The low lux figure is supposed to represent the sensitivity of camera electronics and its ability to film in low light. It will also influence how well the camera performs when using its built in infra red lighting. Much of a CCTV camera's life is spent filming in low light at night so low light performance is quite important. We have recently added a CCTV camera buying guide which includes a section on Low Lux levels.
There are no ISO standards for measuring CCTV camera lux levels so you need to be wary of any quoted figures. Something called an IRE (institute of radio engineers) number is used. This is a measurement of brightness and contrast scored as a percentage. 100 IRE is a perfect image. White is white, black is black and there is a clear grey scale in between. 50 IRE means white is light grey but there is still a grey scale down to black. A 50 IRE image has 50% of its contrast and brightness and would be suitable for security purposes. Eventually with lower IRE numbers you can only make out white from black with no greyscale.
The minimum IRE number you are prepared to accept makes a big difference to the minimum Lux figure. You also need to decide how wide angle and large the lens is going to be. Large wide angle lenses let lots of light in so if they are used the low lux figure will be artificially improved.
Finally the shutter speed needs to be set. A long electronic shutter speed allows the camera to see in low light but any movement will be blurred so it wouldn't be suitable for security use. Because there are no ISO or industry standard parameters quoted low Lux figures should be largely ignored.
Many people think TVL is the holy grail of CCTV cameras - it isn't as can be seen from our CCTV buying guide, so retailers use cheap electronics with a high number of TV lines. These tend to be less sensitive than lower TVL electronics so the low light performance is poor. High TVL and good low light performance is harder to achieve and more expensive. For this reason be careful to avoid cheap high TVL cameras.
This is a bit of a con. People selling cameras with a minimum Lux of 0 are just saying that the camera has built in infra red illumination. All cameras with their own light source are designed to work in total darkness. What you actually want to know is how sensitive the base electronics are and so how well the camera will be able to use that infra red lighting.
A good CCTV camera should be able to see much better than the human eye in low light conditions. As day becomes night you should look at your monitor, think it is still daylight then walk outside and be surprised how dark it is. When looking at different electronics we use our eyes. We don't rely on theoretical numbers, we look at practical results out in the field.