What is the difference between lux and lumens?

If you're looking to understand a light bulb's brightness, you may see two metrics that may confuse you - lux and lumens. Both are related to brightness, but measure slightly different things.

In this article we explain the differences between the two and how you can make sense of the metrics!
 

The difference between lux and lumens by definition

The core difference can be summarized as follows:

• Lux is a measure of illuminance, the total amount of light that falls on a surface
• Lumens is a measure of luminous flux, the total amount of light emitted in all directions.

In the graphic below, each yellow dot represents a unit of brightness. Lux is the number of dots that fall on a particular surface, while lumens is the total number of dots that are released from the light source. The closer to the light source, the higher the lux reading. This is due to the dispersion of light as one moves away from the light source.

For most directional light sources such as LED spotlights, the center of the beam typically has the highest lux reading. As you move farther away from the center, lux will decrease.

When to use lux vs when to use lumens

Lux is important for knowing how bright a particular surface appears. Without a sufficient lux level, it can be difficult to read or take good photographs.

Common lux levels are listed below:

Dark, cloudy day: 1,000 lux, or 20 micromoles per second per sq meter
Indirect daylight: 10,000 lux, or 200 micromoles per second per sq meter
Direct daylight: 100,000 lux, or 2,000 micromoles per second per sq meter

Lux is a measure of how much light falls on a particular surface, and can be the result of multiple light bulbs and even daylight mixed in.

On the other hand, lumens is important for knowing how much light a single light source emits. This is useful for comparing the total amount of light a bulb emits, but depending on its light distribution and space size, whether it is "enough" for a space or task cannot necessarily be determined.
 

Measurement method differences between lux vs lumens

Since lux is a measure of how much light falls on a surface, handheld light meter or spectrometer can measure the amount of light that falls on a surface. These are typically lower cost and even be connected to smartphones and used in the field.

Lumens, on the other hand measures the amount of light that is emitted by a single light source, and in all directions. Therefore, slightly more sophisticated instruments are required. Typically a goniosphere or integrating sphere is needed. These devices capture the light emitted in all angles, and then measures the collective light emitted.