CCTV CAMERA RESOLUTION

Resolution is the quality of definition and clarity of a picture and is defined in lines

meaning more lines = higher resolution , means better picture quality.

The Resolution depends on the amount 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 a higher resolution. In other words the resolution is directly proportional to the number of pixels in the CCD (Charge Coupled Device) chip.

In many data sheets, two type of resolution, vertical and horizontal are indicated.

Vertical Resolution

Vertical resolution stands for the number of horizontal lines

The Vertical Resolution is limited by the number of horizontal scanning lines. In PAL it is 625 lines and in NTSC it is 525 lines. Using the Kell or aspect ratio factor the maximum vertical resolution is .7 of the number of horizontal scanning lines. Using this the maximum vertical resolution is

PAL 625 X .75 = 470 lines
NTSC 525 X .7 = 393 lines

Vertical resolution is not critical as most camera manufacturers achieve this figure.

Horizontal Resolution

Horizontal resolution = no. of vertical lines

Theoretically horizontal resolution can be increased infinitely, but the following two factors limit this

It may not be technological possible to increase the number of pixles in a chip.
As the number of pixels increase in the chip, the pixel size reduces which affects the sensitivity. There is a trade off between resolution and sensitivity.
If only one resolution is shown in the data sheet, it usually is the horizontal resolution.

Measuring the Resolution

There are different methods to measure resolution:

1. Resolution Chart

The camera is focused on a resolution chart and the vertical lines and horizontal lines are measured on the monitor. The resolution measurement is the point were the lines start merging and they can not be separated.

Problems

• The merging point can be subjective as different people perceive it differently
  
• The resolution of the monitor must be higher than the camera. This is not a problem with Black and white monitors, but is a problem with most color monitors as they usually have a lower resolution as compared with a camera.
  

Bandwidth method

This is a scientific method to measure the resolution. The bandwidth of the video signal from the camera is measured on a oscilloscope. Multiply this bandwidth by 80 to give the resolution of the camera.

Eample. If the bandwidth is 5Mhz, the camera resolution will be 5 * 80 = 400 lines

Typical Resolutions of Cameras

Sensitivity, measured in foot candles or also called lux indicates the minimum light level required to get an usable / 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 illumination 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 location of use.

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. To compensate for the mismatch, use the modification factor shown below.