Illuminance vs Luminance – How do they differ?

Do you get confused with similar yet entirely different technical terms when buying any type of lighting? The prime examples are luminance and illuminance. Although most lighting products use these terms interchangeably, the differences between their definitions and measurements may surprise you.

What is Illuminance?

The illuminance is also called the general brightness. You learn about it in lighting design, and refers to the extent to which a surface is illuminated by a light source.

Illuminance is the amount of light falling on a surface and is expressed in lux (lx), where 1 lx = 1 lm/m².

The relationship between illuminance and luminous flux =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 seen, illuminance is not directly linked to the light source producing light. It depends on the surface area and the distance from the surface to the light source. The illuminance will be high if a surface area is near the source of light. As the area moves away from the light source, the illuminance value reduces. But, if we took the illuminance of both lights applied to the same area at the same distance we get the measure of intensity of both light sources. Illuminance you can indirectly get an idea of how intense the light source is however, the surface area must be at an equal distance away from both of the light sources you are comparing.

There are also different types of illuminance. So when light strikes a near horizontal surface (like that of a table) it is referred to as horizontal illuminance Conversely, if the light falls onto a wall, this is called 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?

Another crucial yet frequently overlooked factor you should look out for while buying light bulbs is luminous flux. This refers to luminous flux, which is the measure of visible light emitted by a light source, or the light output of a bulb. It is expressed in “lumens,” a unit you have probably seen a million 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. But you cannot rely only on this factor when deciding on a proper lamp; for example, it depends on the power of the lamp and its color temperature.

Luminous flux does give a measure of the amount of light emitted by a light source, but not whether that is what the human eye sees as visible light. As a result, in many scenarios, you will experience you high-lumen light sources feel dimmer since the actual visible light output from 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.

Conclusion

To sum up, today we brought you everything that you needed to know about luminance and illuminance. Since these terms seem much alike, it is reasonably difficult to differentiate in between the two and be stuck with the incorrect product due to the confusion. However, you can clearly know how these 2 factors differ from each other and choose the right option according to the right quality after going through the guide given here. The primary factor to note, either way, is their relative reliance on the kind of surface. So, when comparing the luminance of a highly polished metallic surface or the surface of a cloth, you can very well know that the metallic plate has more luminance, because it reflects more light when the incident.

Back to blog