Brightness is the amount of light, or other radiation coming from an object; it can be quantified in several different ways. Luminosity is a measure of how bright something is intrinsically. It can be measured in watts, or watts per hertz to describe the luminosity at a specific frequency. For historical reasons, absolute magnitude is used in optical astronomy, while radio astronomers quote the power, which is the luminosity per steradian (multiply by 4π to get back to luminosity). Flux density is a measure of how bright an object seems to an observer on Earth. It depends both on the luminosity and the distance, since distant objects appear fainter. Optical astronomers measure this as apparent magnitude, while radio astronomers use a unit called the jansky. Intensity (or surface brightness) is a measure of the flux density received, not from the object as a whole, but from each unit area of the sky (technically, solid angle). The flux density of an object is thus the product of its intensity times its solid angle.