How does the visual system tune itself to ambient light levels?

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Andrew Steer

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Andrew Steer  asked the Naked Scientists:
Hi Chris,

On the podcast on 19/04/2009 you breifly mentioned the "snow" or
"noise" we see in low light levels. This is something I was conscious of even as a young child, and as I grew older just assumed was a human analogue to the effect you see on a video-camera in the dark with the gain turned way up.

But, a few months ago while laying in bed one night with the green LED from a battery charger on the floor casting some weakish light on the walls and ceiling I noticed another curious effect:

Cover up both eyes (without touching them) using my palms, uncover one eye.

Immediately I see the outlines of the walls and ceiling and wardrobe in the weak light, through a heavy and coarse 'snow'/'noise'. But in a matter of a few seconds, the noise becomes finer-grained and lower
contrast. I can cover up the eye for 10-15 seconds, and uncover it again and see the same thing happen with the noise being very visible at first, then decreasing to a lower, stable, level.

I feel there's unlikely to be enough light in the room for the eye to be
physically adapting to change the noise, and wondered whether the brain (amazing as it is) does some kind of adaptive 'noise reduction' processing (akin to the noise-reduction digital filtering you get on digital cameras these days).

Are you aware of any research/investigations in this field?

I do get the impression that the particular light-level achieved with the LED from this charger gives rise to some of the strongest visible 'noise'.

It is curious that when I cover my eye to make it darker, the 'noise'
decreases. Of course there may be a combination of effects of 'noise'
internal to the eye and small-number-of-photon effects in the presence of low light levels?



What do you think?


Offline Atomic-S

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I am unfamiliar with this phenomenon. However, it may be that the eye requires a significant amount of time to accumulate a useful image in very low light, so that initially it is kind of messed up -- sort of like the time exposure required to get a picture on film in very low light. Also, the fact that you report the phenomenon on the uncovering of one rather than both eyes supports this opinion -- with both eyes, the imperfections of the images in the separate eyes may tend to cancel out so that you are less conscious of them, but with one eye you notice them more easily.