Retina and neural organization (4)

Let’s talk for a moment about light sensitivity. Sensitivity refers to how much light is required to be able tell that light is there – more specifically, the minimum light needed for the cells in your eye to respond. If you are more sensitive to light, then you only need a little bit of light to detect it. For example, the cells in the eyes of nocturnal animals are more sensitive to light than the cells in our eyes. I’ll write about cat’s eyes in the future because they are cool and I love kittens, but note that cell sensitivity is one of the many differences between human and nocturnal animal eyes.

Different cells in your eye have more or less sensitivity to the amount light and color. Rods are one type of cell that responds to small amounts of light or dim light. Notice that the figure below shows that rods are also most sensitive to greens and blues.

 

That is not to say that rods allow you to see in color, rather rods are good in dim light and help you pick up on greens and blues. For example, notice any similarities between these paintings?

 

For centuries artists have intuitively depicted nighttime scenes with blues and greens, and it turns out that the anatomy of the eye is the reason why we pick up on blues and greens the most at night. Painting dark scenes with green or blue is based in neuroscience! Cool right? Brain tip: The human brain is attracted to unexpected or novel stimuli (more to come on the neuroscience behind this).  What that means for color is that you can break the conventions of what we expect to see and produce exciting and attention grabbing effects. Try depicting a night scene in all yellows and reds. For light or time-based designers (video, installation, or performance artists)  you play with dark adaptation.

Dark adaptation is the increase in sensitivity to light that takes place as your eye adjusts to the dark. This is when you first turn off the lights and after time you can slowly start to see. After a few minutes, you start to see better.

So how does it work? What is happening when our visual system is adapting to light?

It has to do with rods and cones (cones are the other type of cell in your eye that is sensitive to light). Rods are very sensitive to light, so when you first go into the dark, your rods need less light than cones (remember seeing blues and greens first). But rods and cones adapt to the light at different rates. Cones adapt the fastest, within about four minutes in the dark you’ll start to see more colors and more defined shapes. However, cone adaptation tapers off quickly, and after about five mins cones won’t adapt anymore. There are significantly more rods than cones (91 million vs. roughly 4.5 million respectively), and rods adapt over the course of about 30 mins. There are so many more rods than cones that rods allow for more detailed vision. From about 4-9 mins there is going to be no change in what you see. But then after about 10-minutes sensitivity picks up again! And the increasing sensitivity goes on for about 30 mins. After that period the rods will be fully adapted. In sum, when you first turn off the lights you’ll be able to see green and blue blurry objects, after five minutes you will be able to see all of the colors that you can, then after 30 mins, all the objects will be as clear and fully colored as possible. If you are working with time-based art, consider playing with how your viewer’s eye adapts to the light in phases. It would be neat to use the viewer’s visual system to do some of the work by allowing the viewer to slowly realize details or relationships that they couldn’t see before.

 

Other cool ways to detect your eye anatomy

Like I noted before, there are more rods than cones and rods are more sensitive to light. Rods are also more spread out around the eye. Cones are located in the fovea or small part in the very back of the eyes where the objects you are focusing on land. That means that we can pick up on small amounts of light, particularly when we aren’t focusing on them. Astronomers used this to detect new stars. Try it yourself: rather than looking directly at a star, notice the stars that are in your peripheral vision. They will be easier to see but blurry.

Thought question: Why does the Navy use red light in a dimly lit submarine? It has to do with different sensitivities between rods and cones and dark adaptation.

    

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