Radiant Flux (often symbolized φe)
the rate of energy (power) emitted/radiated by something,
e.g., Joules per second or watts.
A star's radiant Flux is called its Luminosity.
Spectral Flux (φe,ν or φe,λ)
is the radiant power per hertz (Frequency) or meter (Wavelength).
Spectral Index (α)