Candela (symbol cd) is the original measure of brightness. The word candela means candle in Latin. One candela corresponds approximately to the light intensity of a standard candle. This is also related to some of the other ancient and retired terms for light output, like “candlepower” and “foot-candles”.
Lux (symbol lx) is a key modern unit of illumination. Lux is from latin, and means light. 1 lux is the illuminance produced by 1 candela at a distance of 1 meter from the source. The point here is that Lux is used to measure a very specific part of the emitted output of a source of light (i.e., what falls on a particular point on a surface). In terms of flashlights, this means in practice the centre of the beam – which is where peak brightness can be found (what is commonly referred to as “throw” of a light beam). Lux is closely related to the concept of lumens below, as 1 lux = 1 lumen / m².
The lumen (symbol lm) is the unit for luminous flux. It measures the total amount of light emitted by a light source in all directions. Lumen is thus the measure of the brightness of a light source independently of the direction of the light beam (i.e., it integrates all the light emitter from a source). As indicated above, 1 lux corresponds to the illuminance that is obtained when each square meter of an illuminated surface receives a luminous flux of 1 lumen.
What does all this mean in practice?
Basically, Lux really tells you only what the reach or “throw” of the light is, or how good it is at projecting light over a distance. It is not an overall brightness measure or the total quantity of light produced. As such, Lux readings are strongly affected by the optics of a light – its reflector and lens design. When comparing lights against one another, the Lux numbers represent how much light is hitting the target at the very center area, and does not represent the amount of light surrounding that area.
Lumen, in contrast, is very helpful in understanding the total amount of light a flashlight emits. But it tells you nothing about the pattern of that flashlight’s output – how wide of a beam it throws, and how rapidly the intensity drops off from the center to the periphery. If you had no optics – basically, just a bare emitter – you would have a true “flood” light, that simply emits light in all directions.
Taken individually, these two measures really don’t tell you that much about a flashlight. Taken together, you get a very rough idea of the relative “throwiness” of a flashlight’s output. But you really need to examine pictures of the actual beam of the flashlight, to truly understand its overall pattern.
There’s an analogy here to why I argue that actual runtime graphs are important in understanding how a light works. Simply knowing the starting and ending output levels over a given time gives you a very rough idea of what to expect – but you don’t have any meaningful understanding of how you got there.
One last point to clarify; as per the ANSI FL-1 standards, both center beam intensity (Lux, as indicated in Candelas, or Lux @ 1m) and “beam distance” are reported for flashlights. Beam distance is simply a computed measure from beam intensity. This is because Lux measures follow an inverse square law. This inverse square law says that at twice the distance a light is 1/4 as bright (i.e., one over the square of two). By taking the square root of all of the Lux measurements the numbers can be directly compared.
But beam distance actually takes this a step further, by specifying the specific lux intensity you wish to compare. This is arbitrary, but according to the ANSI FL-1 standard, the computed beam distance value is the distance (in meters) at which the flashlight would produce a light intensity of 0.25 lux. This level was chosen as it is about the level of light that hits ground from a full moon.