Illumination

When objects reflect light they are said to be illuminated. The moon is a perfect example of an object that is illuminated.

The moon does not emit visible light. It reflects light from the sun. This is why it is hard to see the moon during the day.

The brightness of an illuminated object depends on two things.
1. The brightness of the luminous source shining light on the illuminated object
2. The distance between the illuminated object and the luminous source
If a Light source is brighter it will illuminate objects more brightly. This should be common sense for all of us who use light bulbs in our house. The brighter the bulb above your desk, the more brightly your workspace will be lit. The relationship between the brightness of the lighting source and the illumination is direct. If a light gets twice as bright the illumination will double. If a lighting source is three times as bright the illumination will triple.

The Inverse Square Law and Illumination

As light moves through space from its source, it spreads out in all directions. The light twice as far from the source is spread over four times the area, hence it is one-fourth the instensity.

This means that if you double the distance that an object is from its lighting source the illumination decreases by 4 times. If you triple the distance between the light source and the object, the illumination will decrease by a factor of 9 times.

If illumination is directly related to the intensity of the light source and inversely related to the square of the distance the equation to calculate illumination is

Where e is illumination measured in lux or lumens/meters squared
I is the intensity of the light source
d is the distance between the light source and the illuminated object
The concept of illumination is a very important one that must be considered by architects when adding lighting to buildings.
Here are two problems that deal with the placement of lighting over a desk.

Problem #1: A lamp with a 100 lumen light bulb is placed1 meter above a student's desk.How much illumination will the student have for her work?

I = 100 lumens
d = 1meters


e = 100 lum/ (1 meter) squared
e = 100 lum/ 1m2
e = 100 lux


Problem #2: A lamp with a 100 lumen light bulb is placed2 meter above a student's desk.How much illumination will the student have for her work?

I = 100 lumens
d = 1meters


e = 100 lum/ (2 meters) squared
e = 100 lum/ 4m2
e = 25 lux

How do concert lights make different colors on a stage?

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