Ambient light level

Background or general light level of a given area. It is important to remember that light on the scene such as street lighting may have a detrimental effect on picture quality. The illumination should be designed where possible to work in conjunction with the CCTV system.

      

Aperture or F-stop

The ‘opening’ of a lens, a measure of its light gathering capability. Relative Aperture is a ratio between its focal length and effective aperture, measured in F-numbers, generally the lower the better for night-time vision.

      

Cosec²

Cosec is the lens formula used to achieve even illumination, so that returned radiated energy to the camera is the same from wherever the subject is in the scene. This characteristic permits the camera to work within its dynamic range and avoids white out on the screen.

       

Concealed, done secretly. Infra red when emitted beyond 950nM can be considered totally covert. The lamps would not appear to be on when viewed by the human eye. It is important to ensure that the camera sensitivity is sufficient at this level.

     

A logarithmic unit for comparing two voltages, each 6dB doubles the voltage.

       

Used for Night-time CCTV surveillance internally and externally. Infra red is light that the human eye can not see but the monochrome camera can. When IR lamps are used in conjunction with mono CCTV cameras, the camera is given the ability to see where human eyes cannot.

       

Measurement of light in lumens per square metre, unit of which is Lux.

         

A non-linear law that refers in this instance to the relationship between the distance and the illumination. It is important to understand that inverse sq. law applies to infra red in the same way as it does to conventional lighting. To achieve the same level of illumination at double the distance you must sq. the illumination level i.e. 2x2 = 4 times the power.

       

A solid state, long life electronic component that produces light when stimulated by electricity. These are often used in infrared night-time applications when product life and low maintenance is required.

      

Metric measurement of the light striking the surface. Visible light is measured in LUX (a common spec) – non metric units = Foot-candles (Ftcd) Lux relates to human vision – not applicable to IR. Ir illuminators are 0 lux by definition.

         

Black and white, not colour. When IR lamps are used in conjunction with mono CCTV cameras, the camera is given the ability to see where human eyes cannot.

     

Automatically switches on the infrared lights when lights fall to a preset level.

       

The ratio of light returned from a surface expressed as a percentage. Some items are highly reflective to IR light (appear brighter) – trees, grass, snow and natural fibres. Some items absorb IR light (appear darker) such as manmade fibres (e.g. nylon)

      

Measure of noise levels of a video signal; the higher the number the better

       

Often referred to Infrared lamps that emit at approx. 800-850nM. The lamp would give of a low dull red glow as opposed to a clear red glow that is emitted from approx. 700 nM

       

Of a camera – usually specified in LUX and should be the amount of light falling on the image device to produce a video signal of I volt pp (peak to peak). However, it is more often given as the amount of light at the lens. This can be confusing unless the lens specification is provided in detail, including its light transmission value.

       

The sensitivity of the imaging device to different frequencies of light. This is usually in the range of 300 to 1000 nM. Visible light is up to approx. 760 nM. Infra red lighting is between 730nm and 1000nm, IR lighting over 950nm is totally covert (no glow can be seen by the human eye)