Science Buddies: "Ask an Expert"

I am using a histogram to measure the brightness level of the red in red-eye photographs. For my independent variable, I am having the subject look at different angles around the room. I have noticed that when the subject is looking at a certain angle (The 20 degree angle), the red-eye is replaced with a glowing yellowish-white color. This yellowish-white color only replaces the red in the eye that is closest to the camera. This is the subject's left eye. This yellowish-white color has occurred in all fifteen of my trials, at only the 20 degree angle.

When I put these digital photographs onto the computer and use a histogram, I measure the brightness level of the red in the subject's pupil. My question is: Do you know what causes this yellowish-white color? Also, should I write the brightness level of this color on the data collection sheet? I have noticed that the brightness level of this color is the maximum brightness level shown on the histogram. If I don't measure the left pupil (The yellowish one), should I measure the subject's right pupil, which is red, instead?

Thank you for reading this,
Naeh.

Hi Naeh,

Presumably anyone who is interested in red-eye would also be equally interested in any similar artifact. It sounds reasonable to discuss the yellow color in your report; however, since it's possible that whatever is causing the yellow may be different from what's causing the red-eye, you probably ought to treat them separately in your report. For the yellow case, recording both the brightness of the yellow and the red in the other eye might be a good idea.

What's more, it sounds like you've discovered something neat. It could be fun to try to determine what's causing the yellow color.

I'm no photography expert, and there may well be someone here who can provide more insight. That you only see it in one eye is surprising to me: I'd expect the difference in angle between human eyes is small compared to the repeatability you can achieve when having people look in a specific direction over many trials.

The first thing to investigate is whether it could be related to some geometry of the room itself. If you haven't done so already, try rotating both the subject and the camera by a few different angles (so that 0 degrees is facing in a different physical direction) and see what happens. Or, try moving to a different room.

Also, see if you can achieve the same effect in the other eye by rotating in the opposite direction. If you can get the same thing to happen in both eyes in different locations at different geographical angles, then you can claim that it's a real effect.

If you do decide to look into the yellow color in more detail, I'd love to hear what you find out. If you get a chance, drop us a note here.

Best,
Erik

Naeh,

I believe the source of the color that you have observed is the "blind spot" in the eye, where the retinal nerve fibers gather together to become the optic nerve and exit the eyeball to pass the captured images back to the rest of the brain.

Here is an excellent website with information about the retina:

http://webvision.med.utah.edu/sretina.html

The blind spot is approximately 15 degrees off axis from the fovea (the high resolution "center of vision"). It has a distinctly yellow white color compared to the rest of the retina, especially the darker color of the fovea.

The fact that only one eye shows the effect at a time is another clue... since the blind spot occur on the opposit side of of each eye... thus it can only be observed as "white eye" in one eye at a time.

The picture at the URL I listed above clearly shows the color difference between the reddish fovea and yellow-whitish blind spot.

Good Luck with your project.