Flux surface surrounding a charge "q" is "q"/ε. In free space the electric displacement is given by the constitutive relation D = ε E, so for any bounding surface the D-field flux equals the charge "Q" within it. Here the expression "flux of" indicates a mathematical operation and, as can be seen, the result is not necessarily a "flow", since nothing actually flows along electric field lines. The magnetic flux density (magnetic field) having the unit Wb/m (Tesla) is denoted by B, and magnetic flux is defined analogously: with the same notation above. The quantity arises in Faraday's law of induction,
Spectral flux density flux density will suffice. Spectral flux density In spectroscopy, spectral flux density is the quantity that describes the rate at which energy is transferred by electromagnetic radiation through a real or virtual surface, per unit surface area and per unit wavelength. It is a radiometric measure, as distinct from measures that characterize light in terms of the brightness to the eye, or photons. In SI units it is measured in W m, although it can be more practical to use W m nm (1 W m nm = 1 GW m = 1 W mm) or W m μm (1