Warning: Sometimes you have to hit “refresh” a few times for the charts to load on this page. Sorry :(. I don’t know what I’m doing wrong on word press to cause this…

What is special about red?

Red is special for many reasons. It is my daughter’s favorite color and it is mine too. (Have you read Red is Best by Kathy Stinson? I recommend it!) One reason many people with achromatopsia like red tints in their sunglasses is probably that the rods in human retinas are much less sensitive to red light than to other colors of light. This means that rods are not so easily overwhelmed and blinded by red light as by blues or greens.

The chart below shows how sensitive rod cells are to different wavelengths of light in the visible spectrum. I plotted the data from Dartnall, Bowmaker, and Mollon (1983). You can see that peak sensitivity is on the border of blue and green and then tails off towards zero (being so low as to not be detected by Dartnall, Bowmaker, and Mollon) for reds above 620 nm.

Noir 90 vs Noir 95

Noir describes filter #90 as “45% Medium Red”, meaning it blocks 55% of light and lets 45% through. Noir describes filter #95 as “14% Medium Red”, meaning it blocks 86% of light and lets 14% through. One would naturally expect that filter #95 is the darker lens, and so many people are surprised to find when they try both filters that filter #90 is actually “darker”.

What is going on? As we all know, its not just how much light the lenses block, but what color they are that matters. The problem is that there is a lot of nuance to the color of lenses that we can’t describe with simple phrases like “medium red”. It turns out that filters 90 and 95 are very different reds, despite both being described as “medium red”.

To get the full picture we can look at the spectral curves for both filters. A spectral curve describes how much light a lense lets through for each wavelength of light. I have plotted the spectral curves for filters 90 and 95 in the chart below. (Thanks to David Bothner, Marketing Director at Noir for sharing the underlying data with me!)

As you can see from the chart, filter 90 blocks ALL light wavelengths below 580nm, meaning it blocks ALL blue and green light. This makes it what I’ll call a “pure” red. Filter 95, on the other hand, is not a “pure” red, because it lets in a little bit of blue and green light (3–5%). Filter 95 does block more light overall because it blocks more red light than filter 90. But it appears lighter to someone with achromatopsia because rod cells in our retinas are so much more sensitive to the little bit of blue and green it lets in.

My daughter finds filter 95 to be too light to wear outside, but thinks it great for indoor use in bright lighting. In this setting, where its okay that the lens is not darker, my daughter likes the fact that the 95 filter lets in a very little blue and green light. She says that it doesn’t “change the colors” so much. This makes sense. Inside without glasses, blues will look lighter than reds to my daughter (as rods are more sensitive to the blues). This is still true when she wears filter 95. But if she wears something like filter 90, suddenly the shades reverse and reds become lighter than blues. Blues become dark because they are totally blocked by the 90 filter. 

NoIR Filters versus Custom Tints

A handy thing about spectral curves is that we can use them to compare NoIR lenses to custom tints. For example suppose that like my daughter you like filter 95 for indoor use but want it with an Rx. Then which custom tint should you try? BPI FL-41 seems to be a good choice. My father put my daughter’s indoor glasses (FL-41 dye tinted to about 45% transmission) into a spectrometer to measure their spectral curve and you can see they are pretty similar to NoIR 95:

The next chart compares my daughter’s outdoor glasses (FL-41 dye tinted to about 5% transmission) to NoIR 99. 

You can see that NoIR 99 lets more yellow and orange through. One thing that the chart hides, however, is how they compare in the blues and greens because the transmission is too close to zero to see any difference on the chart. So let me give you the numbers. At a 500 nm wavelength, transmission is:

Lens	Transmission at 500 nm
#93 4% Dark Red	0.001%
#99 4% Dark Red	0.3%
My daughter’s outdoor glasses (FL-41, 5%)	0.2%
BPI Deep Red #37880  (BPI Monochrome 600)	Claim: zero

As you can see, filter 93 and BPI Deep Red are “pure” reds that really let in no green or blue at all. Filter 99 and FL-41 tinted to 5% transmission are not quite pure reds. They let in very little blue and green (less than 1%) but they do let in some. Joe couldn’t tell much difference between filters 93 and 99. My daughter likes the 93 filter better in bright sun because the 99, while pretty good, is not as dark as she would like. However, the 99 does  “change the colors” less than filter 93. Our  bright red car still appears dark in the sun with #99 filter or FL-41 custom tint, but pops brighter with the 93. This makes sense, as after all, filter 99 does let in 300 times more blue/green than filter 93.

My advice: if you like 93 better than 99 then I guess that you’ll like BPI Deep Red better than FL-41, and vice-versa. But it could be that they are not much different. 

What do NoIR’s other filters look like?

Here are spectral curves for some NoIRs other filters, beginning with the lighter reds. 

You can see that none of these lighter reds are “pure” reds, they all let in at least some blue and green light. Next, are the darker NoIR reds:

Its hard to tell from the chart above, but filters 90 and 93 really are “pure” reds letting in almost no blue or green. Filter 99 is close to a pure red, but not quite. It does let in a little blue and green (less than 1%). Next are filters 7 and 33. Filter 7 is 2% dark amber which my daughter didn’t like, but Frances Futterman says that others do.

And here is the spectral curve for BPI Deep Red #37880, which is one of the dyes recommended for custom tinting by  Dr. Gunilla Haegerstrom–Portnoy. BPI claims it is a pure red, blocking all light below 600nm:

Let me know if you would like the underlying spreadsheet behind these charts. If there is demand, I’ll try to put it on the website too. 

Update May 2020: I’ve just learned from the ever flowing source of knowledge that is the Facebook Achromatopsia Support Network about an amazing resource. Chadwick Optical has a tint library with spectral curves for a wealth of different lenses. Check it out!