Video: Illuminance

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Suppose you have have a light source– a lit bulb – and a surface some distance away. Light from the bulb will illuminate or shine on the surface. Illuminance is the rate at which light energy lands on a 1-m2 area of the surface. Illuminance is a "Surface Thing" and depends upon two factors – the rate at which light energy is given off by the bulb (a "Source Thing") and the distance the surface is from the source.
Illuminance is directly proportional to the Source Thing – the rate at which light energy is emitted (given off) by the bulb. Technically, we refer to this as luminous flux. While not precisely accurate, it is useful to think of it as the power of the bulb; and it is roughly proportional to the bulb's electric power rating in Watts. So a 120-Watt bulb emits energy at twice the rate as a 60-Watt bulb and causes twice the illuminance on the surface. And if the Source Thing is halved (for instance, by using a 30-Watt bulb), light energy is emitted at half the rate and there will be one-half the illuminance on the surface.
Illuminance also depends on distance and is in fact inversely proportional to the square of the distance – the distance between the source and the surface. Light from a bulb travels outward in all directions. As it travels further from the source, it is spread over a larger spherical surface and is thus "diluted by distance". The rate of light energy landing on a 1-m2 area of surface becomes less and less the further the surface is from the source.

This inverse square relationship means that a doubling of the distance causes the illuminance to be be reduced by a factor of 4 (÷22). Moving a surface from 1 m to 2 m from the source makes illuminance four times less. And three times the distance results in 1/9th the illuminance (÷32). Four times the distance results in 1/16th the illuminance (÷42). But a two-times closer surface would have four times (x 22) the illuminance. Moving the surface from 1 m to 0.5 m from the source makes the illuminance four times greater.
If both the bulb's power and the distance to the surface change, predicting the illuminance can be done in two steps. First consider the effect of bulb power (the Source Thing); that is make a change in the I value to account for a change in the bulb's power. Second, account for the effect of distance upon the I value using the inverse square law. Get organized and make the two changes separately to determine your final prediction of illuminance.
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