Lux, Lumen, Candela, it's all ... latin !
The LUMEN (symbol "lm") measures the quantity, the flux of light emitted by a source, whatever the direction of emission. Like the amount of water coming out of a tap.
The CANDELA (symbol "cd") is the unit of light emission in a cone of one steradian (from a particular direction). It's a luminous intensity.
The LUX (symbol "lx") measures the quantity, the flux of light received per unit area. Like the amount of water received on 1m². It's a measure of illuminance.
The full moon, in a cloudless sky, produces an illumination of 0.2 Lux...
.
Not that much...
And yet! A walk in the moonlight doesn't even require a flashlight...
What about Watts?
For decades, the unit used for light bulbs was the Watt (W). This corresponded not to the amount of light emitted, but to the amount of electricity consumed by the bulb.
The efficiency of an incandescent bulb is around 5% for light, the rest being dissipated as heat.
The advent of LEDs, with their very different energy efficiency, makes this use of the Watt inappropriate.
Lumens, color temperature, bulb base, energy class and wattage "equivalent" are now indicated:
E27 (or E14) are just the diameter in mm of the bulb base, the E being there just to say "Edison".
