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Colorblind Glasses
Written December 15, 1991

A cartoon, The Far Side.  Gary Larson shows a dog saying its bedtime prayers.

“. . . And please let Mom, Dad, Rex, Ginger, Tucker, me and all the rest of the family see color.”

That got me to thinking.  If I had a dog's eyesight and wanted to determine the color of an object, how would I go about it?

Well, being involved in the technical end of television, I'd probably shoot the object with a color TV camera and analyze the signal with a vectorscope.

There's a simpler method:  look through color filters.  A blue object will look black if viewed through a green or red filter, but normal if viewed through a blue filter.

This method might not work for dog vision.  If dogs' eyes are sensitive only to a single wavelength of light, it's as though they're always looking through a filter of that color, and filters of other colors will only darken the picture.

But what about colorblind humans?

It's my understanding that in the most common type of colorblindness, the patient can see blue but can't distinguish between green and red.  I'm guessing that the receptors for green light and red light are both present and functioning, but the patient's brain can't tell them apart.  They must both look like a sort of dark yellow.

2023 UPDATE:  This could be an evolutionary throwback.  Most mammals have only two types of color receptors in their eyes, one for blue (peaking around a wavelength of 445 nanometers) and one for yellow (555 nm).

The ancestor of our particular branch of apes apparently experienced an accidental gene duplication, giving them three types of receptors:  blue and yellow and yellow. 

Over time, it became advantageous for the two yellow types to diverge slightly, specializing so that one peaked at 535 nm (green) and the other at 575 (red).  Now these apes could distinguish those two colors — as can most modern humans.

Why would this evolutionary adaptation be helpful for survival?  Perhaps because it makes it easier to spot ripe red fruit in green foliage.

COLORBLIND: YELLOW & YELLOW  

NORMAL: GREEN & RED

How a normal person might see a picture.

How a colorblind person might see the same picture.  The green grass and the red flag stripes appear to be the same color, as do the greenish-blue sky and the reddish-blue flagpole.

Now what if we gave the patient a set of 3-D movie glasses?  I'm referring to the old kind from 40 years ago, with green cellophane over the left eye and red over the right.

Actually, it works better if the left eye has a cyan filter that lets through both green and blue light but blocks red, and the right eye has a magenta filter that lets through both red and blue light but blocks green.


Above:  How the picture appears when a person with normal sight uses the glasses.  Below:  How it appears to a colorblind person.

 

 

 
Both filters admit blue light, so blue objects look normal.  White objects will look bluer than normal, but the patient will still be able to distinguish them from blue objects because there's also some "dark-yellow" light getting through the filters (actually green through the left, red through the right).

But green objects will look brighter to the left eye than to the right, and red objects will look brighter to the right eye than to the left.

I'm not colorblind myself, but I do have a little swatch book of color filters for theatrical lighting, so I was able to get some idea of how this would work.

It turns out that while looking through the cyan and magenta filters, both green and red objects take on a peculiar "specular" appearance.  The brain doesn't know how to interpret an image that's much brighter in one eye than the other, so it guesses that the image must be that of a shiny object.  (If you hold up a piece of aluminum foil, the different angles of reflection to your two eyes might reflect the bright window to one eye and the dark wall to the other, thus producing a similar effect.)

So a colorblind person using my glasses would feel that he was wearing blue-tinted sunglasses, except that a few dark-yellow objects have this specular appearance.  If such an object brightens when he closes his left eye, it's red.  If it darkens, it's green.  After some experience, it's possible that the brain would automatically determine which eye is getting the brighter signal, so that without the necessity of winking, the colorblind person could actually "see" full color!

There are some limitations, of course.

Indoors or at night, these glasses would be as much a hindrance to the wearer as sunglasses would be.

Although bright green and bright red objects look specular, more subtle shades don't stand out unless you do the alternate-eye wink.

It's possible that colorblind eyes are actually not sensitive to both green and red.

It's possible that an adult brain could never learn to interpret a brighter-in-the-left eye object as a green object rather than merely a specular object.

The idea is as simple as bifocals, so Ben Franklin probably already thought of it and found it impractical.

And a person wearing 3-D glasses all the time would look a little goofy.

 

TBT

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