L31, 32_17080_DLF Components and Calculation

8/10/2019 L31, 32_17080_DLF Components and Calculation

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The concept of DaylightFactor (DF) was developedin the United Kingdom inthe early 20th century.

Daylight Factor is a ratiothat represents the amountof illuminationavailableindoorsrelative to theilluminationpresent

outdoorsat the same timeunder overcast skies.


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A daylight factoris the ratio of internal light leto external light level and is defined as follows

DF = (Ei / Eo) x 100% where,

Ei= illuminance due to daylight at a point on tindoors working plane,

Eo= simultaneous outdoor illuminance on a

horizontal plane from an unobstructedhemisphere of overcast sky.


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Daylight Factor is typicallycalculated by dividing thehorizontal work plane illuminationindoors by the horizontal

illumination on the roof of thebuilding being tested and thenmultiplyingby100.

FOR EXAMPLE, if ther

20,000 lux available ouand 400 lux available given point indoors, thDF for that point wocalculated as follows:

DF = 400/20,000 *1DF=2.


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Due to the variability of out-door lightinglevels it is difficult to calculate interior

lighting in photometric illumination term


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The daylight factor concept is valid (the ratioremains constant) only under overcast skyconditions when there is no direct sunlight.

THREE COMPONENTS WILL CONTRIBUTE TTHE DAYLIGHT FACTOR:

1. Sky component (SC)2. Externally reflected components (ERC)3. Internally reflected components (IRC)

DF = SC+ERC+IRC


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DAYLIGHT ENTERINGINABUILDI


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THREE COMPONENTS OF D YLIGHT


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Direct (sky) component: SC


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Externally reflected component: ERC


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Internally reflected component: IRC


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The Magnitude Of Each Of The Components Depends On TheFollowing Design Variables:A. SC-the area of the sky visible from the point considered a

average altitude angle (i.e. the luminance of sky at that p

window size and position in relation to the point, thicknwindow frame members, quality of glass and its cleannes

external obstructions.B. ERC-the area of external surfaces visible from the point

considered and the reflectance of these surfaces.

C. IRC-the size of room, the ratio of wall, etc. surfaces in relto window area and the reflectance of these indoor surfac


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The concept is used to establish desirable or min. daylight requirements i

rooms of various uses.


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The Building ResearchEstablishment BRE(formerly the BuildingResearch Station BRS)developed a set ofprotractors which givedirect reading of thesky component inpercentages. There areten nos. of suchprotractors, of which

five are for the uniformsky and five for the CIEsky:

Series 1 Series


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Follow this link for detailed description, if needed http://www.up.edu.ps/ocw/repositories/pdf-archive/DAYLIGHTCALCULATIONSReadOnly/body.html
http://www.up.edu.ps/ocw/repositories/pdf-archive/DAYLIGHTCALCULATIONSReadOnly/body.htmlhttp://www.up.edu.ps/ocw/repositories/pdf-archive/DAYLIGHTCALCULATIONSReadOnly/body.htmlhttp://www.up.edu.ps/ocw/repositories/pdf-archive/DAYLIGHTCALCULATIONSReadOnly/body.htmlhttp://www.up.edu.ps/ocw/repositories/pdf-archive/DAYLIGHTCALCULATIONSReadOnly/body.html


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PROTRACTORS


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PROTRACTOR


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Refer to the elevation view of a room in Figurea. Mark the work plane and the pointconsidered (O). Connectthis point to the window sill (P) and the window head (R) to

(PO) and (RO) respectively. Make a tracing of protractor

Place the tracing with its centreon point (O)and its basealongthe w

plane, with scale 1upright. Readthe valueswhere lines (PO) & (RO)intersect the protractor scal

The differencebetween the two values gives the initial sky compone

Find the average altitude

Readthe anglesof elevation of lines (PO) & (RO), onthe innerdegree Addthe two angles and dividethe sum by 2.


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a.

(R)(O)

(P)


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D t i th ti f t (d fi d l th f i d )


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Determine the correction factor (defined length of window)

Refer to the plan view of the same room in Figure b.

Mark on the plan the point considered (O).

Connect this point to the two sides of the window opening (MO) & (NO)

Place the protractorwith its centre on point (O), with the baseparalle

window and scale2towardsthe window.

Interpolate an imaginary semicircle between those designated Oo, 3

and 90o, corresponding to the average altitude previously obtained.

Readthe values along the curves on the inner scale. Interpolate if neces Find the sum of the two readings, if they were taken on either side

centre line.

Find the differenceof the two readings, ifthey were both taken on th

sideof the centre line.

SC= Initial sky component X Correction Factor


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b.

(M)

(N)

(O)


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If there is an obstruction opposite the window, then thelimit for the sky component will be, a line drawn from(O) to the top of this obstruction with reference to Figand b.

The sector between this line and the sill (RO) line will givERC. Follow the same procedure as for the sky compone

Multiply the result by the reflectance of the obstruct

known) or by 0.2if the CIE-sky protractors were used. Using the uniform sky protractors, take half of the reflec

value or 0.1as the multiplying factor.


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Find the window area and find the total room surface area (floors, ceiling, a

including the windows. Calculate the window: total surface area ratio. Find ton Scale A.

Find the area of the walls and calculate the wall area: total surface area ra

this value in the first column of the small table.

Locate the wall reflectance and read the average reflectance where the colu

row meet. Note that ceiling reflectance is assumed to be .7 and floor to be .1

Locate the average reflectance on scale B and lay a straight edge from B t

value obtained in step 1.

The intersection of scale C is the average IRC if there are on external obstruct

If there are external obstructions locate its angle on scale D by measuring it f

horizontal at the centre of the window.

Lay a straight edge on D through the point on C and read the value on scale E

An average Maintenance factor(0.75) is applied to the IRC as a correction fact The DF is obtained by adding SC+ERC+IRC

Use of the Nomogram


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Where:

W= Window area (m2), A = Total internal surface area, wall, floors ceilings and windows

p1 = Area weighted average reflectance of areaA, (using 0.1 asreflectance for glass).

p2= Average reflectance of surfaces below working plane, p3 = Average reflectance of surfaces above working plane, C= Coefficient of external obstructions.

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Summary


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L31, 32_17080_DLF Components and Calculation

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