Light measurement is complex. It is difficult to compare products when manufacturers provide performance in different formats. Wattage is the measure of how much electrical power a light source uses, not how bright it is. For a true comparison of output, lumen is the best measure to use.
The lumen is the measure of luminous power of a light source as perceived by the human eye. Lumens describe how much light in total is emitted from a light source.
The foot-candle is the amount of light that actually falls on a given surface. The foot-candle is equal to one lumen per square foot. A typical office environment may be around 50 fc, warehouse around 15 fc.
Just like the foot-candle, the lux is the amount of light that actually falls on a given surface (one lumen per square metre). One foot-candle = 10.764 lux. A typical office environment may be around 500 lux, warehouse around 150 lux.
It is important to understand that a light source spread over a large area will measure less foot-candle/lux than the same source focused on a small area. However the lumens remain the same. When using lux or foot-candle charts, be careful to compare fixtures with similar beam angles and distances.
The candela is the power emitted by a light source in a particular direction as perceived by the human eye. A domestic candle emits light with a luminous intensity of roughly one candela.
Nits are equivalent to candela per square meter. Nits are often used by screen manufacturers to illustrate how bright a screen is.
When comparing products it is important to use the measure relevant to the task in hand:
Fixture output comparison – lumens.
Light levels at a given distance – foot candle or lux.
Screen brightness – nit.
Because the light output of an LED fixture varies with temperature and the percentage of that variation (the smaller the better) is an indicator of the quality of that LED lighting fixture. When referring to light output measurement from a LED fixture one must use hot lumens. Hot lumens are measured after the LED light fixture has reach thermal equilibrium. Normally that should be the case after the fixture has been on at full for at least thirty minutes to two hours or more depending on the size of the fixture (the other way is to measure the temperature of the heat sink of the fixtures and wait 10 minutes after the temperature has stopped climbing).