Sunday, May 10, 2020

Published 8:04 PM by with 0 comment

How Do Enchroma Colorblind Glasses Work?

You might have seen clips of people putting on glasses and seeing color with them. How is it possible to cure colorblindness simply by wearing glasses?
For reference on what I'm talking about, Enchroma and Pilestone are the two companies I've seen that sell these glasses. They look just like normal sunglasses in pictures (see below), but I have never tried them to know what they look like in person.


Simple model for color vision

To start with, it's worth walking through a simple but sufficiently accurate model for color vision. Humans have a variety of photoreceptors in their eyes. These are effectively little sensors that respond differently when light of various colors hit them. Here's a crude model of the ones primarily responsible for color vision:



Those three photoreceptors are called 'cones': S is effectively 'blue', M is effectively 'green', and L is effectively 'red'. When light hits your eye, each of those photoreceptors responds to the light based on how much overlap there is between the sensor sensitivity and the intensity of the source. For example, consider the following two light sources:



The total signal from each sensor (area under curve obtained from multiplying light source intensity by sensor sensitivity) scaled to arbitrary units for source 1 is:
  • S: 0.00
  • M: 0.69
  • L: 1.00
The specific numbers aren't important here...the unscaled sum of the values is sort of how 'bright' the source is...the ratio of the sensor responses effectively tells you the perceived color of the light. If those are different for different light sources, the sources will appear to be different colors. 
  • S: 0.01
  • M: 0.77
  • L: 1.00

Simple model for color blindness

Now...how does color blindness affect this? There are many different types of color blindness. One common type that is easy to demonstrate these glasses with has the following effects on your eyes' sensors:



The M cone has shifted to longer wavelegths. The L cone already covers those though. Thus, the M and L cones now might respond very similarly to different light sources causing people with this issue to confuse those colors. This is a type of 'red-green' colorblindness. Let's calculate the total signals like earlier using these new sensors:
  • source 1
    • S: 0.00
    • M: 1.00
    • L: 1.00
  • source 2:
    • S: 0.01
    • M: 1.00
    • L: 1.00
Notice the issue? The S response is so small that we can basically ignore it, and the M and L sensors effectively have the same response to both light sources while someone with normal vision does not. A person with this set of cones will have more trouble differentiating these sources than a person with normal cones.

Simple fix using glasses

What if we could alter the responses of the M and L cones to make them see differently? Let's try applying a filter that blocks light between 550 and 570 nm:



Calculating the total signals for both light sources using these filtered sensors gives us:
  • source 1
    • S: 0.00
    • M: 0.86
    • L: 1.00
  • source 2:
    • S: 0.01
    • M: 0.96
    • L: 1.00
Notice now that the effective ratio of the L to M cone is different in this case for the two sources than it was in the colorblind case above with no filter? That means that with the filter in place, these two light sources will be perceived as different colors.

That's it. The glasses can appear to cure colorblindness simply by filtering light in areas where the cones have heavy overlap.

One important note...simply seeing two sources as the same color isn't color-blindness. Metamers can exist for normal vision also. The issue here is perceiving two sources as the same color when people with normal vision don't, and having a limited ability to resolve colors because of this.

One other note...while the glasses can help wearers differentiate colors that were difficult before using the mechanism I described here, they don't actually 'cure' colorblindness in that they don't make the wearer's cones match those of a person with normal vision.






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