Therefore any given observation only detects some fraction of the total flux. In practise, the observer uses a filter to isolate some waveband of interest, this then determines the magnitude system . The discussion is here confined to the area around the optical part of the EM spectrum. The commonest system of broadband filters, due to Johnson and Morgan, is the system of UBV filters, see figure below. A V-band magnitude is then referred to as m V or just V, for an apparent magnitude, and M V for an absolute magnitude.
V stands for visual, B for blue and U for ultraviolet.
Therefore observations are calibrated with respect to a set of standard stars whose magnitudes in this system are defined. It was later extended into the infrared. The constant defining the absolute magnitude scale was originally defined so that the star Vega had visual and blue apparent magnitudes but has since been redefined so that the Sun has visual and blue absolute magnitudes .