When comparing multiple lighting products, it is useful to understand the terminology that various lighting manufacturers use to describe the output of their products. Depending on the intended application, certain units of measure may be much more relevant than others. For example, if you need a floodlight or close range work light, a product’s Lumen rating will be important for you to consider. If you require a light for long range search and rescue, security, or surveillance applications, a product’s CandlePower rating will be a more relevant measure than its Lumen rating.
The lighting industry uses several methods to measure light. There are two units of measurement that are often used to describe high intensity lighting devices: Lumens and CandlePower.
Lumens are a measure of how much light comes from just the bulb itself. Lumens are measured by putting the bare bulb in a device called an Integrating Sphere that can detect the light energy hitting its surface from all directions. Properly measuring lumen output is quite difficult and therefore is not done by many flashlight manufacturers. As a result, flashlight manufacturers almost always list the lamp vendor’s data which is always in Lumens because this is the most appropriate measure for a lamp.
Lumens do not take into account the design of the reflector, optics, or focusability of the beam of light. This is important to note because not all of the bulb’s energy ends up being directed into the beam and onto your target.
Lumens are a useful measurement for floodlights and household light bulbs that will be used to illuminate a wide area directly around the bulb. Lumens become less valid when you need to know how much of a bulb’s energy is actually being collected and directed towards your target. That’s where the second unit of measure, CandlePower, comes in.
CandlePower measures the intensity of the light that hits a target at a distance. CandlePower takes the performance of the whole fixture into consideration, which makes this unit of measure more relevant than Lumen output for searchlights and other focused light sources that will be used to illuminate specific targets at long ranges.
CandlePower is measured using a light meter that detects intensity on target in Lux (SI unit) or Foot-Candles (non-SI unit). The light meter is set at a distance from the light fixture and the beam of light is pointed at the Light Meter’s probe. To calculate CandlePower, multiple the Light Meter’s Lux reading by the Distance (in meters) squared between the light and the meter’s probe. (If using Foot-Candles, multiple this reading by the Distance in feet squared.)
It is not possible to convert Lumens to CandlePower because these units measure different aspects of a light’s output. Depending on what kind of fixture a bulb is placed in, a bulb with a high lumen rating may not necessarily have a high CandlePower rating. For example, you can take two identical bulbs with the same lumen rating, place them into two different fixtures, and get two different CandlePower measurements.