What is the difference between lumens and lux?

2019-09-05

If you want to know the brightness of the light bulb, you may see two metrics that may confuse you - lux and lumens. Both are related to brightness, but the measurements are slightly different.

In this article, we will explain the difference between the two and how to understand the indicator!

By definition, the difference between Lux and Lumens

The core differences can be summarized as follows:

Lux is an indicator of illuminance, and illuminance is the total amount of light falling on the surface.

Lumens are measures of luminous flux and are the total amount of light emitted in all directions.

In the chart, each yellow dot represents a unit of brightness. Lux is the number of points that fall on a particular surface, while lumens is the total number of points that are released from the light source.

The closer to the light source, the higher the lux reading. This is due to the scattering of light as it travels away from the source.

Therefore, when you look at the lux level of a light bulb, you must always ensure the relevant distance. For example, you might see "1000 lux at 4 feet" - if you only see the lux level, you won't know the measured distance and you won't be able to make a valid comparison.

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

When to use lux vs when to use lumens

Lux is very important to understand the brightness of a particular surface. This is a vital indicator if you want to know how bright the surface is, such as the desktop, reading material or photography theme.

Without enough lux levels, it may be difficult to read or take good photos. Common lux levels are as follows:

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 the amount of light falling on a particular surface, and may be the result of multiple bulbs or even daylight mixing.

On the other hand, lumens are important to understand the amount of light emitted by a single source. This is useful for comparing the total amount of light emitted by the bulb, but depending on its light distribution and spatial size, it is not necessarily certain whether space or task is "sufficient".

Differences in measurement methods between lux and lumens

Since lux is a measure of how light falls on a surface, even a small handheld photometer or spectrometer can measure the amount of light falling on a surface. These are usually lower costs and can even be connected to a smartphone and used in the field.

Lumen, on the other hand, measures the amount of light emitted by a single source and in all directions. Therefore, a slightly more complicated instrument is required. Usually a goniosphere or integrating sphere is required. These devices capture light from all angles and then measure the collective light emitted.