# Units in optical and radio spectral data cubes: Flux vs Brightness

I have noticed that the units used in spectral data cubes coming from optical IFU instruments are usually in $erg/s/cm^2/Ang/pixel$. To my understanding this is a measure of Flux. Right?

However, in radio astronomy, spectral data cubes coming from interferometers is usually in $Jy/beam$. To my understanding, this is a measure of surface brightness. Right?

Why optical and radio spectral data cubes don't use the same kind of unit (either flux or brightness)?

The answer is partially historically and partially scientifically. Flux, i.e. energy of radiation integrated across a solid angle (steradian) is basically counting all photons coming from a solid angle, while specific intensity (units of specific intensity are $erg/s/cm^2/Hz/sr$), is the energy per frequency along a line of sight! A surface brightness is flux divided by solid angle, i.e. the mean intensity within this solid angle. This evenly smears out all photons in this solid angle.