One lumen is defined as one candela-steradian, or the amount of light power needed to send 1 candela of light in all directions (isotropically).įor those of you that don’t remember your geometry, steradians are used to describe the surface area of the unit sphere (4 ?) much like radians are used to in describing the circumference of a unit circle (2 ?).
However, I must admit, since they are just units related by a unit-less constant, it really doesn’t matter. Somewhere along the way the switch was made to use the unit lumen, presumably because it maps nicely to that other unit of raw power that we know so well, the watt. visible light power) as defined by our set of ideal human eyes. The lumen was created as the measure of total luminous power (i.e. The candela is a unit that describes light in a given direction, but doesn’t necessarily take into account the spread factor or anything else.
Instead, if you go buy a light bulb, you will no doubt find the number of lumens listed on the package next to the number of watts. Unfortunately, you don’t see the unit candela used much anymore. In addition, there is a precise definition of light frequency spread around the colour green that goes with it – for now I won’t attempt to confuse with that aspect so just think of it as good old fashioned visible light. You can see that would be a very useful measure if you were planning a dinner party during medieval times and you wanted to estimate how much light you needed to light the banquet hall and to subsequently tell your local candle maker how many candles to make.
It is often referred to as luminous intensity. You probably notice the similarity between “candela” and “candle.” Effectively one candela is the amount of visible light emitted by one candle in a given direction.As you might expect, this was really important in the days before electricity when rooms were lit by candle light. First, we need to start with the unit called the candela.