The Chart, the Dome and the Bar

I didn’t really know how to define low vision, so I looked it up.

Think of the familiar eye chart, invented by Snellen. With normal vision when wearing your glasses or contact lens you will be able to read the eighth line down on the chart. This is 20/20 vision. You can distinguish between lines 1.75 mm apart at a distance of 20 feet (1.75 is around the width of a matchstick I measured). The metric version is from 6 metres, so 6/6 vision equates to 20/20.

If someone has 20/40 vision, it means that at 20 feet they can only read what someone with normal vision can read from 40 feet.

The World Health Organisation classifies 20/70 to 20/160 as modest visual impairment, 20/200 to 20/400 as severe, and 20/500 to 20/1000 as profound. Beyond that is considered near-total impairment.

When it comes to reading, aids for those with low vision include large print, and of course magnifiers. On my visit to Cam Sight I was shown may electronic magnifiers, such as the ‘CCTV’ I talked about in a previous post. Cam Sight also demonstrated a wide range of optical devices. Some of these are magnifying classes, somewhat in the Sherlock Holmes style, but often with built-in illumination Another kind which I found interesting are dome and bar magnifiers.

A dome magnifiers is a lens roughly in the shape of half a sphere, a bit like a paperweight. You place it directly on what you want to read and slide it over each bit you want to magnify.

Where a dome magnifier is half a sphere, a bar magnifier is half a cylinder. Bar magnifiers are useful for reading line-by-line, such as in a directory.

These devices are always in focus and they collect light so the image is very bright. I also found them very nice to use, they are very elegant and they have a pleasing ‘chunkiness’ about them’.


About partialinsight

One evening I had a stroke. Half my sight vanished overnight. Adapting made me grasp how amazing the visual system and brain are. It also taught me to understand disability completely differently and I'm grateful for the lesson.
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