Lumen vs Lux: What's the Difference?

When shopping for a light bulb or reading about good light, you've probably come across the terms lumens and lux. They sound similar, but they describe two very different things.

Understanding the difference is essential, especially when it comes to lighting for health and wellbeing.

Jan Sleepman

What are lumens?

Lumens (lm) measure the total amount of light a lamp produces. Think of lumens as the light output of the source itself. The higher the lumen value, the more light the lamp emits in all directions.

However, lumens don't tell you how much light actually reaches you.

What is lux?

Lux (lx) measures how much light, or how many lumen, falls on a surface. For the health aspects of light, that surface is usually your eyes.

One lux equals one lumen spread over one square metre.

This means lux depends not only on the lamp's output but also on factors such as, the distance from the light source and the direction of the light.

In other words, lux tells you how much light is actually available where you are.

Why lux matters more for health

For your biological clock, it doesn't matter how many lumens a lamp produces; it matters how much light reaches your eyes.

Imagine sitting beneath a very bright lamp. If the lamp is high above you or poorly directed, only a limited amount of light may actually reach your eyes. Conversely, sitting close to a window on an overcast day may expose you to thousands of lux, even though the sky doesn't appear particularly bright.

This is why chronobiologists and lighting professionals often talk about lux at eye level, rather than the lumen rating printed on the packaging.

A practical example

Imagine you buy a lamp that produces 2,000 lumens.If the light is spread across a large room, the illumination at your desk might only be 200 lux. Move the same lamp closer or focus the light more effectively, and your desk could receive 500 lux or more.

The lamp hasn't changed; only the amount of light reaching the surface has.