Inverse-Square Law and Artificial Lights

Imagine taking a picture on a sunny day. To set the camera for the right exposure you use an external light meter to measure the intensity of the sunlight. After taking the measurement on ground level, you shoot a picture. Then you and your friends, decide to have lunch, take an elevator to the sixth floor of a building that has an outdoor restaurant on the buildings roof. The view on the roof is so spectacular, you decide to take another picture. You take out your light meter and reading light's intensity, you discover the meter's measurement of the sunlight's intensity is the same on the building roof, six floors up, as it was on the ground floor.

That's the major difference between sunlight illumination and artificial lights. The intensity of sunlight doesn't change appreciable as you move up or down a couple of hundred feet. With artificial light this movement towards or away from the light source changes the light intensity dramatically.

If a one-watt light source is illuminating an area of one square centimeter at a distance of one meter, the intensity of light is 1 watt/SQ.CM. If we move the screen back to a two-meter radius the same light would be covering four times the area, reducing the light intensity to .25 watt/Sq. CM.

This is the inverse square law. The inverse square law states that the intensity of radiation is inversely proportional to the square of the distance from its source. So a doubling of distance reduces the intensity by 1/4th.

So a general rule to follow when growing plants under artificial illumination is to keep the plants as close to the light source as possible, but not so close that any heat from the light source will damage the plant.

Light intensity is measured two ways. The Photometric system measures light based on the sensitivity of the human eye. The human eye, like other human senses does not react in a linear fashion to the intensity of energy. The luminous intensity of light is measured in candela.

A one candela light source produces one lumen of light on an area of one square foot at a radius of one foot.

The Radiometric system uses artificial detectors that accurately measure the intensity in terms of energy. Energy is measured in watts. At the earth's surface, full sunlight is estimated at 1000 watts per square meter.

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