Lumen, Lux, and Watt - How to Choose LED Light with Right Brightness

When it comes to LED fixtures purchase, labels including lumen, lux, watt, or lm/W are indicated by manufacturers to show the lighting performance. Another term associated with light ‘brightness’ is candela, a unit that measures light intensity. You may scratch you head about what they mean and which of them can express the brightness of LED fixtures. Actually, all of them are relative with the brightness of fixtures on different aspects. Perception to brightness of fixtures varies from people, as it is based on individual visual experience. This post will explain the definition, differences and the relationships of terms about brightness. After having the knowledge about former terms, you can choose the LED fixtures with desired brightness.

Here is picture about the relationship between lumen, candela, and lux.

Lumen

Lumen measures the total light emitted by a light source per unit of time. This metric describes the total output from all directions. Thus, the luminous flux measured by lumen is not emitted evenly. In short, lumen is used to describe the total quantity of light emitted by the light source that you want to illuminate an area. It is often misunderstood that brilliance will increase with the increased lumen. However, total light output from all directions is only partly perceivable by human eyes and the light is emitted unevenly. Therefore, even lumen increase significantly, brilliance that human perceived may remain the same.

As light intensity is not even in all directions, ‘Candela’ is used to measure the luminous intensity. In other words, candela describes the total light emitted in one direction or certain solid angle. Compared to conventional light sources, LEDs emit and direct the light to where is needed through the composition of lens and package structure. In consequence, less light loss and more concentrated light are achieved. That is reason why LEDs are more effective than conventional light sources.

Lux

Lux is the unit of illuminance. Lux refers to the total luminous flux on a surface. One lux is equal to one lumen per square meter (lm/m²). Lux is also used to measure the light intensity. Lux levels are related to the distance between light source and the item being lit. If the item is moved further from light source, the lux levels will be reduced. For example, if one candela of light falling on an item, there is one lux at one meter (distance between light source and the illuminated item) while it will be 0.25 lux at 2 meters. Both candela and lux are used for measuring light intensity. The difference is that candela does not change with the distance but lux does.

A table about the illuminance in different environments is as followed:

Watt

Watt is the measurement of the total wattage (power or energy ) that is consumed for emitting light by light source. When we purchase LED fixtures, watt is the common term that indicates the ‘brightness’ of them. In general, wattage of LED high bay for industrial warehouse ranges from 100W to 200W. Wattage of LED flood light for outdoor sporting, however, can be up to 2000W. But watt cannot indicate the light output directly. To connect the light output with wattage consumption, lumen and watt are used together in a term - lm/w, to express the efficiency (efficacy) of light source. Lm/w means the ratio between total light output and energy consumption of light source.

In conclusion, a table of descriptions of lumen, candela, lux, watt, and lm/w are as followed:

Lumen has better description of lighting output than watt and it indicates the actual brightness of lighting fixtures. But as consumer, what we want is the cost-effective lighting fixtures. Lm/w is an excellent metric to help us choose LED fixtures accurately.