Thursday, February 23, 2017

Nuances of Blue Vision

In the Jan/Feb 2007 issue (#425, p. 3) of ISCC News, I described a personal journey with Benham disks under colored lights.  The below testimonial describes a journey that has a point of similarity (a variety of saturated colors evoked by a flickering flame), but which is offered with complete anonymity of the author, for reasons that will be obvious.

“I just read a fascinating article by Esther Inglis-Arkell at http://io9.gizmodo.com/why-viagra-tints-your-vision-blue-1685176169.  The article begins with the question: ‘One of the lesser-known side effects of Viagra is blue-tinted vision. How does a pill that's supposed to help men maintain an erection cause them to see the world as too blue?’ The answer is that Viagra inhibits two related enzymes in the human body, phosphodiesterase 5 and phosphodiesterase 6. The effect of inhibiting the former is the desired effect of smooth-muscle relaxation so as to promote an erection. The effect of inhibiting the latter is to enhance visual sensation by the rods. I accept all the biochemistry proffered here, but I believe the visual effect of Viagra deserves a personal description, perhaps similar to the journey of Don Juan (by Carlos Castaneda or otherwise…)

“To say ‘blue-tinted vision’ is misleading, especially given the article’s graphic of a blue-tinted pair of sunglasses. The light always appears brighter than expected, rather than darker as suggested by the sunglasses.

“Here are my own experiences, which may relate to rod vision but through a rather complicated mechanism.

“Within about an hour after taking the pill, if I sit in a relatively dark room and look at a patch of daylight through a window, a blue-white haze suffuses the patch and extends beyond its boundaries. Within the patch itself, the white is very bright and looks as if the scene outside is covered by an optical brightening agent and illuminated by a UV-active light source. Although the effect is stronger for peripheral vision, it persists when I stare directly at the patch.


“There is also an artifact due to moving objects, which I call the bright spider web effect.  If I suddenly move a paper or other object that has an edge, an electric blue-white image of the edge will remain for about half a second.  It is thin, possibly with corners (following the shape of the edge), and seems to reside in my peripheral vision: I have not had a direct look at it with my fovea. It doesn’t occur every time an object moves in my visual field, but it can replicate multiple times in a single motion---hence the web-like appearance. I’ll need to study it further.  One exaggerated form of the effect occurred recently: When I moved my forearm in front of a bright background, a fuzzy, electric blue-white stripe remained that was much wider than the bright spider web: at least half an inch across, and sharper on the side where my forearm was.  Like the spider web, it persisted for perhaps half a second.  I found I could repeat the effect.

“ Finally, there is an effect that seems to have nothing to do with blue tints. While I was sitting at night in front of a bonfire at a spa, the flickering flames elicited bright, Benham-like colors: Saturated greens, reds, yellows, and blues, winking in and out as the flicker went on.  I knew this was the result of the pill because I had experienced no such effect upon visiting that fire the previous night without having taken the pill.

“In closing, I can confirm the reassurance of Inglis-Arkell: The visual effects of Viagra, like the performance-enhancing aspects, wear off after a few hours.  Not to worry. Enjoy the experience.”

Anonymous





Looking through these glasses will not produce the blue effect described here [image from Inglis-Arkell, op. cit.].

Monday, November 14, 2016

Paradox Lost

In the Spring 2016 issue (#474) of ISCC News, I described what appeared to be a paradox that emerged from an exercise for students [1]: “If the wavelength of the green line of mercury is 546 nm in a vacuum, what is it in water? In heavy flint glass? [410 nm, 331 nm].” Would change of refractory material abutting the eye’s photoreceptors cause the color of a light to change as drastically as suggested here? More precisely, would changing the refractive index change the tristimulus values of a light with emitted spectral power distribution E(λ)?

Unlike in the wavelength domain, the change of refraction index would not affect the frequency of the transmitted light. I concluded that the proper choice of integration domain in which to perform tristimulus integration should be a matter of experiment.  This conclusion was incorrect. As often happens when one poses a paradox, closer examination provides a resolution. The tristimulus values should not change if the integrals are performed properly.  I describe the resolution in a tutorial note recently posted for publication in Color Research and Application [2].

It is reasonable and respectful of physical principles that a tristimulus value [say, M = the integral over λ of  E(λ) m(λ)] should not depend on the domain of integration used to obtain it. In [2] it is shown that the domain invariance from wavelength λ to frequency ν = c/λ is assured if the spectral power distribution of the test light in frequency is represented as E*(ν) =|dλ/dν| E(λ(ν)).

 Now imagine a transformation from frequency to two alternative wavelength domains λ1 = c1/ν  and λ2 = c2/ν , where c1 and c2 represent speeds of light in different refractive indices (possibly functions of ν ).  Here, M doesn’t change either; rather, the spectral power distribution E*(ν) is multiplied by the Jacobian |dλ1/dν  |-1 or | dλ2/dν |-1, respectively. (Things get a bit more complicated when λ1 or λ2 is a nonmonotonic function of ν , but the principle is the same.) Of course, changing from λ1 to λ2 is equivalent to changing from λ1 to ν  and then from ν  to λ2.  There is no predicted variation of M, by construction.

So my “gentle paradox” evaporates by construction. Obeying the rules of mathematics leads back to where one started: tristimulus values don’t depend on the refractive index of the transmitted medium.
 
Where does that leave the role of experiment? If one were to measure the camera version of tristimulus values in different refractive media (say, with M being a measured camera value for a light E through a camera sensitivity function m), we would expect no change in those values.  But experiment can trump theory. If M were observed to change, then a new paradox would surface. From “paradox lost” we would see “paradox regained.”

References:
[1]. HQ Fuller, RM. Fuller, and RG Fuller, Physics Including Human Applications. Harper and Row, 1978. Revised electronic version copyright 2009 by RG Fuller. http://physics.doane.edu/hpp/Resources/Fuller3/pdf/F3Chapter_19.pdf exercise 2, p. 436.
[2] MH Brill, Ways to define a tristimulus value. Color Res. Appl. 2016, Early View, DOI 10.1002/col.22079.

Michael H. Brill
Datacolor