How to Measure Light Output

The terminology used for light output measurements can vary on each package’s specifications, so while you stand in a hardware store comparing light bulbs, not knowing what the difference is between candela, lux, and lumens, and cursing school for never teaching you Latin, don’t let your frustration get the best of you. Not knowing the difference between lighting measurement terms can be as maddening as not knowing the difference between the metric and imperial systems.

But worry not – we will define candela, lux, and lumens, so that the next time you buy lightbulbs or fixtures, you understand the specs without being fluent in a dead language.

Candela

While candela means candle in Latin, the measurement stands for candlepower. Candela, represented by cd, measures how much light the fixture emits in a particular direction, which is also known as luminous intensity.

Candela is named appropriately as it dates to when candles were the most common source of light. One candela is roughly equivalent to the amount of light a candle projects in a single direction. If you have a bulb turned on in your room that only measures one cd, and you block part of it with your hand, the part of the light that is not blocked will still measure one cd because candela only measures in one direction. The candela unit is best for measuring fixtures with a focused beam of light.

For example, a spotlight in the yard that points toward your house would be better measured using candela units since the goal is to direct the beam of light in one direction. A high candela value would be more important in that situation than a high lumen value. 

Lumen

The term lumen, represented by lm, measures the total amount of visible light that a lighting fixture emits in all directions, which is also known as luminous flux. Lumens are the most common lighting measurement.

One candela is the equivalent to 12.57 lumens. If you covered half of that same bulb in your room that measured one cd, the lumen value would decrease to 6.28 lm because it measures the amount of light in every direction, which is now cut in half due to the blocked illumination.

Let’s say you are tasked with illuminating a large space, like a garage or a conference room. In either case, you would care more about the fixture’s lumen value than its candela value. A high lumen value will guarantee that the light is illuminating in every direction to light your entire space efficiently.

Lux

In Latin, lux means light, but in terms of measurement, lux measures how much light hits a surface per unit area to illuminate it. In other words, it measures illuminance.

One lux, represented by lx, is equivalent to a single lumen per square meter. A greater lux value is important when choosing a source for task lighting. Lamps are most commonly measured using lux. 

To Put It Simply…

• Candela

  • Candela measures how much light the fixture emits in a particular direction

  • Higher candela values are more important for torches, spotlights, laser pointers, or any lighting device that needs illumination in one direction for great distances

• Lumen

  • Lumens measure the total amount of visible light that a lighting fixture emits in all directions

  • Higher lumen values are important for light bulbs that need to give off a lot of illumination in an entire space

• Lux

  • Lux measures how much light hits a surface to illuminate it

  • Lux values are important for lamps to determine how much light will illuminate your surface area

If you would like to know more about light output measurements, please contact one of our friendly experts at 1-800-624-4488 or by the convenient pop-up chat feature at the bottom right corner of our web pages.