Some people are probably already familiar with the terms above, even if you’re not an electrical engineer or lighting designer. I'm proof of this... As an HVAC engineer, I’ve definitely heard of them all before, but don’t personally use them in a routine capacity (with the exception of Watts, of course, which can be expanded to include anything that uses electricity).
Engineers are commonly looking for Watts/sf (W/sf) of heat contribution from the lighting system to include in heating and cooling load calculations. But by definition, the actual installed W/sf of any lightings primary output is light, with waste heat only caused by inefficiency in the fixture.
Lumens, Footcandles and LEED
Lumens and FC come into play, for me, when looking at fixture types for LEED SSc8 (Light Pollution Reduction) where percent of lumens designed must meet a certain NADIR (straight down) requirement depending on lighting zone and not exceed a certain FC level beyond the site boundary.
So if I’ve already lost you… let’s just take these terms one at a time and start out with lumens.
A Lumen (lm) is the amount of light falling on a 1 square ft area 1 foot away from a standard candle, or the quantity of light that leaves a lamp in all directions.
When a lamp is rated in lumens, that value is a measure of the total light output from that lamp (i.e. a lamp is rated at 65 W & 750 lm). As lamps and fixtures age, their total lumen output can also decrease (lumen depreciation). Initial lumens indicate how much light is produced once the lamp has stabilized – in the case of fluorescent and HID lamps – and mean lumens indicate the average light output over the lamp’s rated life.
A standard candle is putting out about 13 lumens. Other examples include a 100 W incandescent bulb (1,700 lumens) and a 32 W fluorescent bulb (2,800 lumens).
Footcandles are a measure of light intensity. A footcandle is defined as the quantity of light falling on a 1 square-foot area from a 1 candela light source at a distance of 1 foot (lumens/sf), which is literally the illumination level 1 foot from a standard candle.
Footcandles can be measured both horizontally and vertically. If you didn’t pick up on that, we introduced yet another new term in that definition and referenced lumens, which we covered above. Candela is a measure of ‘directional brightness’ or intensity of a light source. FC decreases as you go farther away from the source, as the square of the distance.
Illuminance is the light intensity measured on a plane at a specific location, and measured in footcandles. The metric unit of measure for illuminance of a surface is Lux, which equals the light level on a surface 1 meter from the 1 candela source (0.0929 FC = 1 Lux).
Finally, the watt is a unit of measuring electrical power. Watts do not relate to light output level, as I alluded to before, but define the rate of energy consumption by an electrical device in operation. When you look at your electric bill you’re seeing two charges for consumption. A demand charge in watts (actually, kW) and watt-hours (kWh), or the measure of the amount of time a certain amount of watts was needed.
So a lamp is putting out lumens, the intensity coming out of the lamp in any particular direction is measured in candelas, and footcandles are the lumens/sf on the work plane. And watts only come into the equation when we’re curious about the efficiency of a lamp. So incandescent bulbs generate 12 lm/w while LEDs and halogen sources can produce 40 lm/w (not to mention fluorescents at 60 lm/w)… what would say is the most energy-efficient?
Of course lighting design - which takes into account things like light loss factor, wavelengths and lens patterns - is a bit over my head and this is just some basic knowledge.
But hopefully it helps shed a little light on where these terms come from and how they relate to each other.