Sometimes it is more convenient to display graphical spectra with vertical axes that show the relative spectral flux density. In this case, the spectral flux density at a given wavelength is expressed as a fraction of some arbitrarily chosen reference value. Relative spectral flux densities are expressed as pure numbers without any units.
Spectra showing the relative spectral flux density are used when we are interested in comparing the spectral flux densities of different sources; for example, if we want to show how the spectra of blackbody sources vary with absolute temperature, it is not necessary to show the absolute values. The relative spectral flux density is also useful if we wish to compare a sources flux density at one wavelength with the same sources flux density at another wavelength; for example, if we wish to demonstrate how the Suns spectrum peaks in the visible part of the EM spectrum, a graph of the Suns relative spectral flux density will suffice.