Friday, December 12, 2008

In the dark

In the dark, a couple of wierd things happen besides not being able to see very much.

With conventional analog video or digital/IP video, the image certainly looks worse as contrast fades and AGC (Automatic Gain Control) kicks in, amplifying the weak signal but simultaneously, the noise and hurting the signal to noise ratio. I refer to this ants crawling all over the screen, but please stop me if I get too technical. Snow storms, or as one colleague puts it, the equivalent of the 'shhhhhh' sound in the audio world. The other thing is the shutter speed slows to let in more light, which naturally leads to blurring (especially noticeable with Megapixel cameras, but by no means exclusive to them). This is not noticeable in a static picture, but when you most need it, when a person or car goes by, you have to ask yourself was it a bird, a plane or some other extra-terrestrial object? The point is that for security and surveillance video it might still be 'good enough'. You might say 'I can see what I need to see', and quite rightly, no-one should argue as long as you're happy.

But a second thing happens in the IP video world, the noise in the image makes the compression engine go wild, because (i) it thinks almost the entire image is moving (like a PTZ) and (ii) there is no stable part of the image which it can simplify through redundancy (like a person walking down a corridor). This makes the compression engine beg for more bandwidth in order to adequately represent the image and this puts a strain on the network and consumes more disk space (which is often half the cost of a CCTV system). If you do not feed the engine's appetite with more bandwidth, it will do the only thing it is allowed to do - oversimplify the video, either by making it horribly blocky or just unashamedly drop frames leading to jerky video. This is particularly evident in WANs and wireless networks, but I've even seen it on a LAN with one switch.

The third impact of low light is that machine vision, that gave us modern day video analytics, simply collapses in a blubbering heap. Near-zero contrast and noisy motion all over the image overload the analytics so that it can neither detect key objects nor track them. Video analytics just doesn't work in the dark.

In our experiments with active IR illumination, the improvement in the quality of the image is jaw-dropping. You have to see it to believe it. PowerPoints, video clips and brochures just don't cut it. It's like looking at a monochrome image in broad daylight, because, well, that's basically what the illuminator is giving you. We've watched bitrates slashed by 30-80% down from their peak values in the dark, just because the video is clean. Finally, and I don't quite know how to quantify this better, video analytics doesn't work in the dark, but it does in the light.

Of course you don't have to use active IR illuminators, you can use white light, which also acts as a deterrent. However IR consumes very little power and generates little to no pollution. Another option is thermal imaging, or passive IR. This comes at a very different price point but is excellent for long-range detection that 'something is out there'. You don't know what yet, but it warns you to take a closer look, possibly with a camera and lighting that reveals more detail.

So lights aren't just for better video, although that's a good enough reason when you do a side by side comparison. It affects network bandwidth, disk space consumption and whether or not your analytics dollars are going to deliver on their promise or find nothing, or maybe worse, swamp you in thousands of false alarms. Nasty, nasty ants.

Click here for a video clip


Geoff Moore said...

Hi Bob
The only weird (and a little worrying) thing about everything you've posted, is that people don't realise it already :) !!
No light, no pictures.
We've had great results with Dinion-XF cameras in very low light with IR, but one of the things I think people overlook a lot when using IR is to make sure they use the right power illuminator for the job. Too often folks just put up the biggest one they can get, and that's fine if the target is always a long way off, but in many typical applications you're going to have targets moving towards the camera or away from the camera, but the light doesn't know that! So if you use too powerful an illuminator you end up with white-out when the target gets close up.
Like working out what lens you need to see a target at a particular distance, you need to work out how much illumination you need to *light* the target at a particular distance, taking into account typical reflectivity of the target and its surroundings (not forgetting how shiny rain-wet surfaces can be.


Geoff Moore said...


You have a really nice style of writing I have to say and You have a gift of writing about technical things in very simple way. In this article for example You catch people's eyes on many aspects, which often are overlook. Good job. Keep up a good work Dr Bob.