Luminosity

Luminosity is generally understood as a measurement of brightness. Each discipline, however, defines the term differently, depending on what is being measured.

In astronomy, luminosity measures the total amount of energy emitted by a star or other astronomical object in SI units of joules per second, which are watts. A watt is one unit of power, and just as a light bulb is measured in watts, so too is the Sun, the latter having a total power output of 3.846×1026 W. It is this number which constitutes the basic metric used in astronomy and is known as 1 solar luminosity, the symbol for which is . Radiant power, however, is not the only way to conceptualize brightness, so other metrics are also used. The most common is apparent magnitude, which is the perceived brightness of an object from an observer on Earth at visible wavelengths. Other metrics are absolute magnitude, which is an object's intrinsic brightness at visible wavelengths, irrespective of distance, while bolometric magnitude is the total power output across all wavelengths.

The field of optical photometry uses a different set of distinctions, the main ones being luminance and illuminance. Astronomical photometry, by contrast, is concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation. In the field of computer graphics the concept of luminosity is different altogether, a synonym in fact for the concept of lightness, otherwise known as the value or tone component of a color.