Reference: Color Vision

[Andrea B.]

Color Vision by Bruce MacEvoy, 2015

 

This is a free, online reference which covers physiology of the human eye, color perception, color physics, color theory, color models and more. It is very well done. And given that it is online, it has liberal use of links for easy look-up. Other references are listed.

 

I stumbled on MacEvoy's work when looking for more about the Abney Effect. This is the misperception that hue has shifted when a color becomes more desaturated. I first became aware of this effect when looking at our UV botanical photographs which are "click-white" standardized to a yellow/blue/grey palette. I was always wondering why some of these photos seemed to have areas which were pale purple rather than pale blue. Turns out I was experiencing the Abney Effect. What I though were pale purples turned out to be desaturated blues.

 

Here is a blue gradient I made to show the Abney Effect. The right side is stepped in RGB increments of 32. Every RGB values on this chart has B=255 (when viewed as a TIF in Photoshop).

 

Do you see some pale purples in this chart?

[Andrea B.]

One of these colors is desaturated cyan and the other is desaturated blue.

However, to me, these colors appear to be pale blue and pale purple.

 

What do you see?

[Alaun]

Yes, there are purples!

 

Very nice reference, but -perhaps not fair- the first paragraph I read had an incorrect definition:

 

The ratio between the speed of light in air and its speed through a transmitting medium — which determines the amount of bending produced in the light beam — is the refractive index of the medium. The baseline wavelength and speed of light are usually measured in air at the earth's surface.

 

Instead of air it should read vacuum ;-) But the "usually measured in air" takes care of it :)

[Andrea B.]

We should write him and request a correction?

[Alaun]

Common perception: the second dots are pale blue and pale violet/purple

[Alaun]

No, as it is as he writes "commonly measured in air", so the error is not that big (unless you consider the colors of our sky throughout a day).