CCTV camera lux levels

A CCTV camera's low lux figure represents the sensitivity of the electronics and the camera's ability to film in low light or under infra red lighting. As such it is a very important variable to consider, especially as much of a CCTV camera's life is spent filming in low light at night. CCTV camera lux levels can however be a little confusing and at times misleading.

In general terms the lower the number the more sensitive the camera and the better it is. However there are no ISO standards for measuring CCTV camera lux levels so you need to be wary of any quoted figures. The next bit is a little technical so apologies in advance but you need to know how lux levels are arrived at. There are 4 or 5 main parameters which you need to know when determining how a low lux figure was arrived at. Alter any one of these and the results vary tremendously.

The first thing you need to decide is what is the minimum video signal you are prepared to accept from the camera's electronics. This is measured in IRE, or Institute of Radio Engineers units. 100 IRE is a perfect video image, white shows as white, black as black and with a clear grey scale in between. At 50 IRE the white would show as a grey, black would still be black and there would be a grey scale between the two. You would still be able to use a 50 IRE image for low light security purposes, it still has 50% of it's original contrast and brightness. Go down to say 30 IRE and the image potentially becomes useless for security purposes.

The Lens used for the test makes a big difference. The amount of light the lens allows in is measured by the lens' F number. A small F number lets more light in than a large number. Something around F1.4 would be typical of a wide angle lens on a CCTV camera. Tests could be carried out with a very specialist lens which has a figure of less than 1. These are more suited to a laboratory and cost thousands of pounds. So figures achieved using this type of lens would give falsely optimistic readings when compared to the lenses used on an actual CCTV camera.

Shutter speed. By using a very slow shutter speed a camera can detect objects in lower light but the problem is if they objects are moving you will get motion blur. So you need to know what shutter speed was used to ensure the reading is applicable to the real world where movement occurs.

Gain. This is where the camera amplifies the signal to boost low light performance. Background noise is also amplified as well as the signal. By using a very high gain you can achieve fantastic low lux performance figures but eventually the signal to noise ratio becomes too low and the image quality is unacceptable for real world use.

The contrast or reflection ratio of the object being filmed. A white square on a black background can be distinguished even when the image quality is poor. What you actually need to be able to detect are grey scale changes because that is what is needed when using a camera for security purposes.

There are other variables as well but these are the main ones. The following is a guide to allow you to understand quoted minimum lux level figures:

Min 1.0 lux - 0.1 lux. These cameras will loose image as the light falls to a point where the screen becomes black. They will not be able to make full use of any built in infra-red light source.

Min 0.01-0.05 lux. These cameras will see better than the human eye in low light conditions and perform well when using their built in infra red illumination.

Min 0.001- 0.0001 lux. These figures will almost certainly have been arrived at using misleading methods and are unlikely to represent their actual performance capability. Treat them with suspicion.

Min 0 lux (IR on). This figure is very misleading. Any cameras equipped with IR can see in total darkness. The problem is if you don't know how sensitive the camera electronics are they may not be able to make best use of both the ambient and infra red light.

In truth when it comes to judging CCTV camera lux levels we use our eyes and judge as we find. We carry out actual comparison tests in real world scenarios. We test cameras in the way you are going to and the results can be quite surprising. Electronics combinations which supposedly give miraculous results fail miserably when we test them at night with images which would be of no use at all. As with most things in life we find you get what you pay for and it's not without reason that our cameras usually cost us twice what we could spend when specifying them with manufacturers to achieve supposedly similar results.