Luminous flux (luminous power) is the measurement of the (perceived) power of light. It is adjusted to take the human eye’s sensitivity to the different wavelengths of light into account. Luminous flux differs from radiant flux in this manner, since radiant flux is a measurement of the total power of light emitted independent of wavelength.

What is the SI Unit of Luminous Flux?

The unit of measurement that is associated with luminous flux is the lumen (lm). A single lumen is the amount of light that produces a single candela of intensity over a solid angle of one steradian. Some unit systems associate power measurements with the lumen. For such measurements, a single lumen is the same as 1.46 milliwatts of radiant power when measured at a wavelength of 555nm (the middle of the visual spectrum of light).

When Is Luminous Flux Used?

Luminous flux is most commonly used as a measurement of the useful light power that a light source, such as a bulb, emits. In recent years, consumers have used the luminous flux as a comparative measurement for light bulbs, these include fluorescent and incandescent bulbs. The measurement is independent of the light source’s brightness, as that is a function of the person’s distance or detection means from the light source.

How to Convert Luminous Intensity to Lumens

Step 1 – Calculate the distance between the light source and the location in lumens. The variable “r” represents the distance between these two points.

Step 2 – Calculate the area that the light will cover over the distance calculated in step one. If the light source has a stated “beam angle,” it can be used to determine the width of the light that is transmitted over the distance.

Step 3 – Calculate the area of the illuminated region divided by the distance squared, which is also referred to as the “solid angle” and its measurement unit is steradians (sr).

Step 4 – Multiply the result by the luminous intensity that is measured in candelas. Lumens = solid angle * candelas.