Choosing the correct monitor

August 2001 Surveillance

Monitors are available in a variety of sizes. Choosing the proper size for the proper application is a relatively easy process if two simple rules are remembered, as detailed below:

These are:

1. The distance between the monitor and the person viewing the monitor will have direct effect upon the clarity of the picture as seen by the viewer. The closer the operator is to the monitor, the smaller the screen should be, and the farther away the operator from the monitor the larger the screen should be to maintain proper resolution.

2. A monitor with 700 or more lines of resolution will produce a picture in direct proportion to the number of lines of resolution transmitted to it. In other words a 700-line monitor will produce 700 lines even if a 1000 line camera is used. Now this does not mean that a 700-line monitor will produce a better picture out of a 300-line camera transmission.

The size of the monitor is determined by the size of the viewing area of the screen as measured from the bottom left-hand corner to the top right-hand corner.

As a guide the following can be used to help choose proper monitor size:

(Monitor screen size in inches) - 4 = viewing distance x 0,3 in m (±25%)

For example: 9" monitor - 4 = 1,5 m (±25%)

12" monitor - 4 = 2,4 m (±25%)

Location and layout

Location and the console layouts are important in control room designs. A misconception in the CCTV industry is that the more monitors installed in a single viewing room, the more an operator can watch. A perfect system will have one monitor which can be viewed at any time. The maximum number of monitors will have one master monitor, usually a 12 to 15", and up to four secondary smaller monitors, usually 9".

The average person cannot watch more than four pictures simultaneously with any comprehension of what is happening. The angle from which a monitor is viewed becomes less effective for every 5° it is rotated from centre. In other words, design the console or viewing area in such a way that the monitors to be viewed by the operator are aimed straight on from wherever the operator is sitting. Another consideration is the monitor height. If an operator has to look down more than 45° to the monitor then much of the viewed picture becomes useless.

In a system where many cameras are needed and which does not permit the use of only four monitors, peripherals such as the quad splitter, multiplexed splitter or video switchers may be used. An interesting point to note is that the longer an operator watches a monitor the less he/she sees. Usually, after approximately one hour, the average operator's mind has mentally shut off the monitor.

Colour pictures do increase the amount of time that an operator can effectively watch a continuous video scene by almost double. Alarm interfacing can play an important part in the design of CCTV systems. A good solution to operator fatigue is that operators should be rotated between posts more often.

Monitor resolution

The resolution of the monitor is determined by the number of horizontal sweeps. The higher the number of sweeps the higher the resolution of the monitor. Black and white or monochrome monitors are now available from 800 lines to 1000 lines while colour monitors are available from 300 lines to 650 lines.

For further details contact Abie Ali, Frank Street on tel: (011) 838 4515.

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