Low Light Problems

A brief primer on the reasons for the noise observed in footage recorded in low light conditions.

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Low light conditions in video surveillance is always a problem. When the light available diminishes, there are generally only a few knobs to turn – gain and shutter speed.

Slow Shutter Woes

Regular photography in low light conditions usually calls for a tripod and a timer, this produces wonderfully high fidelity images of things that are stationary, unfortunately moving object – people and vehicles – are reduced to a blurry smear making it extremely hard to recognize and identify people in the frame. If you need to recognize the person in the frame, you need a high shutter speed.

Grain in the Gain

If the shutter is fast (to avoid blurring), the sensor will only receive a fewer of photons in each frame. If the sensor produces an output, say, from 0-255, where zero means “I did not detect ANY light” in other words “black” and 255 means “Lots of light” or “white” then we’d generally like to have our sensor produce values nicely distributed between these values for a regular scene. Every time we sample an area on the sensor we get a slightly different count, sometimes we count 64 photons, the next frame there are 65, then 64, 62, 63, and so on. So the pixel changes from frame to frame. This variation is just random noise that we can deal with in different ways.

But when the light is dim, most of our values are low – say from 0-32 (with a couple of 255’s from distant lights and the odd diode on a piece of equipment). ¬†The noise doesn’t not go away, but since our values are now lower, the impact of counting one more or one less photon is much bigger. The signal to noise ratio goes up.

All these low values would give you an almost black frame, so we simply multiply all our value by 8 to get them nicely spread out between 0 and 255 again. The 255’s are then saturated, but that’s not the biggest problem. Recall how we get slightly different photon counts in each frame? Those small changes are now also amplified by 8, so the pixel is 64, 68, 60, 68 and so on. We also amplify the noise, and that’s why we see those grainy images.

Compressing noise

White noise does not compressed very well. In some audio codecs we identify segments as noise, and ask the receiver to “just make some noise for the next 100 ms, and use this filter to shape it”. In MPEG4 and H.264 we can get some pretty weird results, and JPEGs suddenly grow 100-400% – for frames that contain very little usable information.

What to do about it

There are various filters and algorithms that filter out the noise. You can use spatial and temporal analysis to try and minimize noise, usually performing a sort of averaging in time, but this should be done PRIOR to compression to allow the compressor to function as intended. If a large area of a scene is Рwell Рblack, then why not just accept that it is black, and set all those pixels to zero (decimation)?

Other options are a little more simplistic and expensive (throwing money at a problem usually helps) larger sensors and suitable lenses will certainly allow you to improve fidelity in low light conditions.

Some good examples of the concepts can be found here

Author: prescienta

Prescientas ruler

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