Easy "Red Eye" Exit Pupil Method

Astronomers talk about the "Exit Pupil" of their optical systems. Most commonly it's used with binoculars but it also applies to telescopes. It's used in two ways, (1) to find the best match between the lowest power of your system and your eye's pupils, and (2) in binoculars which have a fixed magnification to get the maximum light available into your eye.

The rule of thumb used is that the exit pupil should be 7 mm. In binoculars, a 7x50 (7 power x 50 mm lens) is considered a near perfect fit for star gazing because it matches the exit pupil. A 10x70 would also be an excellent fit, but an 8x25 wouldn't provide enough light.

For more on this see Visual Astronomy on The Effects of Exit Pupil.

My son recently completed a Science Fair project on night vision which looked at pupil size and age. To get enough data he needed a quick, easy, and reliable method of measuring pupil size in the dark. They couldn't take everyone to an eye doctor and the "slit" method was neither easy nor quick.

What he came up with was the "red eye" method. By taking a flash photo with a digital camera after about a minute of darkness you can then use software like Photoshop or GIMP to measure the diameter of the red eye in pixels. The only other thing you need is an object of known size to find the number of pixels per mm. For this he used a dime (18 mm) on a Popsicle stick.

The photo above shows a 7 year old with an amazing 9mm pupil!

Later, I'll provide more about pupil size and age.

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