GEK1532Differences in Color Vision

Thorsten WohlandDep. Of Chemistry

S8-03-06Tel.: 6516 1248

E-mail: chmwt@nus.edu.sg

http://members.shaw.ca/hidden-talents/vision/color/colorblind1.html http://www.allpsych.uni-giessen.de/karl/colbook/sharpe.pdf

ProblemWe remarked that we cannot find three actual colors that could mix all colors in the CIE diagram.

Historically, the fact that we have three cones was derived from the fact that we can mix (almost) all colors by using only three primary colors, namely the colors discussed for additive and subtractive color mixing.

The existence of exactly three cones was then confirmed by physiological research and the sensitivity curves for the three cones were measured.

Mixture of three colors

STL, Chapter 9

Assume you choose 460, 530, and 650 nm.

We then proceeded to describe all colors by only three values. This should be possible since with three cones we get only three signals from our retina.

We found out that we really need only three values to describe all colors. But no matter which existing color we chose, we always end up with some of the values beeing negative (meaning some of the exisiting colors are not in the gamut of our choice of colors).

Mixture of three colors

STL, Chapter 9

It turns out, though, that to describe colors with three POSITIVE values we need to assume some “theoretical” colors.

www.adobe.com

www.adobe.com

www.adobe.com

Fig. 1-10 of Nassau

additive mixing

Additive vs subtractive color mixing

The CIE diagram is based on additive color mixing

Subtractive color mixing is not easily predictable by the CIE diagram. And the mixture of two pigments often leads to curved lines in the CIE diagram.

subtractive mixing

The reason for this is that in additive mixing we work with three well defined colors.

But in subtractive mixing we have to work with pigments and dyes which occur naturally. These compounds do not just reflect single defined wavelength but absorb differently over the whole visible spectrum.

Different types of color deficiencies and their frequency

All defects we discussed up to now lead to people that are color deficient. People that are color blind are …

Monochromats (achromatic vision) cone monochromats (one cone works)

rod monochromats (no cone) -> photophobic

Defect Frequency of occurrence in males

monochromats 0.01 %

deuteranopes 2 %

protanopes 2 %

tritanopes < 0.1 %

Anomalous trichromacy 5 %

See chapter 14 of Kurt Nassau

The color circle as viewd by red-green deficient people

From Scientific American, Special on Color (German Version)

Normal vision Red-green deficient

See the following website for some pictures that demonstrate vision of color deficient people:http://en.wikipedia.org/wiki/Color_blindness

What colors do you see?

100% Sat

100% Brightness

10% Saturation25 % Brightness

Same Hues, but ordered differently

The “Anomaloscope”

From Scientific American, Special on Color (German Version)

a) Normal vision

b) No red

c) No green

d) Red anomalous

e) Green anomalous

The number of cones

Retina independent color anomalies

With age the lens of humans becomes more and more yellow (same happens with cataracts).

Your brain adapts to that and you still perceive white as white etc.

However, when you paint, the colors you use will contain more yellow (Metamers).

Retina independent color anomalies

Normal a) and b) Cataract c) and d)

Color of object a) and c)

Color of paint chosen b) and d)

From Scientific American, Special on Color (German Version)

a) and b) are metamers for the normal painter

c) and d) are not metamers since the yellow lens filters out different amounts of blue light in the object and paint color (note that the object color contains more blue than the paint color)

Claude Monet

From Scientific American, Special on Color (German Version)

Color defectsRoughly 10 % of males and 0.5 % of females have color defects.

Rarer cases include one sided color defects (unilateral color deficiency) or

Digit-color synesthesia in which digits can elicit a color perception (Nature 406, 365, July 27, 2000).

Do you see the shape?

Hearing Color, Tasting Shapes – Scientific American, May 2003, p. 53 – 59.

Can you recognize the number?

Stare at the plus sign and try to identify the number on the right.

53

333+

How does the retina now work?

There are several phenomena in nature which any theory how the retina works has to be able to explain:

Lightness constancy

Color Constancy

Weber’s law

Lightness and Brightness

Lightness: Attribute of a visual sensation of an illuminated area that describes the intensity of the stimulus in relation to a white illuminated area. (light - dark)

Brightness: Attribute of a visual sensation that describes the intensity of the stimulus (bright - dim).

Weber’s law

abab lnln

beba a ln

Seeing the light, Fig. 7.4

We call this a logarithmic scale:

On the left the reflected light intensity increases by equal amounts.

On the right the ratio of adjacent intensities is constant.

b2 22

2 1

b

b 212

221 lnln

lnln

b

bb

b

Weber’s law

xe

xaa

ln

Seeing the light, Fig. 7.4

We call this a logarithmic scale:

ln(1) = 0 = 0*ln(2)

ln(2) = 0.693…= 1*ln(2)

ln(4) = 1.386…= 2*ln(2)

ln(8) = 2.079…= 3*ln(2)

On the left the reflected light intensity increases by equal amounts.

On the right the ratio of adjacent intensities is constant.

Lightness ConstancyWeber’s law states that we see brightness in logarithmic scale.

However, we know as well that we perceive something white always as white, no matter how bright the illumination is. This phenomenon is called Lightness constancy.

Lightness constancy thus means that we see objects always in relation to the surrounding. So when the illumination changes, the brightness (absolute intensity) changes, but not the lightness (the ratio of different brightnesses).

Good illumination Darker illumination

http://www.purveslab.net/seeforyourself/

Color Constancy

Color constancy describes the same effect for the perception of color: Colors tend to stay the same, independent of the intensity of the illumination (remember that a change in color in the illumination will of course change as well the perceived colors).

Why is this advantageous to a human beeing?

Two examples were lightness and color constancy do not work!

1. Lightness changes not uniformly everywhere.

2. At dim light, the rods are starting to work and add their signal to the cone signal.

Lightness changes not uniformly everywhere

The rod system

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

Kurt Nassau, Fig. 1-16

Scotopic vision: night vision, based on rods; maximum sensitivity: around 500 nm

Photopic vision: day vision, based on cones; maximum sensitivity around 550 nm

Mesopic vision: transition from photopic to scotopic vision, both systems operate (e.g. at dusk)

Scotopic vs Photopic Vision

http://www.cquest.utoronto.ca/psych/psy280f/ch3/purkinje/ps.html

Purkinje shift

Scotopic vision: max sensitivity ~500 nm Photopic vision: max sensitivity ~550 nm

Mesopic vision: Humans have characteristics of tetrachromat

Short Summary

• Color deficiencies

• Weber’s law and laws of color and lightness constancy

• Color of the cone system

• Influence of the rod system on color