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

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.