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Illuminance VS luminance – What Is The Difference?

When you are buying a light of any kind, you will come across a lot of technical terms that seem similar but have completely different meanings. The best example of that is between luminance and illuminance. While both of these terms are interchangeably used by many brands, you will be surprised to know the difference between their definitions and measurements.

In a lot of cases, you will either find one or the other mentioned with a light bulb, or a light source of any kind. So to know what product you are buying and whether it is the right one for you or not, you should know the difference between illuminance and luminance.

What is Illuminance?

Illuminance is a pretty basic term and you probably would have read about it in school while studying physics. Illuminance is basically the intensity or amount of light coming into contact with a surface area like the area of the wall or a desk. As you can notice, this term is dependent upon the surface area over which the light is falling, not the source of the light itself.

Therefore, illuminance has nothing to do with the capacity of the light source to produce light. It will still vary based on the surface area or the distance between the light source and the surface area.

If the surface area is close to the light source, the illuminance value will be high. And as the area is moved away from the light source, the value keeps on decreasing. But, if we were to compare the illuminance of 2 lights over the same surface area from the same distance, we can get an idea about the overall intensity of each light source.

So indirectly, the illuminance can give you an approximate idea about the intensity of the light source, but the surface area has to be at an equal distance from the sources that are in comparison.

There are also various types of illuminance. If the light falls on a horizontal surface like a desk, it is known as horizontal illuminance. On the other hand, light falling on walls is called vertical illumination.

How to Measure Illuminance

Measuring illuminance is a pretty easy task. The unit used for the measurement of illuminance is lumens per square foot or footcandles. You can also find the ratings mentioned in lumens per square meter if the manufacturer follows metric units for measurement. And to measure illuminance, a device is required which is called a light meter.

A footcandle is basically the unit of illuminance produced by one lumen of light coming in contact with a unit square foot area of the surface. You can break it down to the amount of light you see on a one-foot surface from a candle in the dark.

You can also get an idea about illuminance by comparing the light available around us day and night. On a bright day, there is about 150000 lux light whereas, in cloudy weather, this value drops down to 1000 lux. On the other hand, moonlight has an illuminance of 0.3 lux.

What is Luminance?

Luminance on the other hand is a completely different term and you should pay attention to the difference between the two to get a better understanding. By definition, luminance is the amount of light that is reflected from a flat surface. Any object that you see around you has light reflecting from its surface, thus being visible to your eyes. So the amount of light that is reflected from its surface basically gives you a value for its luminance.

To put it in simple words, luminance is what we see and it is how we can perceive the world around us. You can also call luminance brightness since it varies how bright an object will appear to you. The most basic example of that is your smartphone. When under direct sunlight, you will have to increase the brightness of your smartphone to be able to see its content. But, since the amount of light being reflected off its surface does not change, you are not changing its luminance, but its luminous flux.

To understand it more clearly, consider the same brightness level of your smartphone in a dark room now. As you can imagine, a particular brightness level would be too bright in a dark room, but won’t be bright enough under direct sunlight. It is because brightness is a qualitative measure of the appearance of an object which depends upon contrast. So, luminance and illuminance, even though they are different, are somewhat dependent on each other.

How to Measure Luminance

Unlike illuminance, luminance is measured in the unit candelas per square foot, or candelas per square meter for metric calculations. You will also see FL or “Foot Lamberts” units mentioned at some places for luminance, but it is the same as candelas per square foot.

Again, let us check the variation of luminance based on the brightest object we have around us, the sun. The luminance of the sun is about 1,600,000,000 candelas per meter squared. On the other hand, the luminance of the moon is only about 2,500 candelas per meter squared.

So, when you are looking directly at the sun, you are basically looking at about 1.6 billion candles placed within a meter squared surface area. This should give you an idea about how the unit of luminance works, and what luminance is exactly.

As you can imagine, any light source that has a higher luminance value offers a brighter output. So, you should always prefer the luminance rating over illuminance when choosing a light bulb, or any other type of light source. It should also prevent you from buying an overly bright light source for your application.

What is Luminous Flux?

The luminous flux is also an important factor that you would probably check out before buying a light bulb. The amount of light emitted by the light source is called luminous flux or luminous power of the bulb. It is simply the measurement of visible light coming out of the source. This is probably the most important factor since it is measured in “lumen” units, something that you must have seen hundreds of times before since the capacity of a light source is always mentioned in lumens.

The number of lumens or the amount of luminous flux produced by a light source basically determines its brightness levels. So, you should definitely check the lumens rating while you are choosing a light source. However, you should not base your choice based on this factor alone.

While luminous flux does give you a rating of the light produced by the light source, it does not specify whether it is the rating of just the light visible to the human eye or not. So most of the time, you might find high lumen light sources that appear dimmer in comparison since the actual value of the light produced by the source which is visible to the human eye is lower.

What are The Differences Between Illuminance and Luminance?

So far, we have seen what luminance, illuminance, and luminous flux represent and what these factors mean by definition. So, you can easily tell these features apart now and get an idea about what they represent on a product. But to actually understand the differences between luminance and illuminance, one has to compare their key points side by side. Therefore, we are offering the following difference table that will clearly let you understand the differences between luminance and illuminance.

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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