I hesitate to use analogies but . . . think of lumens as a flow of photons, like so many gallons per minute of water coming out of a garden hose. Visualize footcandles as the local pressure caused when this stream is directed onto something, like a concrete driveway, or in the case of light, on your scrim or on the sensor in your footcandle meter. If you put a nozzle on your hose you can narrow the stream, producing a large impact. Or, you can spread it out into a fine spray over a large area that has little force. The same stream (lumens) can produce a large difference in impact (footcandles) depending on how the stream (beam of light) is shaped.

The relationship between lumens and footcandles varies with the area illuminated. If a source of light produces one lumen of light flux that evenly illuminates an area of one square foot, the illuminance or intensity produced over that square foot will be one foot candle. If that lumen is evenly spread out over one square meter, the illuminance is one lux in the metric system. Since one square meter is equivalent to about ten square feet, one lux has about one-tenth the intensity or brightness of one footcandle.

It must be noted that the lumen, as well as the footcandle which is derived from the lumen, is a photometric unit of energy or a unit designed to describe visible light, the light we as human beings see. Therefore, the spectrum of the energy that is collected by the instrument measuring the lumen (or footcandle) is filtered to the color response of the human eye. The photopic response curve for the eye in daylight conditions is a highly peaked curve that reaches its maximum sensitivity in the yellow greens, near the wavelength of 556 nanometers. Your footcandle meter has a strong yellow green filter placed over its sensor and is much more sensitive to greens than it is to reds or blues.