What Is the Difference Between Lux vs Lumen?

Heather Clouse
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How to Read Lux

Lux and Lumens are the two most popular units of measurement when it comes to indoor and outdoor lighting. The two units differ only in the way they measure the intensity of light, and the differences are quite small.

Lux measures the intensity of the light only from the source itself. If you have a candle burning in a dark room, you will have about 400 Lux illuminating an area of 1m2 around the candle.

Lumen measures the intensity of the light as perceived by human eyes. This is a more important measurement when defining the brightness of a light source.

For instance, if you have a candle in a dark room, you won’t see much besides a weak light around the candle. But when you put the light from a candle next to that from a 60W incandescent bulb, you will add up the number of Lumens that reach you to be equivalent.

This does not mean that you can mix the units. The rule of thumb is that you should use Lumens when designing an outdoor light source, and lux when designing an indoor light source.

When comparing two different lights, Lumens is the measurement that matters.

What Are Lumen?

Lumen is a measurement of light and luminous flux. It is the total amount of light/luminosity from a source. It describes the way light is emitted or reflected from a source.

The higher the lumen, the more the light available. Lumen is one of the units of photometry, a branch of so-called kinematics. For example, a light bulb of 100W produces about 1,600 lumens.

Also, the SI unit of luminous flux is the lumen.

According to the International System of Units, the SI unit of luminous flux is the lumen (symbol: lm). The unit is a derived unit defined as the total luminous flux emitted by a source or incident on a surface.

It is measured in SI by using suitable instruments.

In everyday use, the lumen is often used as a unit for the perceived brightness of light sources.

And the source of light is often specified, for example, 50 lm (lumens) fluorescent lamp.

Understanding Brightness

This can be quite tricky for people who have never had to understand brightness. Just know that the term lux is used when talking about illumination from light sources in the entire environment. When looking at just a light source, you will hear people talk about lumens.

Essentially, these terms are interchangeable. The more lumens there are, the more the light is going to illuminate the area. This means that lux is related to the area it illuminates – as the distance from the light source increases. In the same way, a source with a high lux rating will reach a certain distance, at which point it will experience a drastic drop in the lux rating.

In a simple environment:

If you have one lux meter into a small environment, you will experience 1000 lux.

If you keep it in the same place, the light will decrease if you increase the distance from the lighting source. The opposite will happen too – adding the distance between the lighting and the lux meter will make the light increase.

If you have multiple lux meter in a small area, you are likely to have an average of 750 lux.

If you have a source that illuminates an area with a high-lumen rating, you will have a dimmer light at a distance than an average source. Even if you only have one lux meter in a big area, you can expect to have 300 lux.