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The Difference Between The Lux and The Lumen.

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Of Luxes and Lumens...

          We recently received an E-Mail from a person who was trying to locate fluorescent bulbs bright enough to illuminate a subject within a range of 5000 to 10000 luxes. This person said that he was having trouble finding a fluorescent fixture that had any rating with regard to luxes.

          And he never will. . .

          That is because a lumen describes a quantity of light emitted from a source and a lux describes the illumination of an object.

          A light bulb manufacturer can only state what light is emitted from the bulb. Ultimately, the illumination of an object is dependant upon the transparency of the medium, the color of the object, the angle at which the light strikes the object and the distance between the object and the light source.

          A light may illuminate your desk nicely because the desk lamp holds the bulb at 14 inches from the surface. However the same desk lamp with the same bulb would not illuminate your desk well if that lamp were placed 50 feet away! The output of the light (lumens) did not change but the illumination (luxes) changed dramatically! You can see that no light bulb manufacturer could possible predict all the variables to assign a rating in luxes.

          Both of these terms, the lumen and the lux are in the metric system. The lumen was set in 1948 by international agreement as a certain amount of light that is emitted by a specific light source (molten platinum). The lux is defined as the light per unit area or lumens per square meter. They are by no means the same thing!

          Roughly speaking , illumination decreases proportionately to the square of the distance from the light source. As the angle that the light hits the source increase, the illumination decreases further roughly proportionate to the cosine of the angle of incidence.

          And . . . Illumination decreases further if there is obscuration in the medium. (What does that mean? It means the sun does not shine as brightly on us when it is a cloudy day… )

Published 2002.02.24

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