# colorblind glasses: how do they work?

This video was in the news last week. In it, a grandfather is given a gift of special glasses that somehow alleviate the colorblindness he’s suffered all his life. Apparently they work really well, as he’s moved to tears by the profound difference in what he sees when he looks through them (see at 0:50).

So how do they do what they do?

Suppose you can’t differentiate green vs red - you’re presented with two identical objects - one is green - the other red - but they both look exactly the same shade of brown to you.

You look at them through a green filter, the red one will be darker, because the green filter blocks the red light bouncing off the red object (even though you still can’t perceive the colour, you can perceive the reduction in lightness). Correspondingly, if you look at them through a red filter, the green one will look darker.

So they enable people to differentiate between colours by selectively darkening one part of the spectrum.

IIRC, whatever they do, they don’t actually let a color-blind wearer pass that test with those dots that form a number if you can distinguish them from the other dots.

The technical name is an Ishihara test.

Parenthetical note: the Ishihara test is a major plot element in Jasper Fforde’s “Shades of Grey”, a dystopian-future novel in which society is divided into castes, based on which color a person can see.

My understanding is that the problem with color blind people is that the color sensing cones in their their eyes pick up not only the color they’re supposed to, but adjoining wavelengths as well. It’s not a lack of sensitivity, it’s a lack of discrimination, so that the red cones which should respond only to light of around 650 nM also react to light of 550 nM, which muddies up the colors. The glasses work by using narrow bandwidth filters, so that red light only between say 640 to 660 nM gets through, green light only 500 to 520 nM gets through, and so on for all the colors.

It’s a notch filter at a narrowband range in the spectrum. It increases the ease of classification between reddish and greenish hues that are muddled in some types of colorblindness (the more common types). Rather than having a single “hump” in your responses to a range of wavelengths, the filter lowers sensitivity in the middle, creating two humps. It is certainly not a cure, and colorblind people that I know are “meh” about them when they try them.

They also make “color blindness inducing glasses.” I’ve tried them (color normal). Pretty cool, but not something I’d buy for fun.

I saw that clip on FB or somewhere with the headline something like “Grandfather sees color for the first time” nope, that’s not what’s happening :smack: Seeing shade of gray is one rarer type of CB, but these won’t fix that at all.

I understand what you’re getting at, but color normal people’s red (L) cones respond to both 550 and 650, these aren’t narrowband receptors.

I saw a video of a color blind guy wearing his Enchroma glasses and being able to read the tests.

Some info here: (basically what has said before – filtering some of the code overlap)

Brian