TECH TOPICS - CCTV

Or what you didn't know about CCTV cameras

CCTV CAMERAS AND THE GREAT RESOLUTION ROBBERY

In the course of trying to find new CCTV cameras for home security use and onboard rocket video use, a few interesting points of interest about the actual capability of the CCTV cameras advertised came to light. It would seem that important aspects of a CCTV camera's specification such as resolution and lux rating are not always quite what they may seen in an advertisers marketing blurb.

Resolution

One thing that isn't immediately apparent, is that resolution is not as simple as the headline spec on a manufacturers CCTV camera may have you believe.

The number of lines (or to be more accurate, line pairs) is not sufficient to provide a figure of the resolution of an analogue CCTV camera when dealing with modern digital based (CCD and CMOS) image sensors. The number of lines has to be used in conjunction with the number of pixels to provide a more accurate figure for the resolution of the CCTV camera. Lines, or line pairs can only be used as a resolution figure for an older analogue camera.

Take an example of a 512 x 582 pixel camera capable of 330 lines (line pairs). This means that the visible picture only fills about 88% of the full PAL picture width and height. (A full PAL picture is 625 scan lines, but of that, only 576 scan lines are actually visible. The width is 720)

Each line pair is effectively one black and one white line next to each other. Each pair takes two of the sensors pixels to resolve. Hence it is impossible to resolve more than 291 line pairs with a 582 pixel camera. The claim of 330 lines is really a cheat - because the 291 lines only cover 88% of the picture - the remaining 39 are imaginary and cover the lost 12%. I.e. the sensor could resolve 330 lines if it were extended to cover the whole picture area.

On this basis a 380 line camera could actually have worse resolution than a 330 line camera - simply by covering less area of the picture.

Basically the number of pixels sets how good the camera can be. The line value is actually showing that the lens is not degrading the resolution.

The clipping of the picture area at this sort of level is not normally a practical problem because most TVs screens also clip the picture by this amount (unless you have a true TV monitor or one of those wide screen jobs).

Another thing to watch out for is cameras with a quoted vertical resolution of more than 576 pixels - each PAL frame actually only has 576 visible lines - so any more than 576 will be clipped. So 582 vertical pixels is definitely very good.

Lux Ratings

Lux ratings are a similar cause of not always entirely accurate information. The lux ratings quoted in CCTV camera specifications are almost always the lux ratings for the best lenses attached to the CCTV camera. In reality, the lenses fitted to a CCTV camera are almost never the best lenses, and as a consequence, the practical lux rating is almost never as good as the quoted lux figure. Sometimes, a really lousy lens is fitted to CCTV cameras, and the practical lux rating of a CCTV camera is considerably worse than that quote in the camera's specifications.

Generally, body cameras (e.g. those that are fitted in a large box with an interchangeable lens mount) are able to be fitted with better quality lenses than board cameras, and subsequently, better lux ratings can be achieved - in some cases as good as the lux ratings quoted in the camera specifications. Many of the board cameras use 3.6mm f2.0 lenses, and the specifications frequently quote lux ratings at f1.2 - in English, this means that the lux ratings of the cameras are not as good as claimed, since they are not using the best lenses.