Due to the large number of different photometric systems adopted by astronomers, there are many expressions of magnitudes and their indices. Each of these newer photometric systems, excluding UBV, UBVRI or JHK systems, assigns an upper or lower case letter to the filter used. For example, magnitudes used by Gaia are 'G' (with the blue and red photometric filters, G BP and G RP) or the Strömgren photometric system having lower case letters of 'u', 'v', 'b', 'y', and two narrow and wide 'β' (Hydrogen-beta) filters. Some photometric systems also have certain advantages. For example, Strömgren photometry can be used to measure the effects of reddening and interstellar extinction. Strömgren allows calculation of parameters from the b and y filters (colour index of b − y) without the effects of reddening, as the indices m 1 and c 1.