Thoughts on Low Light Cameras

For security applications, I don’t rely on a cameras low light performance to provide great footage. I recently installed a camera in my basement, and naturally I ran a few tests to ensure that I would get good footage if anyone were to break in. After some experimentation, I would get either

  • Black frames, no motion detected
  • Grainy, noisy footage triggering motion recording all night
  • Ghosts moving around

The grainy images were caused by the AGC. It simply multiplies the pixel values by some factor, this amplifies everything – including noise. Such imagery does not compress well – so the frames take up a lot of space in the DB, and may falsely trigger motion detection. Cranking up the compression to solve the size problem is a bad solution, as it will simply make the video completely useless. I suppose that some motion detection algorithms can take the noise into consideration, and provide a more robust detection, but you may not have that option on your equipment.

Black frames is dues to the shuttertime being too fast, and AGC being turned off. On the other hand, if I lower the shutter speed, I get blurry people (ghosts) unless the burglar is standing still, facing the camera, for some duration (Which I don’t want to rely upon).

A word of caution

Often you’ll see demonstrations of AGC, where you have a fairly dark image on one side, and a much better image on the other, usually accompanied by some verbiage about how Automatic Gain Control works its “magic”. Notice that in many cases the subject in the scene is stationary. The problem here is that you don’t know the settings for the reference frame. It might be that the shutter speed is set to 1/5th of a second.  The problem is that a 1/5th of second shutter speed is way too slow for most security applications – leading to motion blur.

During installation of the cameras, there are too common pitfalls

  1. Leave shutter and gain settings to “Auto”
  2. Manually set the shutter speed with someone standing (still) in front of the camera

#1 will almost certainly cause problems in low light conditions, as the camera turns the shutter waaay down, leading to adverse ghosting. #2 is better, but your pal should be moving around, and you should look at single frames from the video when determining the quality. This applies to live demos too : always make sure that the camera is capturing a scene with some motion, and request an exported still frame, to make sure you can make out the features properly.

A low tech solution

What I prefer, is for visible light to turn on, when something is amiss. This alerts my neighbours, and hopefully it causes the burglar to abort his mission. I also have quite a few PIR (Passive InfraRed) sensors in my house. They detect motion like a champ, in total darkness (they are – in a sense – a 1 pixel FLIR camera), and they don’t even trigger when our cat is on the prowl.

So, if the PIR sensors trigger, I turn on the light. Hopefully, that scares away the burglar. And since the light is on, I get great shots, without worrying about AGC or buying an expensive camera.

The cheapest DIY PIR sensors are around $10 bucks, you’ll then need some additional gear to wire it all together, but if you are nerdy enough to read this blog, I am pretty sure you already have some wiring in the house to control the lighting too.

So – it’s useless – right?

Well, it depends on the application. I am trying to protect my belongings and my house from intruders, that’s the primary goal. I’d prefer if the cameras never, ever recorded a single frame. But there are many other types of applications where low light cameras come in handy. If you can’t use visible light, and the camera is good enough that it can save you from using an infrared source then that might be your ROI. All else being equal, I’d certainly chose a camera with great low light capabilities over one that is worse, but rarely are things equal. The camera is more expensive, perhaps it has lower resolution and so on.

Finally a word on 3D noise reduction

Traditionally noise reduction was done by looking at adjacent pixels. Photoshop has a filter called “despeckle” which will remove some of the noise in a frame. It does so by looking at other pixels in the frame, and so we get 2 dimensions (vertical and horizontal). By looking at frames in the past, we can add a 3rd dimension – time – to the mix (hence 3D). If the camera is stationary, the algorithm tries to determine if a change of pixel value between frames, is caused by true change, or because of noise. Depending on a threshold, the change is either discarded as noise, or kept as a true change. You might also hear terms such as spatiotemporal, meaning “space and time” – basically, another way of expressing the same as 3D noise reduction.


Author: prescienta

Prescientas ruler

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