Inverse Square

One reason larger aquariums need more light is the inverse square law. Simply stated, light becomes less intense the further you are from the source. This is common sense, of course, but the inverse square law tells us how much the intensity decreases: double the distance and intensity drops to a quarter of its previous level. The effects in an aquarium can be visualized from the table below, a table of light levels at various points below a fluorescent fixture.

The following table demonstrates how light intensity diminishes in relation to various distances from a light source. The researchers took measurements with a light meter underneath a shop light fluorescent fixture with two 40-watt cool-white lamps.

Lighting Table

Height

Foot-candles

6 in

100

320

520

320

100

12 in

140

250

300

250

140

18 in

130

180

200

180

130

24 in

110

140

150

140

110

12 in

6 in

Center

6 in

12 in

Horizontal Distance from Center

Making Your Aquarium Look Real 57

Making Your Aquarium Look Real 57

Data from Barbara Joe Hoshizaki, and Robbin C. Moran, The Fern Grower's Manual. (Portland, OR: Timber Press, 2001), 31.

You can see that a considerable drop in intensity occurs as one moves down and away from the center of the light source. To maintain even illumination over the entire aquarium usually requires multiple lamps, either fluorescent, metal halide, or a combination.

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