Or, given the foot-candles, we are equally adept at solving backwards for the lumens . So now we know how to express numerically the brightness of a light source and we also know how to quantify the light emanating from that source as it illuminates a distant surface. First, let's stick with the square foot concept and make our newest unit a measure of the light coming off 1 ft2 of surface. Let's use the lumen again and declare that 1 square foot of surface radiating 1 lumen of light is producing 1 foot-Lambert . To tie all this together neatly we need just a few more terms, the first of which is the word flux.
Technical people like to say flux when they are referring to a flow of energy.
Another phrase popular with scientific types is solid angle . In fairness we now have to acknowledge that all of the brightness units we've developed so far count on one foot. And although citizens of the United States and a few other enclaves remain comfortable with this 12-inch length, the rest of the planet may find it bewildering. Notice that our expanded lumen definition has already freed us from reliance on any particular distance unit. Any consistent unit will serve.