Archived posting to the Leica Users Group, 2004/02/02

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Subject: [Leica] Re:Color vision
Date: Mon, 2 Feb 2004 14:53:24 EST

In a message dated 2/1/04 10:58:46 PM, Adam writes:

<<  The human eye uses more than three types of 
> color sensors - but then nature is not constrained by accountants or
> engineers.

Really? What are there? I thought there were (in most people) three different
sets of cones with different spectrum sensitivities and rods which are 
luminance sensing.

I'm not an expert though, and I don't play one on TV.

What I have found fascinating is that we have built-in edge-detection and
sharpening, which definitely supports our pattern recognition functions.

The last time I studied this area, I recall that there were five to seven 
different types of cones sensitive to various ranges of color in the human eye. 
More may have been discovered recently. The idea that there are three types of 
cones, each sensitive to a primary color stems from research done in the 
1880s. At that time Young and Helmholtz independently attributed color vision to 
the operation of three types of cones in the eye (red, green, and violet). This 
conclusion was based on the finding that most colors could be matched 
additively by various amounts of the three primary light sources. This theory was 
seized upon by both printers and photographers since it prescribed the simplest 
technique for reproducing color. The theory gained such wide currency that most 
elementary physiology textbooks stated that the eye worked in this manner and 
that's what most of us learned in school. Unfortunately, the tricolor theory 
could not explain color vision anomalies such as red green blindness, blue 
yellow blindness, or achromatism. For example red/green color blind person can see 
yellow quite clearly although the trichromatic theory presupposes that yellow 
is a mixture of red and green and both receptors must be working. Yellow/blue 
color blindness is much rarer but produces the additional complication that 
the red and green sensors are working but the resulting yellow color cannot be 
distinguished from blue, although red and green can be clearly distinguished.

Despite the seductive simplicity of the tricolor approach, visual 
physiologists were forced to conclude that the actual visual mechanism may be different. 
Some theories proposed combination red/green sensors, yellow/blue sensors and 
a luminosity sensor. (Sort of like the way your color TV signal is modulated.) 
Others added additional sensors which modulated the color response of three 
primary sensors. Still other theories added additional primary sensors. The 
jury is still out on the exact mechanism of color sensation. One thing all modern 
theories have in common, however, is that considerable image processing is 
done in the eye and the brain before colors, edges, movements, and shapes are 
perceived. Human vision is MUCH more complicated than we were taught in Biology 

All that being said, the tricolor approach to color representation has been 
wholehearted adopted by technology because it provides the easiest and cheapest 
method of adequately capturing and reproducing MOST colors. The three colors 
chosen form a rough isosceles triangle superimposed over the color diagram 
shown in most textbooks. Any color inside the triangle can be represented by 
various amounts of the three chosen primaries. If you have a Mac computer, the 
Color Picker extension lets you see this triangle and adjust the actual hue and 
saturation of a color on the screen. But there are colors outside the triangle 
which the three chosen primary colors cannot reproduce. You can get these 
colors by choosing another set of primaries but then some colors that you could 
specify previously are unobtainable. Preferences for the color rendition of 
specific color films (Fuji vs. Kodak) are in part due to slight differences in the 
manufacturer's choice and balance of primary colors. The answer to more 
accurate color rendition is to use more than three primaries, perhaps four or five. 
This is the answer that most high end inkjet color printers have chosen to 
get around the limitations of subtractive pigments or dyes.

So we just have to live with it. As long as our color films use only three 
color layers or our digital cameras use three types of color sensors, there will 
be colors our eyes can see but our cameras cannot reproduce.

Larry Z 
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