Light intensity is measured in a variety of different ways. The most common measurement scales are "lumens" and "candlepower." Unfortunately, for the average person, these two measurements do not measure the same thing. It can be very frustrating to try to compare two different lights that are rated using different methods. However, comparing lumens and candlepower is easy once you understand how the two scales work.
What are Lumens?
Lumens measure the total output of light from a light source. Light from any source emits at a certain angle. Lumens measures the total amount of light energy that is emitted within that angle. Lumens are also used to calculate efficiency of a light source. The lumen output of a light source indicates how much energy is being converted to light. The lumen output compared to the total energy usage of the light indicates its overall efficiency.
What is Candlepower?
Candlepower is one of the most misused terms used in light measurement. In a strict sense, candlepower measures the light intensity of a beam of light. It is useful to compare how concentrated a light is. The primary unit of candlepower is called a "candela." Candela measures the light intensity within a certain angle of emission.
How Lumens and Candlepower Relate
Candlepower and lumens can be roughly compared. One candela is equal to about one lumen per steradian. A steradian is a unit that measures the angle that a light emits. If a light emitted in all directions at once, it would emit in a perfect sphere. One steradian is equal to 8 percent of the surface area of that sphere. If a light is emitting one candela in all directions it is emitting about 12.5 lumens of light.
Comparing Lights Using Candlepower and Lumens
When comparing candlepower and lumens, it is important to remember that they measure different qualities of a light, and are not interchangeable. When considering what specifications to look for, consider the purpose of the light. If you are looking for a light to illuminate a large area, or are concerned about energy efficiency, then lumen rating is probably the most important thing to consider. If you are looking for a spotlight, or any other application that requires a very intense beam, then candlepower is probably more important to consider.