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How to Use a Digital Lux Meter for Flash Photography

A lux meter is designed to measure the illumination in a location in a unit known as lux, or one lumen per square meter. It is the metric equivalent of the Imperial foot-candle. Photography, however, uses luminance as a standard of light measure, so the readings from a lux meter must be converted before use. Illumination is the amount of direct light from a source. Luminance is the amount of reflected light, which is typically measured from a gray card that has an 18 percent ink density.

Instructions

- 1
  Take your lux measurement.
- 2
  Convert lux to foot-candles, since most camera lighting charts use the Imperial standard. For precise conversion, multiply the lux value by 10.76, or estimate and just add a zero to the end. So, roughly, one lux would be 10 foot-candles. Then, convert to foot-Lamberts by multiplying by pi, or estimate by multiplying by 3. So one lux would give 30 foot-Lamberts of luminance.
- 3
  Set your camera's flash according to the luminance that you want. Figure your total luminance which, for a bright, sunny day is typically 800 foot-Lamberts. Subtract the number of foot-Lamberts you figured from the lux meter, and set your flash to provide the difference.

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