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Brightness or luminance?

Updated: Mar 10, 2023

You have probably noticed that there are several different types of light measurements in FizziQ: luminance and brightness. What is the difference between these two types of measurements?

Quantifying light is neither easy nor intuitive. Should we refer to the amount of energy consumed by the light source, the amount of photons it emits or our perception of radiation? Should we consider the sender or the receiver? Photometry defines the measurements that characterize these different elements.

Consider first a point light source. Luminous flux, or visual flux, is defined as the effective light power for the eye, that is, what the eye is capable of interpreting. This measurement therefore depends on the wavelengths emitted. The luminous flux is expressed in lumens (lm) and a lumen roughly corresponds to the effective power emitted by a candle. A 75 watt incandescent bulb produces approximately 900 lumens, as much as an LED bulb consuming 13 watts.

A point light source emits in all directions, and to take into account the fact that certain directions may be obscured, we therefore prefer to use the concept of light intensity which is the luminous flux per unit of solid angle. This intensity is expressed in candelas (cd). For example, marine lighthouses, visible more than 30 km away, have a light intensity greater than 200,000 candelas.

In the real world, the sources are not point. If two bulbs emit the same luminous flux but have different sizes, the smaller one will appear brighter, hence the notion of luminance. Luminance is the light intensity emitted per unit of emitting area. It is expressed in candelas per square meter. The higher the luminance, the brighter the light will appear. The sun at its zenith has a luminance of one billion cd / m2, a white sheet of paper in the sun, 25,000 cd / m2, and the full moon, 2,000 cd / m2.

Finally, for the moment we have only characterized the emitting sources but we can also be interested in the amount of light that a given surface receives when it is illuminated by several light sources. The amount that is used is the brightness which is the luminous flux received per unit area. A lux is the illumination received by an area of ​​1m2 on which a flux of 1 lumen falls. When studying solar panels, one will be particularly interested in lighting.

So brightness or luminance? It all depends on what you want to measure! FizziQ can provide both.

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