Proceedings of: Workshop on Improving Building Design for Persons with Low Vision

Visual Acuity

So what is visual acuity? When we’re defining low vision on a basis of [Snellen] test, what is it we’re talking about? Well, visual acuity is just a measure of the limit of your vision resolution (slide 7).

And this is what is called the ETDRS chart (slides 7 and 8). It’s a new design of the eye chart. And if you visit any low-vision clinic or you participate in any type NEI-funded clinical trial, you’re probably familiar with this chart. Okay, 20/20 visual acuity means that the smallest letter that a person can identify is five arc minutes – and I know there are [engineers] in this room, so you know what that means and I’ll explain for those who aren’t (slide 9).

Five arc minutes of visual angle and size and has a critical detail of one arc minute. Okay, if you drop a triangle from the eye to the letters – so the letters at the base of the triangle point to the triangle at the eye. That angle at the eye is what we’re talking about – the size of the angle.

There are 360 degrees in a circle; there are 60 minutes in a degree. So one minute is pretty small. If stars are separated by one minute apart, you can see them as two stars, if you have 20/20 vision acuity. So this is the definition of limited resolution. If they’re closer together than one minute apart, it looks like one star. They blur together.

Okay. The absolute limit to visual acuity, if the optics of the eye were perfect, would be about 20-over-eight. And that corresponds to the distance between pixels in your retina. Okay, these are the photoreceptors. They’re sort of like – think of those like pixels in your camera. We used to say [film] in your camera. But nobody knows what we’re talking about. So each photo receptor we thought of as a pixel. So the limited resolution induced by the pixelized nature of the retina is about – would give you about 20-over-eight, whereas 20/20 is a more practical definition of normal vision as the optics of our eye are anything but perfect (slide 9).

Now, conventionally, we specify visual acuity as the minimum angle of resolution, which abbreviated is M-A-R – MAR (slide 10). And it’s the ratio of the distance to the letters on the chart divided by the size of the letters. And when we’re talking about Snellen notation, which is the 20/20, it is the Snellen notation that dates back to the 1800s (slide 11). He was a Dutch ophthalmologist who invented the eye chart and visual acuity measurements.

The standardized distance is 20 feet. Those of you who develop offices know that nobody has a 20-foot exam room, but we still specify the distance, as if we’re testing at 20 feet. Sometimes people who are purists will use mirrors to optically get the 20 feet, but for the most part, 20 feet is just an idea. So you measure the distance.

Twenty, the bottom number, the other 20, that’s the size of the letter. And the size is specified also as a distance. And it’s the distance at which that letter [transcends] five minutes of arc. So if it’s a bigger letter, you have to put farther away in order for it to be five minutes. So 20/20 says you can resolve this 5-minute arc letter at 20 feet and the size of the letter is five minutes of arc at 20 feet. Twenty-two-hundred means the letter is five minutes of arc at 200 feet, which means it’s 10 times bigger (slide 12). So, if 20/20 the smallest detail you can resolve is one minute of arc, in 20/200 the smallest detail you could resolve is 10 minutes of arc, okay? One-sixth of a degree. That’s pretty good vision, but you’re legally blind if that’s the best you can do. There are a lot of animals that would die to have 20/200 vision.

Okay, and the reason we have some problem with a small amount of visual acuity loss – small, relatively speaking – is because our whole society is built around normal vision. Newspapers, magazines are printed with print size that’s only three times the resolution limit of the average person – 20/20. It’s the size of 20/60 so it can be read comfortably. For most people who would have 20/40 acuity, they would struggle with 20/60, because it’s like trying to read the – if you have 20/20 vision – trying to read the back of the [one dollar] bill because the print size is too small.