Difference Between Brightness and Luminosity

Brightness vs Luminosity

Brightness and luminosity are two very important concepts of light and radiation. There are tremendous amounts of applications of brightness and luminosity in fields such as astronomy, physics, astrophysics, cosmology, agriculture, meteorology and even photography. It is vital to have a good understanding in the concepts of brightness and luminosity in order to excel in such fields. Brightness and luminosity are mostly discussed with regard to light, but these theories can also be applied to any form of electromagnetic radiation. Some of the theories of luminosity and brightness are easily understood, but some theories require some understanding of advanced mathematics and physics, these advanced theories are out of the scope of this article. In this article, we are going to discuss what luminosity and brightness are, their definitions, what are the calculations associated with luminosity and brightness, their applications, similarities and finally the difference between brightness and luminosity.


Luminosity is a frequent term used in fields such as photography, graphic designing and astronomy. In some cases, the word is used in other context than the literal meaning of the word. The proper definition of luminosity is the energy radiated per unit time. The unit of luminosity is watt. Alternatively, we also can take the units as joules per second. In photography, luminosity is used in the context of luminance, which is measured as the candela per unit area. However, luminosity of an object does not depend on the distance from which it is observed. Luminosity is an intrinsic property of the object. In astronomy, the luminosity of stars is measured using the unit called solar luminance (L0). This is equal to 3.846×1026 W. Luminosity also corresponds to the absolute magnitude of a star, which is defined as the apparent luminosity of the visible light region in the electromagnetic spectrum. There is also a concept of spectral luminosity, which is defined as the energy radiated per unit frequency per unit time. The luminosity of an object, which has a surface area A, and the surface having a uniform temperature T is given by E =σ AT4, where σ is the Stephan-Boltzmann constant and the temperature is measured in kelvin.


Brightness is a term frequently used in the study of photography, astronomy and any optical phenomena. The term brightness usually refers to the amount visible light. Brightness is formally defined as the energy carried by electromagnetic waves going through a unit area per time. Imagine a point object of luminosity L, which means it radiates L watts per second. When a hollow imaginary sphere is drawn at a distance r from the center object, the area of the sphere is 4πr2.Therefore, the energy carried per unit area per unit time is L/ 4πr2. Brightness is measured in watt per square meter. Both radiated waves and reflected waves can contribute to brightness. The brightness of an object follows inverse square law.

What is the difference between luminosity and brightness?

• Luminosity is an intrinsic property, which means it is independent of the distance and other factors; on the other hand, brightness is a function of both luminosity and the distance from the object.

• Brightness is measured in watt per unit area, while luminosity is measured in watt.

• Both radiated waves and reflected waves contribute to brightness, while luminosity only depends on the radiated waves.