Now that we all know the difference between Watt and Lumen, it’s time to learn about another unit you can sometime find in LEDs’ specifications: the candela.
We have first to remember an old story from school: the solid angle. Roughly, it can be compared to “3D angle”; Wikipedia (http://en.wikipedia.org/wiki/Solid_angle) describes it as a “measure of how large an object appears to an observer”. And in the same way that “2D angles” do not have physical units but are still measured in degrees or radians, the solid angle has no physical unit and is measured in steradian (abbreviation: sr). The most useful equation to remember is, by far, the solid angle of a cone:
And the 2 values to remember are:
- the solid angle of an entire sphere:
- the solid angle of a half-sphere:
And now, back to our candela: the “light intensity” is the quantity of photons passing through a given solid angle. The units are W/sr in radiometric system, and lumen/sr in photometric system. The latter is also called candela.
In a LED’s specification, this value is intended to tell you how much light is directed in the LED-axis direction.
Seems easy, isn’t it? Next time, we will speak about optics and intensity diagram, and you will understand why this parameter is so much misleading if you don’t use it properly…