光度在科学的不同领域中有不同的意义。

在光度学（photometry）中，"光度"（luminosity）经常与亮度（luminance）弄混。亮度（luminance）是光源在给定方向上单位面积单位立体角内所发出的的光通量，单位是尼特。光度（luminosity）并不是一个物理量，这个词用于光度函数。

人眼能相当精确地判断两种颜色的光亮暗感觉是否相同。所以为了确定眼睛的光谱响应，可将各种波长的光引起亮暗感觉所需的辐射通量进行比较。在较明亮环境中人的视觉对波长为555.016nm的绿色光最为敏感。设任意波长为的光和波长为555.016nm的光产生同样亮暗感觉所需的辐射通量分别为和，把后者和前者之比

In astronomy, luminosity is the total amount of energy emitted by a star, galaxy, or other astronomical object per unit time. It is related to the brightness, which is the luminosity of an object in a given spectral region.

In SI units luminosity is measured in joules per second or watts. Values for luminosity are often given in the terms of the luminosity of the Sun, which has a total power output of 7026384600000000000♠3.846×10 W. The symbol for solar luminosity is L_⊙. Luminosity can also be given in terms of magnitude. The absolute bolometric magnitude (M_bol) of an object is a logarithmic measure of its total energy emission.