Everything about Lumens, Lux, Candlepower

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SilverFox

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Hello Flys4Fun,

Welcome to CPF.

Your references seem to cover the topics pretty well, so I will just add a bit of perspective to what they have already defined.

Lumens refer to the amount of light radiated from the source and is weighted to the photopic response of our eyes.

Lux is the metric measurement of foot candles.

Lux and fc measure the amount of light illuminating an object, and is also weighted to the photopic response of our eyes.

To properly measure lumens, you need a spectrometer and it really helps to have an integration sphere. This equipment tends to be a bit expensive and is usually not available to the hobby person.

To measure lux and fc, you need a light meter. These are less expensive, but they are generally tuned to measure how our eyes respond to a tungsten light at 2856 K. If you light source is operating at a different CCT, or in a different range of the spectrum, the accuracy of your light meter may not be very good.

There are some approximations to convert from luminance (lux or fc) to radiance (lumens) and visa versa, but unless you have an evenly radiating lamp (i.e. no reflector or optic) it is simply a good guess based on how well your light meter responds to the light it is measuring. Since the lighting we use in flashlights is directed, involves reflectors and/or optics, and generally involves a hot spot surrounded by a spill of lower intensity, it becomes difficult to do and in general is not done with great degrees of accuracy.

Hope that helps...

Tom
 

dano

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This topic comes up about once a week. W/in the updated CPF, please use the search function, which will yield excellent results.

-dan
 
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