Use of light measuring instruments

The human eye is unreliable as an indicator of how much light is present. For accurate results in the measurement of the illuminance at a surface it is necessary to use a reliable instrument. Light meters are available for this purpose. 

1 Cover the cell with opaque material and alter the zero adjustment until the pointer reads zero on the scale. 

3 Select the appropriate scale on the instrument, i.e. that which gives the greatest deflection of the pointer or where the reading is closest to the upper end of the range. 

4 If readings are to be taken during daylight two readings are necessary  

The result required, i.e. the measure of the artificial light, is tiie difference between the two readings. If the two readings are large and approximately equal it will be necessary to re-check the artificial light reading after dark. 

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