scotopic vision happens when there is not enough light for the cone cells in the eye to work. The rods, though, need much less light to function.
The rod cells are not sensitive to color, though. This is why in very dim light we see in black and white.

Scotopic vision happens with illumination levels between 10-6 and 10 cd/m2. If there is more light than that, photopic vision dominates.

Another particularity of scotopic vision is due to the fact that in center of your eyes there are not many rod cells. This is why if you want to see something at extremely low light levels it is better not to look at it directly.

There is a theory for why the rods and cones in our eyes are unevenly distributed.

The reason we have well-developed color vision is so that we are able to discern the quality of our vegetarian foods. We need to be able to look at a banana and tell if it is rotten or not. Most pure carnivores, on the other hand, have poorly developed color perception, since all they really need to do is grab some food 'on the hoof'.

However, we also need the ability to very quickly notice a predator in our peripheral vision. Since rods are most sensitive, it makes sense for there to be a greater number of them in around the edges of our eyes so that we might notice a lion about to pounce on us in the dark.

What's interesting is that you can notice this sometimes. Have you ever been in a dark room and had something in your peripheral vision get your attention, but when you looked at where it was, it disappeared?

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