"The intensity of light observed from a source of constant intrinsic luminosity falls off as the square of the distance from the object. This is known as the inverse square law for light intensity."
It's used in astronomy all the time. Anyway, if you take a measurement of light intensity 10" from the source and another measurement 20" from the source, the measurement at 20" will be exactly 1/4 the intensity at 10" if your instrument is accurate and your measurements were precise. So, if your measurement at 10" was 50,000 lux, your measurement at 20" would be 12,500 lux and your measurement at 40" would be 3,125 lux. Your measurement at 15" would be 25,000 lux and your measurement at 30" would be 6,250 lux.
And in answer to your next question, lux is lumens per square meter.
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Taking a look at the numbers without knowing all the "ins and outs", it logically doesn't make sense. But hey, what is logical in this hobby?
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