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

Visual System Disorders

Most people who have low vision have a central vision loss (slide 25). And this is a pie chart showing the various diagnoses of patients who come into these clinics seeking low-vision services (slide 26). 

Macular Disease

More than half – the ones that have some shade of yellow – are due to a macular disease. That’s a disease of your central vision, which is your acuity. Around 10 percent have glaucoma coming in seeking services. Diabetic retinopathy, which represents somewhere in the neighborhood of around 10-12 percent of the retinal vascular diseases is included in here. And stroke is about 2 percent. So by far, most of the people who are seeking services have some type of macular disease. The dry form of macular degeneration is most common, followed by the wet form – inherited forms.

Reduced Visual Acuity

The major causes of functional limitations from low vision or reduce visual acuity (slide 27). And looking at the distributions of acuity in the sample (slide 28), about 35 percent of the people who present to a low-vision clinic have acuity in the range of 20/20 to 20/60. So these people feel their vision’s bad enough that they’re seeking services. Close to 40 percent are the range of 20/60 to 20/200. About 20 percent are 20/200 to 20/500 and about 6 percent are worse than that. Somewhere in the neighbor of about 26 percent are legally blind.

But if you ask these people how’s your vision rate on a scale of zero being poor to four being excellent, the average rating is between poor and fair – no matter what your visual acuity when you come in (slide 29). And you’ll notice even those with the range of 20/20 to 20/60 are included. To be fair, some of these people have glaucoma. Glaucoma can shrink your visual fields way down and not necessarily have to do with central vision. A lot of these people also have dry forms of macular degeneration, which can produce what’s called foveal sparing. The center can still read the eye chart, but it’s like looking through a keyhole. And surrounding that would be a blind area so that they could still have very poor vision, even though they might be able to read quite far down on the eye chart.

Question by [Participant]: Are those corrected numbers – the visual acuity?

Response by Bob Massof: I’m sure that they’re not coming in for refract records. So these are people who have best-corrected visual acuity on presentation. I should say they’re wearing their individual correction. If they needed correction, they don’t get into the database. Now, if they get corrected and back to normal, they’re happy. You’ve cured their low vision. Same with [those who] come in with a cataract and you say, you came into the wrong department. You have to go over here and get the cataract done. They don’t come back. They not in these databases. So these are people (slide 29) who stayed in the database.

Visual acuity is a measure of blur. My daughter’s gotten over it, but I haven’t. And that the – now, I’m just simulating 20/200 visual acuity (slide 30). So if the only thing that was occurring was blur, the picture on the left would [represent] 20/20, the picture on the right is what 20/200 would look like. That’s the difference. And you can overcome that blur with magnification (slide 31). And that’s the main trick that’s used in a low vision clinic: to provide people with telescopes and magnifiers to compensate for the loss of acuity. You can read the bigger letters on the charts and make everything bigger and you can make the compensation.

Reduced Contrast Sensitivity

But the other major cause of functional limitations is reduction in contrast sensitivity (slide 32). We don’t hear about that as much. And information in the image is defined by contrast (slide 33). Whereas, if visual acuity is lost, it looks blurred. When contrast sensitivity is lost, it goes away. You don’t see at all.

And when patients experience the loss of contrast sensitivity, the way they will describe it is glare. Okay, and here’s a simulation of what it looks if you lose contrast sensitivity (slide 34). The only difference between these two images – well, the two differences – one is this is blurred to 20/200, the same as the other one was. But in addition, the contrast of the image is uniformly reduced, as if you lost contrast sensitivity by about – I guess it would be on the order of about 28 log units.

The way contrast sensitivity is measured is with an eye chart, but the letters are all the same size. It’s called the Pelli-Robson Chart (slide 35). All the letters, [which] are set up between the viewing distance and the size of the letter, [are] the equivalent of about 20/400 or 20/800 letters, they try to make them as big as possible. But what varies as you move around the chart is the contrast of the letters.

If you have perfectly normal contrast sensitivity, you should have no trouble reading the letters right here [pointing to bottom of chart]. What you do is just see how far down the track you can read. And each contrast varies in a tenth of a log for each triplet of letters (slides 36 - 42). So you specify contrast sensitivity really as a ratio of the light-to-dark. If the letter’s absolutely black, there’s no light coming off of it, the contrast will be 100 percent, no matter what the background is, as long as it’s not black too.

Contrast is not the same thing as brightness. You can’t improve contrast simply by turning up the intensity of the light. It’s the ratio of the light to the dark. If you turn up the intensity of the room lights, you’re going to reflect the same ratio from the two things. What improves as you turn up the light is your sensitivity to contrast. At low light levels, contrast sensitivity is worse for everybody, whether you have low vision or not; at higher light levels, your contrast sensitivity is better. There are many ways to specify that ratio (slide 43). We won’t go into it. It’s just a light-to-dark ratio which formally you want to use. And it’s important to remember that it’s – those of you who are illuminating engineers – reflectance determines the contrast not the overhead light.