What Is Brightness? Part 2: Candela and Candlepower
As we did in part 1, let’s continue to shine some light on exactly what is brightness.
When you turn on your headlights, your auxiliary lights, or even the lights you use in your house, you get the illumination you need to see where you’re going. But, how do you know the difference in brightness between your spotlight and your reading light? Well, other than looking at it and getting blinded for a moment, the light output can be measured in a unit called a lumen, or lumens when you’re talking about a measurement over one. But, what is a lumen? How do I know that 100-lumens is brighter than 99-lumens? Why is a lumen count even important? And how do I compare it to light that was measured in watts, candela or candlepower?
Now we get into candela, or candlepower. But wait, aren’t they the same?
A candela is another unit used to describe the luminous power per unit solid angle emitted by a point light source in a particular direction. Simply put, it’s the luminous power of a light source in one direction. A candle has roughly the output of one-candela, and if we block any portion of the candle to prevent light going out in that direction, it will still produce one-candela in any direction that isn’t blocked. You can also interchange candela with candlepower. The original definition of one-candlepower is equal to .981-candela, when in 1948 candela replaced candlepower in the international unit definition. More recently candela and candlepower have been used in the same definition. So, candlepower and candela are now the same measurement.
There is a problem in using candlepower and candela as a measure of a light’s brightness. Remember when I mentioned that if you block a candle in one direction you still have the one-candela in any direction that isn’t blocked? Well, you can also increase your candela by focusing your light onto a single target. A given lamp will have a higher candlepower (or candela) rating if its light is more tightly focused. So, while your focused beam of light will have the intensity of 1000-candela on the target you hit, anything beyond the focused target will be less. Great for spotlights, but not so great if your driving light is only focused brightest in a single direction, and not in any other direction.