How to Read Lux
Lux and lumens are the two most popular units of measurement when it comes to lighting. The two units differ in the way they measure the intensity of light, but the differences are noticeable.
Lux measures the intensity of the light only from the source itself and takes into account the area over which the luminous flux is spread. If you have a candle burning in a dark room, you will have about 20-50 Lux illuminating an area of 1m 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. When comparing two different lights, lumen is the unit that matters.
What are lumens?
Lumen is a measurement of light and luminous flux. It is the total amount of light/luminosity from a source per unit of time. It describes the way light is emitted or reflected from a source.
The higher the lumen, the more light is available. According to the International System of Units, the SI unit of luminous flux is the lumen (symbol: lm).
In everyday use, the lumen is often used as a unit for the perceived brightness of light sources. 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 the illumination from light sources in the entire environment. When looking at just a light source, you will hear people talk about lumens.
However, these terms are not 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.
For example, the light will decrease if you increase the distance from the lighting source. The opposite will happen too – decreasing the distance will make the light increase.
Conclusion
In a nutshell, one lux is equal to one lumen per square meter. When you need to calculate or choose the intensity of the light, you want to consider the lumens, as they measure the total amount of light emitted in all directions. Lux measures the amount of light that falls on a surface and is usually not so relevant for practical purposes.