[visionlist] Was »Info on "coloured filters" sought« – some answers

[Prof. Michael Bach]

Dear Vision Friends:

A year ago I had written to this list
> Supposed to write a review on coloured filter glasses ('twas a situation where I couldn't really say no), I have a hard time finding good literature on this. …
and received much help from you, thank you again! The review has now appeared in print (with fantastic help by Klaus Rohrschneider), but only in German :).
Bach M, Rohrschneider K (2018) [Cut-off filter : Physical and physiological basics]. Ophthalmologe 115:922–92
http://doi.org/10.1007/s00347-018-0703-3
Rohrschneider K, Bach M (2018) [Edge filters : Medical indications and clinical application]. Ophthalmologe 115:916–921
http://doi.org/10.1007/s00347-018-0691-3


To give back to this non-German :) community I here briefly list the items where clearcut (and often new to me) findings cropped up. In French quotes »« the original question is given. 

»• Contrast: How can contrast improve, when by design there is less information behind a wavelength-selective filter than before it? One exception: short wavelengths suffer more scattering, so removing them can help. But is there more?«

Answer in 2 words: Wiener filtering. In more words: If the signal spectrum and the noise spectrum differ, then a filter can be designed that increases the SNR. Often removing the blue part of the spectrum can help, but certainly not in _all_ situations does a blue blocker (= yellow tinted filter) increase contrast.


»• In various retinal dystrophies (e.g. cone dystrophy or RP), tinted glasses are supposed to help. How does this work beyond "psychology"? Are there reliable experiments?«

(Only) one clearcut answer: In AMD filters have _not_ found to be beneficial.


»• “Brightening” glasses. There are types of sunglasses making the world appear brighter. How does that work?«

This was described here
Kelly SA (1990) Effect of yellow-tinted lenses on brightness. J Opt Soc Am A 7:1905–1911
http://doi.org/10.1364/JOSAA.7.001905
I favour this explanation by
Chung ST, Pease PL (1999) Effect of yellow filters on pupil size. Optom Vis Sci 76:59–62
Briefly: Blue light affects pupil size more than longer wavelengths. Removing it enlarges the pupil, leading to increased retinal illumination, thus more “brightness”.

»• I don't believe you can “cure” colour blindness / deficiency by tinted glasses.«

Briefly: yes. Longer: I include some pertinent demos in my 2018 paper (I like them :), see above for source). Better: Quite recently the highly hyped Enchroma lenses were tested (too late to include in our reviews)
Gómez-Robledo L, Valero EM, Huertas R, et al (2018) Do EnChroma glasses improve color vision for colorblind subjects? Opt Express 26:28693–28703
http://doi.org/10.1364/OE.26.028693
and they found: "The results show that the glasses introduce a variation of the perceived color, but neither improve results in the diagnosis tests nor allow the observers with CVD to have a more normal color vision." Nice work!

»Possible exception: creating interocular differences.«
Here I found a nice review: 
Sharpe LT, Jagle H (2001) I used to be color blind. Color Res Appl 26:S269–S272
and a great paper
Knoblauch K, McMahon MJ (1995) Discrimination of binocular color mixtures in dichromacy: evaluation of the Maxwell–Cornsweet conjecture. J Opt Soc Am A 12:2219–2229 
http://doi.org/10.1364/JOSAA.12.002219
Gist: “Despite the filter-aided increase in dimension, estimated wavelength discrimination was quite poor in this spectral region, suggesting a limit to the effectiveness of this technique.”
First mention 1857 (!, that’s why Ken called it the “Maxwell-Cornsweet conjecture):
Maxwell JC (1857) Experiments on colour, as perceived by the eye, with remarks on colour-blindness. Trans R Soc Edinb Earth Sci 21:275–298
http://doi.org/10.1017/S0080456800032117


Thanks, enjoy, best seasons greetings to all of you,
Michael


PS: no response necessary :), but for anyone interested I can send PDFs of any mentioned source.
-- 
Prof. Michael Bach PhD, Eye Center, University of Freiburg, Germany. 
Michael.Bach at uni-freiburg.de   <https://michaelbach.de>