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Illuminance vs Luminance – What is the Difference?

Steve Shepherd |

When you buy any type of lighting, do you often get confused by similar but entirely different technical terms? The best examples are luminance and illuminance. While many lighting products use these terms interchangeably, you might be surprised to find that there are significant differences in their definitions and measurements.

What is Illuminance?

Illuminance is a measure of the amount of light that falls on a surface. It is an important concept in lighting design and describes how well a surface is illuminated by a light source.

It indicates how much light is hitting a surface and is measured in lux (lx), where 1 lux equals 1 lumen per square meter (lm/m²).
The formula for calculating illuminance (E) is: E=Φ/A
where Φ is the luminous flux in lumens, and A is the area in square meters over which the luminous flux is distributed.
As you have noticed, illuminance is not directly related to the light source's ability to produce light. It varies based on the surface area and the distance between the surface and the light source. If a surface area is close to the light source, the illuminance value will be high. As the area moves farther from the light source, the illuminance value decreases. However, if we compare the illuminance of two lights on the same surface area from the same distance, we can understand the overall intensity of each light source. Indirectly, illuminance can give you a rough idea of the light source's intensity, but the surface area need to at an equal distance from the light sources being compared.
There are also different types of illuminance. If light falls on a horizontal surface, such as a table, it is called horizontal illuminance. On the other hand, if the light falls on a wall, it is referred to as vertical illuminance.

How to Measure Luminance

Let’s use a common household item—a television—to understand the concept of brightness. A television has a brightness of about 300 cd/m², which means it’s similar to placing 300 candles within one square meter. With this analogy, you can better understand luminance. From this, we can see that any light source with a higher luminance value will provide a brighter output. Therefore, when selecting a light bulb or any other type of light source, you should always prioritize luminance levels over illuminance. This approach will also help prevent you from purchasing a light source that is too bright for your application.

What is Luminous Flux?

Luminous Flux is also an important factor you may check before purchasing light bulbs. The measurement of visible light emitted by a light source, also known as luminous flux or the light output of a bulb, is called luminous flux. It is measured in "lumens," a unit you have likely encountered countless times.
The number of lumens or the light flux generated by a light source essentially determines its brightness level. Therefore, when choosing a light source, you should definitely check the lumen rating. However, you should not base your decision solely on this factor; it may also depend on factors such as the power of the lamp and its color temperature.
While luminous flux does give you a rating of the light produced by the light source, it does not indicate whether it corresponds solely to the visible light perceived by the human eye. Therefore, in many cases, you may find that high-lumen light sources appear dimmer because the actual visible light emitted by the source is lower.
Illuminance Luminance
Illuminance cannot be perceived by the human eye. The luminance can be perceived by the human eye.
Illuminance is the measure of incident light that falls upon a horizontal or vertical surface. Luminance is the amount of light reflected by the surface, ultimately making the surface visible to the human eye.
Illuminance is independent of the surface type. Luminance is dependent on the surface type.
Illuminance changes with the change in distance between the light source and the surface. Luminance changes based on the object’s ability to reflect incident light.
Illuminance is independent of the number of light sources Luminance depends upon the number of light sources
Illuminance is independent of the direction of light source Luminance changes with the change in direction of incoming light


Today, we have provided you with all the information that you need to know about luminance and illuminance. Since these terms feel a lot familiar, it is quite easy to get confused between the two and end up with the wrong product due to the confusion. But after going through the guide provided here, you can easily understand the differences between these 2 factors and pick the right option based on the right quality. The main thing to keep in mind, in either case, is their dependence on the type of surface. So, if you are comparing the luminance between a highly polished metallic surface or the surface of a cloth, you can easily tell that the metallic plate has higher luminance since it can reflect more light upon the incident.

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