There are many terms used in the lighting world. There are terms that seem to be similar and people use them interchangeably, yet the truth is that they have different meanings. Two of these confusing terms are illuminance and luminance.
We will distinguish between these two in the following paragraphs. But first, we need to take a look at another important term, the luminous flux.
What is luminous flux?
When buying a light bulb, you are usually concerned only about the amount of light emitted by the bulb. This amount of light is called the luminous flux or luminous power. It measures the visible light that comes from the source. The measurement used for luminous flux is the lumen.
The number of lumens produced by a lighting fixture determines its brightness. In choosing a lighting fixture, you should not base it on its lumen output alone. The reason for this is that lumens only measure the light being emitted by the light source. It does not measure the amount of light perceived by the human eye in a given application. It can result in an inaccurate image of the performance of the fixture.
In other words, the lumen output should not be the determining factor for the suitability of a lighting fixture. In order to evaluate the lighting fixture, you should measure the illuminance and the luminance.
What is illuminance?
Illuminance is the intensity of light falling on a surface area, such as a wall or a desk. It is not the same as the amount of light produced by a light source. Illuminance is also called incident light. This incident’ is the beam of light actually landing on the surface.
The illuminance will be of high value if the area illuminated is close to the light source. If the light lands on a horizontal surface, such as a desk, it is described as horizontal illuminance. Illuminance landing on a vertical surface, such as a wall, is described as vertical illuminance.
The values of illuminance for lighting applications will depend on the complexity of the visual task that is to be performed. For example, if you simply need to walk down a hallway, you don’t need a high illuminance value compared to reading the fine print.
What is luminance?
Luminance is the amount of light reflected from a flat surface. This light is detected by a typical human eye from a particular angle.
What is so important about light reflected on a surface? This is the light that enables us to see objects in this world. Luminance is what gets into your eye. It is apparent brightness, how bright an object appears to your eyes. The world contains a pattern of varying luminances.
Brightness, though related to luminance is something that is not quantifiable. Brightness is a qualitative measure of how bright an object is which is affected by contrast. For example, in a dark room, the screen of your phone is very bright, but not when you look at it in direct sunlight.
Since luminance is what we see, then every light source has luminance too. The sun’s and the moon’s luminance give us a wide range of brightness which our eye can handle.
Glare is also closely related to luminance.
How do we measure illuminance and luminance?
To measure illuminance, we use lumens per square foot or footcandles (US). The metric version of this measurement is lumens per square meter or lux. A light meter is used to measure illuminance.
To better understand a footcandle, we look at it is a unit of illuminance produced by one lumen hitting a one-square-foot area. Or, literally, it is the amount of light on a surface one foot from a standard candle.
For a better viewpoint, daylight produces illuminances that cover a broad range. On a sunny day, it produces 150,000 lux compared to an overcast winter day which produces only 1,000 lux. Moonlight has an illuminance of 0.3 lux.
A typical home or office will have an illuminance of 300 to 500 lux or between 30 to 50 footcandles.
The measure of luminance is candelas per square meter or foot-lamberts (FL). The sun’s luminance is 1,600.000.000 cd/m2. The moon’s luminance is 2,500 cd/m2.
Looking at the sun is like looking at 1,600 million candles per square meter. This is the reason why you should not look at the sun directly for a very long time.
What are the differences between illuminance and luminance?
Illuminance is not seen by the human eye, but luminance is. Your ability to see is affected by the amount of light delivered into the space and the reflection of the surfaces in the space.
Illuminance measures the incident of light or the light illuminating a surface, while luminance measures what is reflected from the surface.
Illuminance measures light hitting a surface and luminance measures light coming from a surface to our eyes. Why do we need 2 different qualities for them?
Illuminance is independent of the surface. Whether it is hitting a desk or a wall, the illuminance is the same. Luminance is dependent on the surface. Since different objects have different reflection or refraction rates, luminance will differ between them.
Illuminance does not depend on the number of light sources; the direction from which the light comes; the position of the light sources; and the type of surface on which it falls. Whether the surface is a polished steel plate or a piece of black felt cloth, the illuminance remains the same.
On the other and, all the quantities that illuminance is independent of, in needed to measure luminance. This includes the direction from which the light reaches the surface, the angle from which the light is viewed, and the properties that different objects have in themselves. Hence, if you compare the luminance of the surface of a polished steel plate and the luminance of the surface of a black felt cloth, you can easily tell that luminance on the surface of a polished steel cloth is higher.