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

Contrast Correlates to Visual Acuity

In this population [i.e., patients], contrast correlates with visual acuity. If you lose your central vision and you’re forced to use your peripheral vision to do everything you used to do with your central vision – everybody’s contrast sensitivity is worse in the periphery than it is in the center; everybody’s visual acuity is worse in the periphery than it is in the center, so the two tend to go down together more or less (slide 44). But there are people who can lose more contrast sensitivity way out of proportion to the visual acuity loss and vice versa. So even though they correlate, they do have independent effects. I’ll show you those.

Interpreting the Interactions

Now, if we ask: What is the actual distribution of contrast sensitivity in people who have visual acuity loss? Here you can see a normal contrast sensitivity is about 1.3. That’s the lower limit of normal. You get above 1.3, we won’t get too concerned about, but below 1.3 or lower. People in the 20/20, 20/60 range, the average contrast sensitivity is in the range of 1.2 and you can see contrast sensitivity just declines with each of the acuity tabs (slide 45). That’s just a restatement of what we saw before.

However, within these groups, there is a distribution (slide 46). There are people who have in this range – 20/20 and 20/60 – that have contrast sensitivity that is in the pits. They can’t see anything; everything is just totally washed out, even though they have relatively good acuity. And acuity, by the way, is measured with the highest possible contrast you can get. On the visual acuity chart are close to 100 percent contrast. And you can see there are a lot of people – about 40 percent of the people in the range between 20/20 and 20/60 have contrast sensitivity that would be considered moderately impaired.

Those of you who do illumination engineering probably know that as you go to higher spatial frequencies in the images, contrast falls. People who do remote sensing and that type of thing tell us that there’s almost a linear relationship between the contrast in terms of power versus the spatial frequency level. So the more detail there is, the bark on the tree will show more contrast than the tree; what gets lost is the detailed information, the pattern, the herringbone. The things that you designers are paid so much to put into the environment – they can’t see it, so you’re wasting somebody’s money.

And again, if you ask about contrast sensitivity – whether the contrast sensitivity tests out to be in the normal range, or they all say of course, they all came to a low vision clinic, so we kind of expect them to (slide 47). But then there are – it’s kind of a combination of an appearance.

The third thing that occurs is glare (slide 48). And what we find – and most of the glare [does not] come from eye disease in the retina, [but from] the cataract that didn’t get taken out, because if you have low vision, finding a cataract surgeon to take out that cataract can be pretty difficult, because they’ve been burned a few times where they’ve taken out the cataract and the vision hasn’t changed, because the person had a macular degeneration or something and they’re very unhappy. So you become a poor candidate [for cataract surgery], in technical terms, if you have low vision. And so a lot of the glare is coming from the cataract.

Effects of Lighting and Daylighting on Performance

Most patients self-report their vision quality is quite poor – in poor ranges, over 90 percent (slide 49). And important to you is when we ask them, what kind of affect does lighting have on your performance activities? Close to 70 percent say it has a major effect. Lighting is one of the biggest problems. Over 20 percent say moderate effect and only 11 percent say it has no effect. So lighting is a very important consideration.

Question by [Participant]: Day lighting?

Response by Bob Massof: Yeah, daylight – both too much and too little. Too much light and they tend to wash out from this glare. Too little light their sensitivity is reduced and what might look like mood lighting to the average person can be a funhouse.