Measurement of Light Intensity

How do we measure the intensity of a source of light falling on a surface? You know that light spreads out uniformly in all directions from a source. The amount of light shining on a unit area decreases with increasing distance, This decrease is equal to the square of the distance away from the source. Figure shown below shows how the intensity of light decreases with distance from the source.

Light intensity is measured in lux (metre candle). A lux is the amount of illumination shed on a square metre of cutved surface, one metre from a standard candle. Previously, the unlt foot candle (ftc) was used. A foot candle is the amount of illumination shed on a square foot of curved surface, one foot from a standard candle. To get an idealof the magnitude of footcandle, please read the accompanying margin remark.