Lux measures the intensity of light that reaches the eye. Milliwatts, however, measures the light's total rate of energy transfer in joules per millisecond. A lux of the most easily visible light, green light, moves with a power of approximately 1.55 milliwatts. Other lights' visual intensity depends on the wavelength's luminous efficiency, which describes how sensitive the light is to it. A 830- or 360-nanometer wave, for instance, needs a lot of power to appear visible. A 500-nm ray needs less power.
Divide the light's illuminance, measured in lux, by 683. With an illuminance, for instance, of 25,000 lux -- 25,000 / 683 = 36.6.
Look up the eye's sensitivity to the ray's wavelength from the first table in the first link in Resources. If, for instance, the light has a wavelength of 660-nm light, you'll see that the eye's relative sensitivity to it is 0.061.
Divide the answer to Step 1 by this sensitivity -- 36.6 / 0.061 = approximately 600 milliwatts of power.
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