Circles and red/green panels at opticians
September 27, 2007 8:55 AM   Subscribe

Eye tests: What's the purpose of the green/red rectangles with black circles in?

I had an eye test a few days ago. Part of the test involved looking at red and green oblongs on the illuminated panel, which contained two concentric circles in each. Unfortunately, I didn't listen correctly during the testing of my right eye, and thought that I had to tell the optician which rectangle seemed brightest. In fact, she wanted to know in which rectangle (red/green) the circles seemed most distinct, as I learned when she then tested my left eye. I was a little too embarrassed to tell her I'd slipped up earlier, so went with it.

How important is this? Is it likely my prescription will be screwed up?
posted by long haired lover from liverpool to Health & Fitness (7 answers total) 1 user marked this as a favorite
Is it a test for red-green test for color blindness?
posted by ericb at 8:59 AM on September 27, 2007 [1 favorite]

That was in fact a test for color blindness. It's probably no big deal, since corrective prescriptions cant do anything about that, but you may as well 'fess up to your doctor (ie opthamologist, not optician) when you get the chance. They cant help you if they dont know all the facts. Trust me, they've seen a lot worse than someone nervously making a mistake.
posted by elendil71 at 9:15 AM on September 27, 2007

No, it's not to do with colour blindness.

According to this page, from a manufacturer of software versions of the illuminated panels used by ophthalmologists, it's a duochrome test, a rapid method of assessing patients' refractive status.
posted by Aloysius Bear at 9:17 AM on September 27, 2007

It relies on chromatic aberration - the refractive index of a material is a (slight) function of wavelength usually. The idea is that if the prescription is right then the two wavelengths of light will focus just in front and just behind the retina, looking equally blurred. If one is obviously sharper than the other then it means the prescription is a bit off one way or the other.
posted by edd at 9:21 AM on September 27, 2007

You know how a prism separates colors? It's because different frequencies refract different amounts.

All lenses are prisms. What that means is that the ideal focus distance for a lense is actually a compromise. One color will focus above the retina, and the color at the other end of the spectrum will focus below it.

They use green instead of blue because it's the color we're more sensitive to. But if the green image is sharp and the red image is blurry, it means they don't have the focus adjusted properly. What they want is for both of them to be equally blurry, which means they've adjusted the tradeoff properly.
posted by Steven C. Den Beste at 9:23 AM on September 27, 2007 [1 favorite]

My bad, I did a bit more research. Safely ignore my comment.
posted by elendil71 at 10:00 AM on September 27, 2007

Hmmmm... I prefer the one where you stare into a box that looks like one of those ancient picture boxes from the 60s. You see a balloon on the horizon. You stare at the balloon. It magically comes into focus as the machine figures out your exact prescription.

If you're worried about getting the best prescription possible, find a doctor with one of these.
posted by TeatimeGrommit at 12:43 PM on September 27, 2007

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