This is the first article in the "Understanding CCTV Series….". In this series, we try and cover some of the important CCTV components and try and explain some of the issues in simple user friendly language. These articles are abstracts from STAM InSight - The Award Winning CCTV Program on CD ROM, which has many innovative CCTV tools for skill and productivity enhancement.
We start the series with cameras. Cameras is the starting point of the video signal and is therefore a critical component of a CCTV system. The word camera comes from the Latin " camara obscura" and means "dark chamber". Artists in the middle ages used a dark box to trace images. Since then the camera has come a long way. Today there are three types of cameras most commonly used.
film cameras
photographic cameras
video cameras
The construction and type of Charge Coupled Device (CCD) chip used in a camera is important. Some of the better quality cameras have superior chip design incorporating many innovative features like On Chip Lens (OCL), Back Light Compensation (BLC), excess charge drainage technology etc. In this article we won’t look into these aspects, but try and understand some of the important camera specifications.
CAMERA SPECIFICATIONS
Any camera data sheet has a number of specifications shown like resolution, sensitivity, signal to noise ratio, camera voltage, chip type, and operating temperature. Some data sheets are detailed, while others are quite sketchy and cover the bare minimum. To classify a camera, most people will first look at the resolution and sensitivity in the data sheet. These two specifications are the most important. In this article we will discuss these specifications in more detail. There is confusion surrounding these terms and I would like to demystify them by explaining them in simple terms.
RESOLUTION
Resolution is the quality of definition and clarity of a picture and is defined in lines
more lines = higher resolution = better picture quality.
Resolution depends upon the number of pixels (picture elements) in the CCD chip. If a camera manufacturer can put in more number of pixels in the same size CCD chip, that camera will have more resolution. In other words the resolution is directly proportional to the number of pixels in the CCD chip.
SENSITIVITY / MINIMUM SCENE ILLUMINATION
Sensitivity, measured in foot candles or lux indicates the minimum light level required to get an acceptable video picture.
There is a great deal of confusion in the CCTV industry over this specification.There are two definitions "sensitivity at faceplate" and "minimum scene illumination".
Sensitivity at faceplate indicates the minimum light required at the CCD chip to get an acceptable video picture. This looks good on paper, but in reality does not give any indication of the light required at the scene.
Minimum scene illumination indicates the minimum light required at the scene to get an acceptable video picture. Though the correct way to show this specification, it depends upon a number of variables. Usually the variables used in the data sheet are never the same as in the field and therefore do not give a correct indication of the actual light required. For example a camera indicating the minimum scene illumination is 0.1 lux. Moon light provides this light level, but when this camera is installed in moon light, the picture quality is either poor or there is no picture. Why does this happen? It is because the field variables are not the same as those used in the data sheet.
How does it work? Usually light falls on the subject. A certain percentage is absorbed and the balance is reflected and this moves toward the lens in the camera. Depending upon the iris opening of the camera a certain portion of the light falls on the CCD chip. This light then generates a charge, which is converted into a voltage. The following variables should be shown in the data sheet while indicating the minimum scene illumination.
Reflectance
F Stop
Usable Video
AGC
Shutter speed
Reflectance
Light from a light source falls on the subject. Depending upon the surface reflectivity, a certain portion of this light is reflected back which moves towards the camera. Below are a few examples of surface reflectivity.
snow = 90%
grass = 40%
brick = 25%
black = 5%
Most camera manufacturers use a 89% or 75% (white surface) reflectance surface to define the minimum scene illumination. If the actual scene you are watching has the same reflectance as in the data sheet, then there is no problem, but in most cases this is not true. If you are watching a black car, only 5% of the light is reflected and therefore at least 15 times more light is required at the scene to give the same amount of reflected light.
