The spectral brightness, e.g. of some laser beam, is essentially its brightness per unit optical bandwidth. Here, brightness is usually understood what is more precisely called radiance, i.e., we actually have a spectral radiance. This is at least the most common definition, leading to units of W/(sr cm2 Hz). However, there are differing definitions of spectral brightness. It is possible, for example,
- to refer to some normalized bandwidth (i.e., bandwidth relative to the mean frequency, specifying e.g. brightness per 0.1% bandwidth)
- to refer to wavelength rather than frequency intervals (e.g. with units of W/(sr cm2 nm))
- or to quantify the photon number per unit time instead of the optical power.
Also, units of mrad2 are sometimes used instead of steradians (sr).
In addition, different terms are often used for the same quantity, often e.g. in the context of synchrotron radiation sources. Relatively popular are spectral brilliance or simply brightness, even though brightness is usually not associated with a power spectral density.