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Measuring light levels

A recent article in Hi-Tech Security Solutions (August 2002, Measuring light levels) suggested quite correctly that a light meter should be used in order to determine what CCTV camera to use for a specific application. Unfortunately the article contained a number of errors in the light levels given for outdoor conditions. The correct values are as follows:

Full sunlight: 100 000 lux

Full moonlight: 0,01 lux

Clear starlight: 0,001 lux

Overcast sky: 0,0001 lux

Full sunlight does vary according to location and latitude and a value of 60 000 lux could be typical in the UK. A value of 1 million lux as given in the article is not possible. The night illumination levels given above are those used for the calculation of the performance of passive night vision equipment for military purposes. They would be typical for Europe and the bright southern hemisphere night sky could provide higher levels of illumination. The values for overcast sky are also an approximation as light from nearby towns can be scattered by the clouds to increase illumination.

Light meters are manufactured in South Africa by MIT in Pretoria and the low-cost auto-ranging version covers illumination levels from 1 to 200 000 lux. Special meters are required to measure illumination levels lower than 1 lux.

Maurice McDowell

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