For the flux density received from a remote unresolvable "point source", the measuring instrument, usually telescopic, though not able to resolve any detail of the source itself, must be able to optically resolve enough details of the sky around the point source, so as to record radiation coming from it only, uncontaminated by radiation from other sources. In this case, spectral flux density is the quantity that describes the rate at which energy transferred by electromagnetic radiation is received from that unresolved point source, per unit receiving area facing the source, per unit wavelength range.
At any given wavelength &lambda;, the spectral flux density, F, can be determined by the following procedure:
*An appropriate detector of cross-sectional area 1 m is pointed directly at the source of the radiation.
*A narrow band-pass filter is placed in front of the detector so that only radiation whose wavelength lies within a very narrow range, Δ&lambda;, centred on &lambda;, reaches the detector.
*The rate at which EM energy is detected by the detector is measured.
*This measured rate is then divided by Δ&lambda; to obtain the detected power per square metre per unit wavelength range.
Spectral flux density is often used as the quantity on the y-axis of a graph representing the spectrum of a light-source, such as a star.