There are altogether too many quantities used to describe light and I was going crazy trying to remember all of them. Here's a summary: - **Radiant flux**, aka **luminosity** or **radiant power**, is the power of a light source. The radiant flux of a star is the total energy of the light emitted by the star each second. - The **spectral flux** or **spectral power** is the same quantity per unit frequency or wavelength - The **radiant energy** is the radiant flux integrated over time. - **Radiant intensity** is the radiant flux per solid angle. If you multiply this by the solid angle subtended by a sphere ($4 \pi$), you get the radiant flux. - **Spectral intensity**: same quantity per unit frequency or wavelength - **Irradiance** or **flux density**: the radiant flux per unit area - **Spectral irradiance** or **spectral flux density**: same quantity per unit frequency or wavelength - This is sometimes called **radiant exitance** when talking about outgoing light, and irradiance for incoming light - **Radiance** is the radiant intensity per area on the light source, or on a detector. Just read the [wikipedia article](https://en.wikipedia.org/wiki/Radiance). - **Spectral radiance**: same quantity per unit frequency or wavelength There are a bunch of other quantities that start with "lumi", like luminance and luminous intensity. Those are defined according to how the human eye perceives the light spectrum, and are not interchangeable with the objective physical quantities listed above.