How to Avoid Hot Spots when using Infra Red Cameras

As more and more retail shops are selling budget security cameras, the level of expert knowledge provided to the end user is less and less resulting in an increase of next to useless coverage.

A lot of low cost security cameras have inbuilt IR lighting located in a ring around the lens, which sounds like a brilliant idea as these cameras can see in the dark. In fact some have a specification of 0 lux. While it is true that they can work in total darkness (0 lux) what is not stated is at what distance. In the image shown here, you are able to see clearly that it is a garden path but notice the extreme variation of the light. At around 3-4 metres the light is so intense that the bush to the left of the path is washed out. The same would occur to a person standing on the path. With in 2 metres of that, we have almost no light, causing the camera to not give a nice even contrast across the scene.
As a result of this, I do not recommend using cameras with built in IR as they result in a bright light in the centre of the image and not much to the edges, resulting in an over exposed area of the image at the centre, making the person impossible to identify.

This is not limited to infra red lighting as similar results occur under normal white light as can be seen in this illustration of poor lighting in a shopping mall.

Infra red illumination is an excellent tool to the surveillance industry as it does allow a camera to see in complete darkness as if it were day time. I have been in rooms where I was not game to walk for fear of tripping in the darkness and yet the camera could see clearly. This is using a reasonable grade of camera and IR light source. In my experience, typically budget security cameras only have IR coverage for a few metres and as can be seen in this image, give a very grainy result outside of the IR hot spot. Better results can be achieved using a reflected or indirect IR light source as I will explain below.

First a couple of clarifications about IR lighting.

    • For IR to be of any use to a camera, you require the camera to operate in B&W mode.
    • Many of the cheaper cameras claim to do this but are actually always in colour mode. They just turn off the part of the video signal that makes the colour, called the colour burst. A proper day/night camera has something called an IR sweep or cut filter, which during the day cuts out all infra red light from hitting the CCD or CMOS image sensor.
    • [easyazon-image-link asin=”0750678003″ alt=”CCTV, Second Edition: Networking and Digital Technology” src=”” align=”right” width=”100″ height=”134″]Why would we want to do that? Because IR light mixed with colour light causes two problems. The first is hot objects appear grey as they are emitting infra red energy. So a black road in summer looks grey. The other is because white or visible light has a different wavelength to IR, they need to be focused differently on the image sensor, causing the scene not to be sharp. This is documented in one of my favourite books on video surveillance, simply titled ‘CCTV’ by Vlado Damjanovski.
    • IR Light is not visible to the human eye when cast on a surface. However some illuminators do emit a dull red glow when you look directly at them. These are working in the 840-880nm wavelength (Defined by the LED model you use), which is on the upper edge of what the human eye can see. If you use IR LED’s in higher wavelengths around 940nm, no visible light is seen but not all cameras are as sensitive to this range.

Moving on to how to use IR lighting without causing hot spots in the scene.

The best solution is don’t use a budget IR illuminator but I realise that many of my readers are in the price conscious end of the market. However for interest sakes, a good quality illuminator similar to those I have used in prisons can be found from manufacturers like Raytec and Microlight, who also have some great advice on their sites. Some low cost products such as the IR130 or IR110 will give reasonable results when following my tips below.

The best way to avoid a hot spot from a budget IR illuminator is don’t point it directly at what you want to see. Higher end illuminators disperse the light more evenly and over a wider area so can be pointed directly at the scene without causing a hot spot. Instead use reflected light from an adjacent surface such as a wall or ceiling. By using an IR light separate to the camera and shining it at 45 degrees toward the ceiling or off an adjacent wall reflects the light much more evenly over the camera scene but it does reduce the distance of coverage.

I have used this technique extensively in small rooms and prison cells but equally as good results can be obtained externally or in hall ways where there is a surface adjacent to the camera. Basically any surface other than where the camera is looking will work. Shining a touch will give a good indication of the angle to work from, bearing in mind that the camera does not need much IR light to perform reasonably well.

Note that the infra red light does not need to be right next to the camera either. If the camera is looking at object 10 metres away, install the illuminator closer to the area so a larger amount of light is cast where it is needed. Light brightness has the inverse square law applied to it as detailed in the above mentioned CCTV book. In simple terms, if the illuminator is 10 metres away from the object being protected by the camera, it will give ¼ as much light as it would if installed 5 metres away.

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