Our ability to see in a large range of lighting conditions is remarkable. We use cones for bright light and colour, in low light conditions the rods take over. This two-part system covers a very wide range of brightness. Dark Adaptation is the term for how our vision adjusts when going from bright light to low light, like going from outdoors on a sunny day into a dark tunnel. Some may see it this way but to others it may seem like this [showing different shades of darkness/brightness].
This test starts with a bright flash of light at time zero (0), then measures how long it takes to recover vision. Cones are the first cells to recover, taking 5 minutes or so to reach full sensitivity, levelling out at medium light level. Rods are slower to adapt, reaching their maximum sensitivity at 20-30 minutes but at a much lower light level. This is the typical curve for a young person.
This graph also shows measurement of dark adaptation by people from different by decade. The red line shows adaptation rate in the 20s. Note that it takes the shortest time and becomes the most sensitive. The yellow line shows adaptation rate in the 70s. Note that it is slower and doesn’t reach the same level of sensitivity for either rods or cones. So what looks like this in youth, later gets to look something like this.
Since we are discussing dark adaptation, this is a good place to discuss the effect of glare and recovery from glare. In the past eye surgery could increase glare. Now it doesn’t and read more here lasik eye surgery facebook page. There are two categories of glare: one is annoyance glare, in which you are bothered by the bright light but it doesn’t necessarily decrease vision. The other is where the bright light hits a cloudy lens, a cataract, and defuses within the eye causing reduced vision.
Now, let us combine the effect of reduced contrast sensitivity we saw earlier and add the effect of glare. This graph shows how much acuity, using low contrast letters, is decreased with exposure to glare. This curve doesn’t seem that different until you realize where the 20/20 and 20/40 lines are, compared to 20/200, which represents legal blindness. Not only does glare make vision more difficult but aging also restores slows recovery time from glare exposure. This is also a significant factor in driving performance.
Field of Vision
Last, we come to field of vision, that is, how far off to the side you can see. One way of measuring visual field is by testing light sensitivity at multiple points. This is the way we typically test visual field, like in glaucoma. Measured this way, we can see that there is a decrease in sensitivity over all points in the visual field with age.
Another way to measure visual field is by the furthest extent of side vision. With either technique we can map missing areas of vision in relation to things like glaucoma and stroke. Imagine the challenge of driving with half your vision missing. Another approach is to look at it functionally, by measuring what is called useful field of view. This is a multi-part test that basically measures how well you can detect and react to objects in side vision. Compared with other measures of vision like chart-measured acuity, this has a higher correlation with automobile accident rate. We will have more to say about that in a later article on driving performance.
We have seen that over time, in an otherwise healthy eye vision changes in every dimension we discussed: acuity, reading ability, contrast sensitivity, color sensation, dark adaptation, glare and glare recovery and useful field of view. We did not discuss what underlies the changes. It turns out contributions come from each and every part of the visual system. In one video we briefly discuss LASIK surgery on youtube. In future articles we will discuss structural changes in detail, for example, clouding of the lens (known as cataract), decreased sensitivity to the retina and decrease in ability to process information in the brain.