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Measurement of minimum illumination

Like the human eye requires a certain amount of light to perceive images, network cameras also require a certain amount of light to capture images.

There is a minimum amount of light that is necessary for both the human eye and cameras to capture or perceive images. This is what is referred to as the minimum level of illumination.

It is imperative, especially in the surveillance business, that this minimum level of illumination is achieved and preferably surpassed when a network camera is set up. This will ensure that images are of acceptable quality and can be used when video evidence is needed, for instance.

Measuring the minimum level of illumination required for a network camera, or any other surveillance camera setup for that matter, can be a tricky affair but one that can definitely be mastered. Leading vendors in the network camera space advise users on how to set up their products and how to get maximum value out of them.

Light sensitivity can be quantified by various different methods for different cameras. For TV cameras for instance, a fixed level of illumination, usually 2 kilolux is used, while ISO (International Organisation for Standards) speeds are used for still image cameras. For surveillance purposes, the level of minimum illumination, which encompasses both these methods, is used.

Setting up the test

The Axis illumination test for network cameras involves the use of illumination test charts, ie, TE 153, TE 182 and TE 170 resolution charts. With the camera at its default video setting, the charts are illuminated with incandescent light. This is any light with the same type of spectral profile as sunlight, for instance the filament in a light bulb or halogen lamps.

The colour temperature of the light should be at 3100, which is around 200° Kelvin, with a maximum variation in illumination flatness of plus or minus 10% from the centre of the charts. Illumination is measured at the centre of the charts as close as possible and parallel to the chart plane. The camera needs to be adjusted so that the charts fill the image. The total exposure time should be set at no more than 200 milliseconds and the focus of the charts should be kept stable at all times. If used, infrared filters should be enabled throughout the test process.

Measurement

Once everything has been set up and the test is in progress, the criteria listed below are measured. The illumination level where each fails is noted, the highest of which will be the level of minimum illumination.

Luminance

The luminance criterion fails when the bright white in chart TE 153 is no longer at least 50% of the level at full brightness.

Black level

This criterion fails when the 2% reflectance black patch in TE 153 increases by more than 5% relative to the black level at full brightness.

Signal-to-noise ratio

The luminance signal to the noise is measured in chart TE 182 and should be higher than 17 decibels.

Resolution

The resolution should be more than 70% of the level at full brightness, measured in TE 170.

When all these criteria are met at their most minimum level, one can still be assured of a fairly decent image but for superior results, it is important to aim a little higher than the minimum level of illumination.

Roy Alves is the country manager of Axis Communications South Africa.

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