wavelength for a white or grey surface, a flat line is obtained. For a coloured surface on the other hand, troughs (corresponding to regions of low reflection) and peaks (regions of high reflection) are seen. A 'red' surface, for example, generally reflects red light but absorbs blue and green and its reflectance profile might be similar to that shown in Figure 13 of Section 6. However it is not just the reflectance properties of a surface that affects its colour appearance. The spectral composition of the light illuminating the surface is also important. If green light falls on the red surface shown in Figure 13 of Section 6 then it appears black - there is no red light present in the incident light to be reflected. Daylight and the light from tungsten filament lamps contain radiation at all visible wavelengths and therefore allow us to see all shades of colour. Other lamps, such as fluorescent lamps, emit most of their light at a relatively restricted range of wavelengths and can therefore 'distort' the colour appearance of a surface. Some fluorescent lamps, for example, have very little red radiation and make red surfaces look 'subdued'. The general colour rendering index Ra of a lamp is a measure of how well colours can be seen or distinguished from one another. Tungsten lamps have a general colour rendering index of 100; fluorescent lamps can have an index of -50-95, according to their individual spectral distribution.

There are many other factors apart from its reflectance and colour and the light source with which it is illuminated which will affect the visual appearance of a surface. These include its texture, the level and uniformity of the illumination and the colour and luminance of adjacent surfaces. These issues are discussed further in Section 7.


3 Lighting properties of building materials

One of the purposes of this publication is to help designers to assess the lighting properties ofbuilding surfaces. The surfaces might be interior or exterior, although for exterior surfaces this will normally only be required for appearance purposes as with floodlighting. Where an exterior surface is used to reflect light for illumination purposes then the designer is advised to consider it in a similar way to an internal surface.

In assessing a building surface there are three main things to consider, i.e. the surface reflectance, its colour and its surface finish. A further consideration is how the material will change through its life. For example it will get dirty, it may change colour and even the surface finish may change. These changes will affect the lighting properties and will also need to be taken into account at the lighting design stage.

3.1 Reflectance of building materials

Building surfaces come in a wide range of finishes from the diffuse matt surface of a plastered wall, through textured surfaces like brick or stone as well as carpets, and on to specular surfaces like glass and metal. From an indirect illumination aspect it is normally the diffuse reflectance that is used. However the designer should be aware that the reflection factor and the appearance of the surface can change depending on how it is lit. For example if a textured surface is lit obliquely then the textured appearance will be enhanced but if it is lit from all angles then the texture will be suppressed.

The previous section has described diffuse reflectance, and how this is measured is described in Section 5. A further way of assessing diffuse reflectance is to use the Re- flectance Sample card provided, the use of which is also described in Section 5. When assessing diffuse reflectance the sample will need to be lit with diffuse light and preferably with the illurninant to be used in the final design. If a surface is all one material like a concrete or rendered wall then the diffuse reflectance of only this material needs to be determined. If, however, the surface is a combination of two materials then both will need to be taken into account. For example a brick wall is a combination of the bricks and the mortar that holds them together and typically the mortar can be as much as 14% of the total area of the wall. In this case, and any other combination of materials, the area weighted reflectance will need to be used. This is where the diffuse reflectance of the individual materials is weighted by the particular areas for a diffuse reflectance of the whole surface to be determined,

e.g. Area weighted reflectance of a brick wall.

Area of brick = A , Area of mortar =A,,, Reflectance of brick = R, Reflectance of mortar = R,,,

Area weighted reflectance of brick wall

A, +A,,,

This approach must be used wherever there is a combination of surface materials or features which have different reflectances and it is the combined diffuse reflectance that is required. The brick wall has already been mentioned but it applies to other surfaces like a perforated panel which is a combination of the main surface and the very low


reflectance of the holes. It will also apply to whole room surfaces like a wall that includes the wall surface, perhaps a door of different colour and windows - all the elements will need to be taken into account.

With regard to the actual diffuse reflectance of building materials, they cover a very wide range and the designer is advised to seek out as accurate a value as is possible. Also for major surfaces actual samples should be assessed preferably under the illuminant to be used. For example bricks, depending on the particular type, could vary anywhere between 0.2 and 0.7 while timber is likely to fall between 0.1 and 0.4. Clear glass on the other hand is typically around 0.15. Textured surfaces like carpets will generally have a relatively low reflectance unless they are very light in colour. Painted surfaces will also vary widely. For example a white emulsion painted plaster surface could have a reflectance as high as 0.8 while a dark painted surface could be as low as 0.1. The diffuse reflectance of some materials, such as paints, can often be obtained from the manufacturer. However if manufacturers data is not available then one of the methods described in Section 5 should be used.

3.2 Colour of building materials

Building materials can come in a wide range of colours as well as many different reflectances. Bricks for example range from dark reds through to pale yellows as well as the dark blue of engineering bricks. Stone also comes in many colours and shades but like brick it depends on the particular natural material. Man-made materials, however, depend on the materials used in its manufacture or on pigments introduced to create a particular colour. Cement for example can come in many colours depending on the desired effect required by the architect. For painted surfaces then the manufacturer's specification will usually include the colour specification as well as the diffuse reflectance. It is important that an accurate system of colour specification is used to ensure the desired effect as determined by the architect or interior designer - see Section 4.

3.3 Surface finish of building materials

The surface finish of building materials vary from the smooth matt surface of plaster to the highly textured surfaces of shuttered concrete. There are also highly polished surfaces which include some glasses, polished stone like marble and metal panels finished with a powder coating. The nature of the surface finish will affect how the light is reflected from it, but this will also be affected by the way the surface is lit. As has already been explained, if a surface is matt then it will scatter light evenly in all directions. If however, the surface is textured and the surface is lit obliquely then only the elements of the surface which receive light will reflect light, whereas those that do not will be in shadow and appear 'dark'. This combination of light and shade will be effective in en- hancing the appearance of the surface texture but it will reduce the average luminance which can be interpreted as a reduction in its average reflectance. Similarly, if the surface is specular then it may be possible to see a reflected image of the light source. But this will depend on the geometly between the surface, the illuminant and the viewing position. As has been mentioned earlier the designer will need to decide on the primary purpose of the material. If it is for effect, then the lighting should complement the surface material to enhance the effect and the light reflecting properties as it affects the general illumination of the room can be dealt with by consideration of the diffuse reflectance of the material. If however the direction and intensity of the light reflected from the


0-55 1 ' I 6 12 18 24 30 36

Elapsed time (months)

Figure 4. Reflectance reduction against elapsed time, from CIE Publication 97

surface is important, then an appropriate reflection factor or factors will be required. As with the other lighting properties of materials much will be achieved by examining, experimenting and measuring actual material samples to determine the lighting effects.

3.4 Surface deterioration and maintenance

Throughout the life of a building the surfaces will deteriorate through their accuinulation of dirt which in turn will reduce the amount of reflected light. How much this will be and at what rate is difficult to determine without long term tests which in most cases will not be possible. Nonetheless the designer must consider this process if the lighting levels are not to reduce to a point that is unacceptable. The magnitude of these changes is governed by the extent of dirt deposition which will depend on the nature of the surface and the cleanliness of the atmosphere. It will also depend on the importance of the surface with regard to inter-reflected light on illuminances within the space. A further element in this is the regularity that the surface is cleaned or refurbished in some other way. As a guide to the likely change in diffuse reflectance when the surface is used primarily as a source of inter-reflected light the graph shown in Figure 4 has been provided. It shows the likely reflectance reduction factor for clean, average and dirty environments against the length of time between cleaning periods. The data for this have been taken from the CIE Pub- lication 97, which does not take into account the effect of the surface texture to retain or accumulate dirt. The overall message must be that the designer at least considers the likely effect of dirt accumulation in the particular situation and the effect on illuminances within the space. The building owners, or their clients, should also be encouraged to clean the room surfaces as regularly as possible.

3.5 Depreciation of luminaire performance

The accumulation of dust and dirt on all room surfaces is fully covered above but it must not be forgotten that the same dirt deposition may also effect the performance of lamps and luminaires. Detailed guidance on the practical calculation of the effect of dirt on and in luminaires and the resultant loss in performance may be made by reference to section 4.5.2 in the 1994 edition of the CIBSE Codefov Inteviov Lighting.


4 Surface colour specification

In terms of light reflection from building surfaces for illumination purposes, it is the reflectance of the surface that is important. However this takes no account of its colour. Light reflected by a coloured surface will be tinged with that colour, but except in strictly controlled environments, where accurate colour representation is required, this is rarely a problem. The colour appearance of a surface will be affected by the spectral radiation distribution of the light source illuminating it, which can be described simply by its chromaticity co-ordinates, either on a CIE 193 1 x and y diagram or on a CIE 1976 u' and v' uniform chromaticity diagram, see Figures 5 and 6. Chromaticity co-ordinates can also be used for surface colours but only when illuminated by a particular light source. In this case the co-ordinates combine the spectral reflectance distribution of the surface and the spectral radiation properties of the light source. Chromaticity co-ordinates are also the basis by which a light source's colour rendering quality can be determined - for further details see the 1994 edition of the CIBSE Codefor interior lighting.

Paint manufactures often use names to describe the colour of a product but these can cause confusion. For example can a user be really sure of the exact colour of a paint describes as 'emperor' or even 'daffodil yellow'. To overcome this anumber ofnumeri- cal systems have been devised. The following are the most commonly used.

4.1 Munsell system

The Munsell system is the oldest of those currently employed. This was devised by A.H.Munsel1 originally in 1905 and has been extended and refined in various ways since (Nickerson, 1976). The system provides a three dimensional matrix and is shown in Figures 7 and 8. The surface colours are defined in terms of three perceptual attributes: hue, value and chroma.

Hue describes the apparently dominant part of the spectmn occupied by the colour, e.g. red, as distinct from yellow or blue. The various hues are located around the periph- ery of a circle located on a horizontal plane and passing through the Munsell solid. There are five principal hues and five intermediate hues:

Principal hues: red (R), yellow (Y), green (G), blue (B), purple (P) Internzediate hues: yellow red (YR), green yellow (GY), blue green (BG), purple blue (PB), red purple (RP).

Chroma is the strength of the colour and increases radially from the centre of the solid which is the point of neutral grey (zero chroma) to a maximuln which depends upon the hue and reflectance of the surface. Surfaces having zero chroma, and therefore no hue, are denoted neutral (N).

Value is plotted vertically through the colour solid and measures the lightness of the surface from 0 (perfect black) to 10 (perfect white). Value is another way of describing reflectance, but differs numerically from reflectance in an important respect: each of the three Munsell scales is divided in such a way that equal intervals of hue, value or chroma denote approximately equal steps in perceived contrast (though the value scale does not have the same spacing as the hue or chroma scales). The approximate relationship between reflectance R (%) and Munsell value V is given by the equation:


I

Figure 5. CIE (x,y) chromaticity diagram 1931

I 0 0-1 0-2 0 3 0.4 0 5 0-6 0 7

U'

Figure 6. CIE uniform (u',vl chromaticity diagram 1976.


White

scale

Black

Figure 7 Schematic diagram of the Munsell colour system (reproduced with kind permission of the Munsell Color companx Baltimore, USA).

Figure 8. Munsell colour solid with slice removed, showing boundary of 5 Y &ellow) hue plane (reproduced with kind permission of the Munsell Color companx Baltimore, USA).

The Munsell co-ordinates for a coloured surface comprise hue, value and chroma, in that order. For example, Munsell reference 2.5GY 618 indicates that the hue is 2.5GY, a distinctly yellowish green, the value is 6, which means a reflectance of about 0.3, and the chroma is 8 (moderately saturated but not startling).

Figure 8 shows the bounding surface of the Munsell colour solid. This indicates the relative positions of the highest possible chromas at each hue and value. Thus high chroma yellows have high value (reflectance) while high chroma reds and blues have low reflectances. (Web site: http://www.munsell.com)


Figure 9. Natural Colour System hue circle Figure 10. NCS constant hue triangle (reproduced by kind permission of the (reproduced by kind permission of the Scandinavian Colour institute). Scandinavian Colour Institute).

4.2 Natural Colour System

The Natural Colour System (NCS) was developed in Sweden by Hird and Sivilc (1981). In this system, colours are scaled according to their degree of resemblance to the six elementary colours: white, black, yellow, red, blue and green. Of these, the four chromatic colours are those in which no trace of any other can be seen, e.g. the red that contains no trace of either yellow or blue and so on. These four are placed 90" apart on the hue circle and form the basis for red-green and yellow-blue opponent axes, see Fig- ure 9. The white and black are the pure colours of this type that contain no trace of the other or of any of the chromatic elementary colours. Any given colour can have elements of no more than two of the chromatic colours plus white and black. For example, an orange hue might be said to bear 30% of red and 70% of yellow; the sum of the elements of the chromatic elementaly colours is always 100. The hue of this orange sample would then be Y30R. The degree of resemblance to black is called blackness (s), that to white is called whiteness (w), that to yellow is called yellowness (y), and so on. The degree of resemblance to a maximum, or completely chromatic colour, is called chromaticness (c). By dividing an elementary scale into 100 steps, it is possible to get a measure of the dcgrce of resemblance to the elementary colours.

The Natural Colour System (NCS) triangle (Figure 10) is therefore a diagram in which the resemblance of a colour to white, black and the maximum chromatic colour is plotted in terms of whiteness, blackness and chromaticness, and

Figure 10 illustrates the arrangement of samples in the NCS Atlas in a constant-hue triangle with comer points w, c and s. Arrays of constant chromaticness (c) are parallel to the vertical axis. Constant blackness (s) and constant whiteness (w) arrays lie on lines parallel to the sloping sides of the triangle and do not pass through the corners unless blackness or whiteness is zero. It is clear that it is only necessav to specify the hue, s and c in order to specify the colour completely. However, neither the hue circle nor the constant-hue triangles show spacings that are close to visual equality. This reflects the very


different guiding principles upon which it was based compared with the Munsell system. The NCS atlas contains 1412 colour samples and more recently, ICI (Paints) has introduced an atlas based on the NCS system. (Web site: http:llwww.ncscolor.com)

Figure 1 7. Colour solid of the DIN system: T = hue, 5 = saturation, D = darkness (reproduced by kind permission of Ellis Horwood).

4.3 DIN system

The DIN (Deutsches lnstitut fir Normung) system was developed in Germany by Rich- ter and Witt (1986). The three variables used are hue (T), saturation (S) and darkness (D), quoted in that order and the arrangement is shown in Figure 11. The system uses a standard daylight designated D,, light source, and CIE tristi~nulus values for the samples available. Hunt describes tristimulus values in 'Measuring Colour' (1991). There are 24 principal hues, having values of T = 1 for a yellow, via reds, purples, blues, and greens to a yellow-green of T= 24. These principal hues were chosen to represent equal hue differences between adjacent pairs all round the hue circle.

The second variable, saturation, is a function of the distance from the point represent- ing the reference white on a chromaticity diagram, and for colours of the same reflectance (not the same darkness) equal saturation represents equal perceptual differences from the grey of the same reflectance.

The third variable is darkness, rather than lightness, and this is not related in a simple way to reflectance.

The colour solid associated with the DIN system is formed by having the grey scale as a vertical axis, with white (D = 0) at the top and black (D = 10) at the bottom. Different hues are represented by 24 different planes with one edge coincident with the grey scale axis and the same angle (15") between adjacent planes.

An atlas for the DIN system is available, known as the DIN Colour Chart: the sample size is 20 x 28 mm. In the atlas, for each hue page, colours of equal DIN saturation are arranged in columns parallel to the grey axis, instead of in lines radiating from the black point.


4.4 BS 5252: 1976: Framework for colour co-ordination for building purposes

A system widely used in the building industry in the UK is the British Standards system BS5252 (1976) which is a list of 237 surface colours. Though cross-referenced to Munsell - co-ordinates, BS 5252 adopts another colour notation system specially developed to avoid some of the anomalies of the Munsell system.

The three dimensions of colour in BS 5252 are: hue, designated by an even number; greyness, designated by a letter, and weight, designated by an additional number, usually odd.

12 distinct hues are identified plus 'neutral': 00 = neutral 02 = red-purple 04 = red 06 =warm orange 08 = cool orange I0 =yellow 12 = green-yellow 14 = green 16 = blue-green 18 = blue 20 = purple-blue 22 = violet 24 = purple

The estimated grey content of surface colours is represented on a five-point alphabetical scale:

A = grey B = near grey C = distinct hue D = nearly clear E = clear, vivid colour

Generally, greyness correlates well with Munsell chroma but the two concepts are not interchangeable, for black (00E53) and white (00E55), having no greyness content, are clear (group E) not grey (group A).

Surfaces having the same weight generally look equally light, but the correlation between weight and reflectance, even within a given greyness group, is uneven. In groups A and B the correlation is excellent, in group C it is good, but in D and E it is poor. This is because saturated reds and blues tend to look lighter than yellows or greys of the same reflectance: the ohenomenon is ltnown as the Helmholtz-Kohlrausch effect (Hunt. 199 1). , A

Individual colours are identified by a combination of a hue number, a greyness group letter and a weight number, in that order. For example, 12B27 means that the hue number - is 12 (greenish yellow). The greyness group is close to grey) but with a slight hue. The weight number is 27, indicating a darkish tone. The colour is a dark olive green.

Subsets of colours suitable for particular applications have been picked from BS 5252 and published as separate British Standards see BS 4800, 4900, 4901, 4903, 4904. For example, the BS 4800 paint colours are listed in Table 1, with their approximate Natural Colour System and Munsell equivalents and their reflectances.


Table 1. Approximate NCS a n d Munsell references a n d reflectances f o r colours in BS 4800: 1981 Greyness Colonr Hue Approximate Approximate Approximate group designation NCS Munsell reflectance

Neutral Neutral Neutral Neutral

Yellow Yellow Yellow

Red Red Red

Yellow-red Yellow-red Yellow-red Yellow-red Yellow-red

Yellow Yellow Yellow Yellow Yellow

Green-yellow Gren-yellow Green-yellow Green-yellow Green-yellow

Blue Blue Blue Blue

Violet Violet

Red-purple Red-purple Red-purple Red-purple

Red Red Red

Yellow-red Yellow-red Yellow-red

Yellow-red Yellow-red Yellow-red Yellow-red

Yellow Yellow Yellow Yellow

Green-yellow Green-yellow

reference 1501-Y03R 3101-Y26R 5301-R64B 7501-R97B

2002-Y03R 4302-Y09R 6702.G98Y

0906-Y78R 1409-Y83R 3810-Y76R

0606-Y41R 1607-Y41R 4107-Y41R 6308-Y40R 8105-Y53R

0504-Y21R 1811-YOIR 4011-G99Y 621 1-G90Y 8305-G89Y

0807-G73Y 1812-G75Y 3915-G65Y 6313-G57Y 8207-G53Y

1704-B78G 4004-B57G 6405-B14G 8205-B06G

1000-N 1804-R58B

11 18-R07B 3531-R17B 5331-R21B 73 15-R24B

1019-Y86R 3632-Y85R 5136-Y87R

1517-Y35R 4034.Y45R 6525-Y40R

0809.Y32R 2430.Y24R 4340-Y 18R 6724-Y22R

0811-Y16R 1122-Y03R 2536.G99Y 6921.G95Y

1623-G72Y 6626-G49Y

reference N 8.5 N 7 N 5 N 3

5Y 810.5 5Y 610.5 5Y 410.5

10R 911 1 OR 812 1 OR 612

lOYR 9.2511 8.75YR 812 8.75YR 612 8.75YR 412 8.75YR 212

5Y 9.2511 5Y 812 5Y 612 5Y 412 5Y 212

5GY 911 2.5GY 812 2.5GY 612 2.5GY 412 2.5GY 212

5 8 811 5B 611 5B 411 7.5B 211

IOPB 911 lOPB 812

7.5RP 814 7.5RP 516 7.5RP 316 7.5RP 214

7.5R 814 7.5R 516 7.5R 316

7.5YR 814 5YR 516 7.5YR 316

IOYR 912 IOYR 716 IOYR 516 l0YR 3.6

5Y 912 5Y 8.514 5Y 716 5Y 314

2.5GY 814 2.5GY 314


Table 1 continued Grevuess Colour group designation

Hue

Green Green Green

Blue-green Blue-green

Blue Blue Blue

Purplc-blue Purple-blue Purple-blue

Violet

Purple Purple

Red Red

Yellow-red Yellow-red

Yellow Yellow


Blue-green

Blue

Purple-blue

Violet

Red Red Red

Yellow-red Yellow-red Yellow-red Yellow-red

Yellow Yellow Yellow


Green Green

Blue-green

Blue Blue Blue Blue

Pu~~le -b lue

Black

White

Approximate NCS reference 0609-G12Y

Approximate Munsell reference 5G 911 5G 712 5G 314 5G 212

7.5BG 812 7.5BG 514

5 8 9.2511 7.5B 713 7.5B 314

5PB 814 5PB 516 5PB 214

IOPB 516

7.9' 813 7.5P 316

7.5R 4110 7.5R 3110

7.5YR 6110 5YR 418

5Y 7110 5Y 518

2.5GY 618 2.5GY 416

7.5BG 316

7.5B 516

5PB 318

IOPB 318

7.5R 913 7.5R 6/12 7.5R 4.5116

7.5YR 818 2.5YR 7111 5YR 5112 lOYR 7.5112

10Y 914 5Y 8.518 6.25Y 8.5113

2.5GY 8\10 5GY 7111

Approximate reflectance

0.81 0.45 0.10 0.07

0.60 0.22

0.84 0.42 0.10

0.63 0.23 0.07

0.22

0.60 0.10

0.16 0.10

0.33 0.16

0.45 0.24

0.32 0.15

0.10

0.22

0.10

0.10

0.80 0.33 0.18

0.60 0.46 0.24 0.51

0.79 0.64 0.64

0.60 0.44

0.34 0.22

0.22

0.79 0.60 0.31 0.15

0.32

0.05

0.85


4.5 RAL Design System

A further system commonly used by designers is the RAL Design System which was developed by the German Institute for Quality Assurance and Specification about 70 years ago. It is used to define surface colours for paint and other manufactured materials and is based on an atlas of 1688 colours. The method of defining colours is similar to the Munsell system using three parameters for each colour, i.e. hue (red, yellow etc.), lightness (which is similar to the Munsell value) and chroma (similar to Munsell chroma). Whilst the earlier systems described were based on visual organisation, the RAL system is based on a mathematical colour space using the CIE chromaticity system. The RAL coding system uses three number groups which give the position in the CIE colour space (CIELab using either cartesian or cylindrical co-ordinates) and in the RAL atlas. (Web site: http://www.ral.de.

5 Measurement of reflectance and colour

There are several methods by which to measure the reflectance and/or colour of a surface, and these are described below. Whichever method is chosen, it is important to consider the magnitude and importance of the measurement errors which can arise. To minimise the magnitude of these errors, instruments should be calibrated or checked using reflectance or colour standards which have themselves been calibrated to an appropriate degree of accuracy. For example, a manufacturer of textiles or paints will usually use a spectrophotometer for making measurements and check its performance using reference materials calibrated at NPL (the National Standards Laboratory in the UK) or other approved laboratory. A householder repainting the woodworlc in a room, on the other hand, will generally make visual comparisons between the existing paint and a swatch of colour samples from a paint manufacturer; accuracy is not an important consideration in this process. Further information on reflectance and colour standards is given in Section 6.

5.1 Measurement using a spectrophotometer

The most accurate measurements of the reflectance and colour of a surface are made in the laboratory using a specially designed measurement system called a spectrophotometer. A spectrophotometer typically comprises a light source, a monochromator or other means of splitting the light into its constituent wavelengths, a compartment or holder in which the reflectance sample is placed, and a detector. For a specular surface it is only necessary to arrange the detector at the correct angle to the sample so as to collect the reflected beam. For a diffusing material, however, some form of optical arrangement (which could include lenses, mirrors, a small integrating sphere etc.) is needed between the sample and the detector so as to collect all the light reflected regardless of angle, or to collect the light reflected into a defined range of angles. It is important to note that for some materials the reflection properties also vary with the angle of the incident light. To avoid the confusion which may arise from the multiplicity of different geometries under which measurements could be made, a number of standard measurement conditions have been defined.

Those often used include:

Oo/t = light incident perpendicular to the surface, all reflected light collected


Oold = light incident perpendicular to the surface, diffused light collected, specular reflection excluded

0°/45" = light incident perpendicular to the surface, light reflected at 45O to the normal collected

8"lt = light incident at 8' to the perpendicular to the surface, all reflected light collected

These standard geometrical conditions are useful when comparing the reflectance of surfaces with similar light-scattering properties. For a more complete description of the reflectance of a surface (as required, for example, in the design of road lighting) measurements may be made at a range of incident and reflected angles.

Their complex nature means that spectrophotometers are relatively expensive and difficult-to-use instruments and their use is therefore generally restricted to those applications where accurate and precise measurements of reflectance and colour are needed, such as the manufacture of paints and textiles.

5.2 Measurement using a colorimeter

For many applications it is the colour of a surface which is of interest, rather than its detailed reflectance properties. As mentioned above, colour depends not only on the properties of the surface but also the characteristics of the light falling on it. A measurement using a spectrophotometer allows the colour of an object to be calculated for any known illuminating source. Other instruments, called colorimeters, can measure colour directly using detectors which are designed to mimic the response of the tristimulus distributions for the CIE 193 1 Standard Observer. For the results from such instruments to be meaningful, it is important that the illuminating source used during the measurements is the same as that to be used in the final environment. For specification and measurement purposes, one of a range of agreed standard illuminants is generally used, the most common being simulated 'D65' (an approximation to daylight with a correlated colour temperature of 6500 K) and 'A' (a tungsten filament lamp at a colour temperature of 2856 K) (CIE, 1986).

As in the case of spectrophotometric measurements, the results of measurements made using a colorimeter will depend on the geometry of illumination and collection of the light. For simplicity, most colorimeters for measuring the colour of a surface use a fixed geometly, typically 0°/45", and therefore the measured colour may not truly represent the visual appearance of a surface, particularly if it has strong specular reflection or is pearlescent (i.e. its colour varies with angle of view).

Some colori~neters can also be used to measure the average reflectance of a surface. Once again, the results depend on the illuminant and measurement geometly used.

The level of uncertainty associated with measurements made using a colorimeter is generally considerably higher (i.e. worse) than that for spectrophotometric measurements, due to the fact that the detectors never perfectly match the tristimulus distributions. This means that their value for making absolute measurements (as required, for example, in the design of a material to match a numerical colour specification) is rather limited. However they are ideally suited to the evaluation of small colour differences between surfaces and they find widespread use in applications such as the quality control of paints and dyes, where a production sample is compared with a reference sample for quality control purposes.


5.3 Measurement using luminance and Illuminance meters

For aperfectly diffusing surface, the luminance Lv(in cd/m2) is related to the reflectance, r and the illuminance E,,(in lux) as follows:

Therefore if one measures the luminance of such a surface and the illuminance of the light falling onto it, the reflectance can be determined.

For the purposes of lighting design calculations, it is frequently acceptable to assume that all the surfaces within a space act as perfect diffusers. This relationship then gives a method by which to calculate the reflectance of surfaces in the field by the use of a luminance meter and an illuminance meter. However it should be noted that the relationship does not hold for heavily textured surfaces, or those with significant specular reflection. For these surfaces the relationship between luminance and illuminance is a complex one; this method will give erroneous results and therefore should not be used.

It is important to use meters that have been calibrated for the type of lamp which is being used to illuminate the surface, preferably by an accredited laboratory (BS 667: 1996 and BS 7920: 1998). The procedure is as follows: (i) Use the luminance meter to measure the luminance of a representative portion of the surface, taking care to align the meter so that its measurement axis is perpendicular to the surface and ensuring that it does not significantly affect the illumination falling on the surface. (The majority of surfaces approximate a perfect diffuser most closely at angles close to the perpendicular.) (ii) Place the illuminance meter on the surface at the position previously viewed using the luminance meter and record the illuminance. (iii) From these two measurements, calculate the reflectance using the relationship given above.

There will typically be a fairly large uncertainty associated with the result of such a measurement, even in the case of a perfect diffuser, and thus for critical applications measurement in a laboratory using one of the techniques described above may be necessiuy.

5.4 Field measurements using the reflectance card

The reflectance sample card provided with this publication is intended to enable lighting designers and architects to make an assessment of surface reflectance to the degree of accuracy appropriate for use in lighting design software or manual calculation routines. The following procedure is recommended: (i) Hold the card against the surface, at a comfortable viewing distance, taking care not to obstruct the light falling on to the surface and ensuring that the card is uniformly illuminated with diffuse light. (ii) Select the column which most closely resembles the colour of the surface. In situations where there is doubt over which column is closest in appearance, use the two (or more) which are most similar and take the average result. In addition the grey column can be used with any colour surface. (iii) Select the sample(s) within the chosen column(s) which appear to be closest in brightness to the surface (which can be viewed through the holes in the card). Read off the reflectance value(s) for the type of lamp with which the surface is being illuminated. Where the brightness of the surface appears to lie between two adjacent samples in a colour column, the mean reflectance of the two samples may be used.

Liqhtinq Guide: Surface reflectance and colour 23

(iv) If necessary take the mean of the reflectance values from the two (or more) colour columns chosen.

5.5 Visual assessment

The human eye is a very adaptable organ, dealing easily with changes in illumination ranging from moonlight to bright sunlight. However this adaptability makes it relatively poor at making numerical assessments of illumination levels, colour, reflectance etc. Visual 'measurements' can only be made by making comparisons between a surface which has been measured using a physical measurement device and an unknown surface. A typical example i.s colour matching of paints or textiles, where visual comparisons between the test and reference material are an excellent method by which to judge when an acceptable match is obtained. Indeed even the best colorimeters and spectrophotometers find it difficult to exceed the eye's performance in the identification of small differences in reflectance or colour.

As in the case of measurements using colorimeters, luminance and illuminance meters, or the reflectance sample card, the results of a visual assessment of a surface will depend on the illuminating source and the measurement geometry. It is not unusual for two surfaces to look similar or to co-ordinate with each other under one illurninant, but not under another - an effect known as metamerisin. Similarly a glossy and a matt surface are compared, the reflectance of the two may appear similar at some angles but very different at others.

6 Reflectance and colour standards

The reflectance and colour of a surface, whether diffuse or specular, can be measured using a number of techniques, as described in Section 5 . It is very important to many industries that measurements made on a wide range of spectrophotometers and colorirneters agree. Colour matching and co-ordination is very important in many applications and disagreements between instrument scales would make this almost iinpossi- ble.

In order to achieve agreement, reflectance and colour transfer standards are available, traceable to NPL (the UK's National Standards Laboratory). There are wide range of standards available including a set of 12 ceramic colour standards, black, white and grey tiles, and white, grey and coloured Spectralon (a form of scintered PTFE). Some of these are available with both glossy and matt surfaces. The white, grey and black reflectance standards are used to check the linearity of the detector system and scale i.e. to check whether the instrumental readings of reflectance are correct. Holmium oxide reflectance tiles are used to check the wavelength scale i.e. to confirm whether the wavelength dis- played by an instrument is correct. Ceramic colour standards are used to check the overall performance of instruments and their software for the measurement of reflectance and colour. Figures 12 and 13 show typical reflectance as a function of wavelength for grey and coloured ceramic tiles. Figure 14 shows a selection of the standards available from NPL.

Some laboratories use pressed barium sulphate or magnesium oxide as a white diffuse reflectance standard, but these can be very easily damaged and have a short lifetime before degradation, so are not recommended. Other materials such as opal glass are also sometimes used as standards, but they are more susceptible to damage than ceramic tiles or Spectralon standards and their use is therefore decreasing.


I 400 500 600 700

Wavelength, nm

Figure 12. Diffuse reflectance as a function of wavelength for pale, mid and deep grey tiles

Wavelength. nm

Figure 13. Diffuse reflectance as a function of wavelength for ceramic colour standards

7 Surface appearance

Much of this document has been concerned with the physical parameters of surface materials and their relationship to lighting and particularly reflected light. However these do not define with any certainty the exact appearance of the surface. It is not the intention to discuss this complex subject in any real detail but to ignore it would be equally inappro- priate. A surface will appear as it does because of the physical parameters of the surface,


Figure 14. Selection of coiour standards

the way it is illuminated and its relationship to the rest of the scene as well as the visual state of the observer.

The physical attributes of the surface that need to be considered are its reflectance, its surface colour and the surface finish including the structure or make-up of the material which could affect its appearance. Regarding the illumination the level of illuminance, the illuminance distribution over the surface and the spectral distribution of the light will all need to be considered. The luminance and colour composition of the surrounding scene will also have some effect, which in turn will affect the observer's visual state and particularly the brightness and colour adaptation. Brightness and colour constancy may also have an effect, but only if it breaks down, which except in rare cases this is unlilcely to happen in real spaces. Constancy will normally be upheld as long as the observer receives sufficient visual cues as to the way the scene is lit and for the purposes of this document it will be presumed that this is the case.

Considering each of the elements in turn:

Surface reflectance: this is the feature of a surface that determines its degree of lightness or darkness. For example if it is required that a surface, or an interior, appears 'light' then it will be essential for the surface or surfaces to have a high reflectance. A dark surface can never be made to appear 'light' how ever much light is incident upon it. Conversely a high reflectance surface will never appear truly dark just by using a low level of illuminance - it will only appear dingy or gloomy.

Surface colour: this is the feature of a surface that determines its unique colour i.e. its redness, blueness, greenness or yellowness and as long as there is a sufficient illuminance and an amount of neutral or achromatic surface colours within the field of view then the appearance should be largely correct.

Surface finish and texture: this will not affect the total amount of light being reflected from the surface but it will affect its appearance. For example a truly matt surface will have the same brightness from any position of view although its projected area may change. However a surface with some degree of polish will have two elements in its appearance. It will have that light which penetrates the polished skin of the surface and

26 Society of Light and Lighting and Natonal Physical Laboratory

Figure 15. Polished surface illuminated to show colour and highlight.

Figure 16. Textured surface illuminated by diffuse, even illumination.

Figure 17. Textured surface lit with directional, oblique light


reflects from the base colour and emerges from the surface as showing the surface as that colour. On top of that will be a reflected image of the illumination source providing the 'gloss' to the surface. The size and position of this will depend on the geometric relationship between the illumination source and the observer. For textured surfaces, if the surface is lit very uniformly from all directions then the surface will appear as a uniform reflectance - see Figure 15. But if the illuminant has a prominent direction relative to the surface then the surface will take on a very different appearance - see Figure 16. This will be because the light will reflect from the peaks of the surface, as a luminance determined by the base reflectance of the material, but there will also be shadows created by those peaks which will cause the surface to have a generally lower overall brightness. It is the pattern of luminances within the surface that provides the information to the brain that the surface is textured and not just a surface with a lower brightness s e e Figure 17. A textured surface, particularly when lit obliquely, will create a greater visual interest within the scene than the overall brightness would suggest.

Surface illumination: this divides into three main considerations. First is the spectral power distribution of the light source, which will affect the colour

appearance of the surface. For example if a surface is green in colour and it is lit with a source that contains radiation of all wavelengths then the surface will appear green. If however it is lit by a source that has no green radiation e.g. a low pressure sodium lamp, then the surface will appear grey or black. If it is required that surface colours should appear reasonably accurate then the illuminant must have a CIE Colour Rendering Index (R") of not less than 80 and if high fidelity colour appearance is required then the Ro must be not less than 90. Lighting a surface with a coloured illuminant is usually only done when a special effect is required for decorative purposes e.g. exterior floodlighting. In this case great accuracy is rarely a requirement.

Secondly, the level of surface illuminance will also need to be considered. For example if a painted mural on a wall was lit at a low illuminance, e.g. lower than 50 lux, the colours would start to appear indistinct and accurate colour discrimination could begin to be impaired. Perhaps more importantly the colours would begin to loose their vi- brancy. If the full colour impact of a surface is required then an illuminance of around 200 lux or above will be required. The ultimate effect will also be determined, to some extent, by the observer's level of adaptation. For example if a room is lit at a very low general level of illuminance and hence the occupant's adaptation level is low, then a surface lit to say 50 lux could be acceptable. This is the situation commonly found in art galleries, used for the display of light sensitive exhibits e.g. watercolours.

Thirdly the illuminance distribution should be considered. For example if a surface is lit with a high illuminance uniformity then it will appear uniformly bright and uniformly coloured. Alternatively if the surface is lit very non-uniformly, e.g. with an illuminance diversity (minimum / maximum) of say 1: 100 or greater, then the surface will take on an uneven appearance both in tenns of brightness and colour; there are situations where this effect may be acceptable or even required. The question the designer needs to ask is what is the effect that is required? The surface finish will also have some effect on whether a surface appears evenly lit or not. For example if a surface is heavily textured then a high illuminance diversity will often be masked, becoming acceptable or even preferable in terms of its lit appearance.

Effect of contrast between one surface and another: the apparent brightness and colour appearance of a surface will be influenced by the degree of contrast between the


Figure 18. Assimilation. When viewed from abour 2 m the uniform grey background looks lighter with white jointing, darker with black jointing. Although assimilation is predominantly an aspect of lightening or darkening, it is not exclusively so. Thus small scale areas surrounded by narrow, distinbctively coloured joints, say red or blue, may appear tinged with the same hue.

surface and adjacent or surrounding surfaces. For example if an achromatic surface of a particular luminance is surrounded by a surface of a lower luminance the tendency will be for the surface to appear brighter and conversely if the surround is at a higher luminance then the surface will appear darker. The effect is known as brightness induction and has the effect of exaggerating a luminance difference between one surface and another - se Figure 18.

A similar effect occurs between coloured surfaces and neutral surfaces, and between different coloured surfaces. A coloured surface will talce on a hint of the complementary colour of the surrounding surface - see Figure 19. This effect is known as colour induction. Therefore a neutral grey surface if surrounded by a green surface will take on a red or pinkish tinge and the same grey surface if surrounded by a blue surface will talce on a yellowish tinge and vice versa. This means that a greater contrast can be achieved by using this effect.

Visual adaptation (brightness and colour): an element of the human visual system includes an automatic process known as adaptation. In the case of brightness adaptation it works rather like the automatic exposure control on a camera in that it adjusts the exposure value to the average luminance of the scene and the areas of higher luminance appear light whilst the areas of lower luminance appear dark. This means that a particular luminance will vary in brightness if the adaptation state changes. A similar process occurs for colour although this is usually less noticeable. If a person works for some time in a room lit with a particular coloured light source, for example in a photographic darkroom lit with red light. After a time the person will cease to be aware of the red source due to colour adaptation, but when switching to a normal 'white' source the scene will take on a greenish tinge for a short time until the eye readapts. The process of adaptation takes time to change from one level to another and designers need to be aware of this and to engineer the lighting to allow this to happen without loss of visual comfort or visibility. This will be required, for example, in a museum where adjacent galleries are lit to different levels for conservation purposes. To allow the transition to take place a linking corridor should be provided which is lit at an intermediaiy luminance.


Figure 19. Contrast and assimilation effects, The small squares within the pattern are printed the same grey as the separate square but look different, through the combined effects of contrast and assimiia tion.

The above phenomena will all affect the appearance of surfaces, some more than others, but they all need to be borne in mind by designers when planning the appearance of a task and the appearance of a lit environment to achieve the most effective performance and visual quality.

8 Explanation of terms

This Guide uses a number of terms, both scientific and descriptive, that may not be familiar to all readers. The following is provided to help overcome the problem.

Area-weighted reflectance: This represents the average reflectance of a surface which is made up of areas of different reflectance but weighted by the areas of the individual parts.

Colour rendering: A general expression for the appearance of surface colours when illuminated by light from a given illuminant compared, consciously or unconsciously, with their appearance under a reference illuminant. Good colour rendering implies simi- larity of appearance to that under an acceptable light source, such as daylight.

Colonr rendering index: A measure of the degree to which the colour appearance of surfaces illuminated by a given light source conform to those of the same surfaces under a reference illuminant, suitable allowance having been made for the state of chromatic adaptation. The system commonly used is that adopted by the CIE in 1965 (updated in 1974 and 1994) which provides a measure of difference between the two values.

Colour temperature (kelvin): The temperature of a Planckian, or 'full radiator' whose radiation has the same chromaticity as that of the light source being considered.

Correlated colour temperature (kelvin): The temperature of a 'full radiator' which emits radiation with a chromaticity nearest to that of the light source being considered.


Diffuse reflection: Reflection in which the reflected light is diffused and there is no significant specular reflection, as from a matt paint. See also Uniform or Lambertian Difhser.

Gloss: The mode of appearance by which the reflected highlights of objects are perceived as superimposed on the surface due to the directionally selective properties of that surface.

Gloss factor: Ameasure of the specular reflectance of a surface, usually measured at an angle of 45". Measurements are generally made using a glossmeter (a spectrophotometer or calorimeter with a.fixed angle of illumination and view) by reference to a calibrated gloss standard (typically a polished clear glass plate with blacltened base and sides, which has a gloss factor of -100).

Illuminance (lumenlsquare metre): The luminous flux density at a surface, i.e. the luminous flux incident per unit area.

Luminous flux (lumen): The light emitted by a source, or received by a surface. The quantity is derived from radiant flux by modifying the radiation in accordance with the spectral sensitivity of the standard eye as described by the CIE standard photopic observer (V(L)).

Luminance (candelalsquare metre): The physical measure of the stimulus which pro- duces the sensation of brightness measured by the luminous intensity of the light emitted or reflected in a given direction from a surface element, divided by the projected area of the element in the same direction. The relationship between luminance and illuminance for a reflecting surface is given by the equation:

luminance = illuminance x reflectance factor 7,?

Note: for diffuse surfaces reflectance factor is replaced by reflectance

Luminance factor: The ratio of the luminance of a reflecting surface, viewed in a given direction, to that of an identically illuminated 'perfect white' uniform diffusing surface. For a non-matt surface the luminance factor may be greater or less than the reflectance.

Mixed reflection: Partly specular and partly diffuse reflection, as from smooth, glossy paint. See Luminance factor.

Radiant flux (watt): The power emitted, transferred or received as radiation.

Reflectance: The ratio of the luminous flux reflected from a surface to the luminous flux incident on it. Except for matt surfaces, reflectance will depend on how the surface is illuminated but especially on the direction of the incident light and its spectral distribution. The value is always less than unity and is expressed either as a decimal or a percent- age. Note: Reflectance can also be measured based on radiant flux rather than lurninous flux.

Liqhtinq Guide: Surface reflectance and colour 31

Reflectance factor: The ratio of the radiant or luminous flu reflected into a given cone to that reflected in the same cone by aperfect reflecting diffuser, identically irradiated or illuminated.

Spectral (power or energy) distribution: Sometimes referred to as 'spectral composition'. The variation of radiant power (or energy) over a range of wavelengths.

Specular reflection: Reflection without diffusion in accordance with the laws of optical reflection, as in a mirror.

Uniform (or Lambertian) diffuser: An ideal surface for which the radiation coming from that surface is distributed angularly according to Lambed's cosine law (i.e. whose radiance or luminance is the same in all directions and whose luminous intensity at angle 0 to the normal to the surface varies as the cosine of the angle 8) irrespective to how it is illuminated.

Further reading

CIBSE Code for interior lighting (London: CIBSE) (1994) CIE Publication 15.2 (1986): Colorimetry (Vienna, Central Bureau of the CIE) (1986) BS 667: 1996: Specification for illuminance meters (London: British Standards lnstitu- tion) (1996) BS 4800: 1989 (1994): Specification for paint colours for building purposes (London: British Standards Institution) (1989) BS 4900: 1976 (1994): Specification for vitreous enamel colours for building purposes (London: British Standards Institution) (1976) BS 4901 : 1976 (1986): Specification for plastics colours for building purposes (London: British Standards Institution) (1976) BS 4904: 1978 (1997): Specification for external cladding colours for building purposes (London: British Standards Institution) (1978) BS 5252: 1976: Framework for colour co-ordination for building purposes (London: British Standards Institution) (1976) BS 6770: 1988 (1994): Guide for the exterior colours for park homes (mobile homes), holiday caravans and transportable accommodation units (London: British Standards Institution) (1988) BS 7920: 1998: Specification for luminance meters (London: British Standards Institu- tion) (1998) Bean, A R and Simons R H, Lighting engineering: applied calculations (Oxford: Butterworth-Heinernann) (2001) Coaton J R and Marsden AM, Lamps and lighting 4th edn, (London: Arnold) (1997) Hird, A. and Sivik, L, Color Res. Appl., 6 , 129 (1981) Hunt, R.W.G. Measuring colour 2nd edn (Chichester: Ellis Honvood) (1991) Hunter R S & Harold R W, The measurement of appearance (Chichester: Wiley & Sons) (1987) Nickerson, D, Color Res. Appl. 1, 7, 69 and 121 (1976) Padgham C A& Saunders J E, The Perception of light and colour (London: Bell & Sons) (1975)

Chartered Institution of Building Services EngineerslNational Physical Laboratory. Lighting Guide I I : Surface Reflectance and colour

Reflectance sample card

The reflectance sample card is intended t o enable lighting designers and architects t o make visual assessments of surface reflectance t o the degree of accuracy appropriate for use in lighting design software or manual calculation routines.

Table of surface reflectance values

Light source A B C D E F G H

grey brown ochre olive red blue green yellow

Row I Incandescent 10 14 25 18 25 15 13 39 Fluorescent 840 10 13 25 18 23 16 15 38 Daylight D65 10 13 23 17 19 17 16 36

Row 2 Incandescent 15 18 31 25 31 24 21 47 Fluorescent 840 15 18 31 25 29 25 23 47

Davlieht D65 15 17 30 24 25 26 24 45 Row 3 Incandescent 24 27 40 33 40 35 34 56

Fluorescent 840 24 27 39 33 38 35 35 56 Daylight D65 24 26 38 33 35 37 36 54

Row 4 Incandescent 36 41 50 47 53 49 47 64 Fluorescent 840 36 41 50 47 52 50 48 64 Daylight D65 36 40 48 46 49 51 50 63

Row 5 Incandescent 55 58 64 61 65 63 62 73 Fluorescent 840 54 58 64 61 64 63 62 73 Davlieht D65 55 58 63 61 63 64 64 72

Row 6 Incandescent 74 77 79 77 77 78 78 81 Fluorescent 840 73 76 79 77 76 78 78 81 Davlinht D65 74 77 79 77 76 79 78 81

Procedure

i Hold the card against the surface, a t a comfortable viewing distance, taking care not to obstruct the light falling on the surface and ensuring that the card is uniformly illuminated with diffuse light.

2 Select the column which most closely resembles the colour of the surface. In situations where there is doubt over which column is closest in appearance, use the two (or more) which are most similar and take the average result. in addition, the grey column can be used with a surface of any colour.

3 Select the sample(s) within the chosen coIumn(s) which appear to be closest in brightness to the surface (which can be viewed through the holes in the card). Read off the reflectance value(s) for the type of light source with which the surface is being illuminated.Where the brightness of the surface appears t o lie between two adjacent samples in a colour column, the mean reflectance of the two samples may be used.

4 If necessary, take the mean of the reflectance values from the two (or more) colour columns chosen.

Note: Although every care has been taken t o select stable printing inks for the reflectance sample card, the patches are likely to change in colour andlor reflectance if they are exposed t o light for long periods. Users are advised t o store the card in the protective sleeve provided when it is not in use.This will also help to keep the card clean.

In the event that the reflectance sample card becomes dirty or damaged, a replacement copy can be purchased from ClBSE Publications.

CIBSE,ZZZ Balham High Road, London S W l Z 9BS Copyright retained 2001


Reflectance sample card

The reflectance sample card is intended to enable lighting designers and architects to make visual assessments of surface reflectance to the degree of accuracy appropriate for use in lighting design software or manual calculation routines.

Table of surface reflectance values Light source A B C D E F G H

grey brown ochre olive red blue green yellow Row 1 Incandescent 10 14 25 18 25 15 13 39

Fluorescent 840 10 13 25 I8 23 16 15 38

Daylight D65 10 13 23 17 19 17 16 36

Row 2 Incandescent 15 18 31 25 31 24 2 1 47

Fluorescent 840 15 18 31 25 29 25 23 47

Daylight D65 15 17 30 24 25 26 24 45

Row 3 lncandesccnt 24 27 40 33 40 35 34 56

Fluorescent 840 24 27 39 33 38 35 35 56

Daylight D65 24 26 38 33 35 37 36 54

Row 4 Incandescent 36 41 50 47 53 49 47 64

Fluorescent 840 36 41 50 47 52 50 48 64

Daylight D65 36 40 48 46 49 5 1 50 63


Fluorescent 840 54 58 64 61 64 63 62 73

Daylight D65 55 58 63 6 1 63 64 64 72


Fluorescent 840 73 76 79 77 76 78 78 8 1

Daylight D65 74 77 79 77 76 79 78 8 1

Procedure

1 Hold the card against the surface, at a coinfortable viewing distance, taking care not to obsbuct the light falling on the surface and ensuring that the card is uniformly illuminated with diffuse light.

2 Select the column which most closely resembles the colour of the surface. In situations where there is doubt over which column is closest in appearance, use the two (or more) which are most similar and take the average result. In addition, the grey column can be used with a surface of any colour.

3 Select the sample(s) within the chosen column(s) which appear to be closest in brightness to the surface (which can be viewed through the holes in the card). Read off the reflectance valne(s) for the type of light source with which the surface is being illuminated. Where the brightness of the surface appears to lie between two adjacent samples in a colour column, the mean reflectance of the two samples may be used.

4 If necessary, take the mean of the reflectance values from the two (or more) colour columns chosen.

Note: Although every care has been taken to select stable printing inks for the reflectance sample card, the patches are likely to change in colour andlor reflectance if they are exposed to light for longperiods. Users are advised to store the card in the protective sleeve provided when it is not in use. This will also help to keep the card clean.

In the event that the reflectance sample card becomes dirty or damaged, a replacement copy can be purchased froin CIBSE Publications.

Lighting guide 11 : surface reflectance and colour : its specification and measurement

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