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:<math>E_{\text{B-}\!\text{V}} = {B\text{-}\!V}_\text{observed} - {B\text{-}\!V}_\text{intrinsic}.</math>
The [[passband]]s most optical [[astronomer]]s use are the [[UBVRI]] filters, where the U, B, and V filters are as mentioned above, the R filter passes red light, and the I filter passes [[infrared]] light. This [[photometric system|system of filters]] is sometimes called the Johnson–Kron–Cousins filter system, named after the originators of the system (see references).<ref>{{Cite journal |last=Landolt |first=Arlo U. |date=1992-07-01 |title=UBVRI Photometric Standard Stars in the Magnitude Range 11.5 < V < 16.0 Around the Celestial Equator |url=https://ui.adsabs.harvard.edu/abs/1992AJ....104..340L |journal=The Astronomical Journal |volume=104 |pages=340 |doi=10.1086/116242 |bibcode=1992AJ....104..340L |issn=0004-6256}}</ref> These filters were specified as particular combinations of glass filters and [[photomultiplier|photomultiplier tubes]]. [[Michael S. Bessell|M. S. Bessell]] specified a set of filter transmissions for a flat response detector, thus quantifying the calculation of the color indices.<ref name=filters/> 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.
Color indices can also be determined for other celestial bodies, such as planets and moons:
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