determines the color of an object, which in the case of a Star gives its Temperature .
To measure the index one observes the Magnitude of an object successively through two different filters - such as U and B, or B and V, where U is sensitive to ultraviolet rays, B is sensitive to blue light, and V is sensitive to visible (green-yellow) light. The difference in magnitudes found with these filters is called the U-B or B-V color index, respectively. The smaller the color index, the more blue (or hotter) the object is. Conversely, the larger the color index, the more red (or cooler) the object is. As a comparison the yellowish Sun has a B-V index of 0.65, while the blueish Rigel has B-V -0.03, because its B magnitude is 0.09 and its V magnitude is 0.12 (B-V=-0.03).
The passbands most optical Astronomer s use are the UBVRI filters, where the U, B, and V filters are as mentioned above, and the R filter passes red light, and
the I filter passes infrared light. This system of filters are sometimes called
the Johnson-Cousins filter system, named after the originators of the system
(See references). These filters were specified as particular combinations of
glass filters and photomultiplier tubes. Bessel specified a set of filter
transmissions for a flat response detector, thus quantifying the calculation
of the color indices. For precision, appropriate pairs of filters are chosen depending on the object's color temperature: B-V are for mid-range objects, U-V for hotter objects, and R-I for cool ones.
