====== Bands ======
Color or frequency bands are standardized systems of filters in astronomy which are very different for different frequency ranges. In the visibile range, the bands are usually called //spectral color bands// or //photometric bands// or //photometric systems//. In radio they are called //frequency bands// each of which are made of narrower //frequency channels//.

===== - Photometric system =====
The most widely used photometric system is the UBV system, created by the American astronomers Harold Johnson and William Wilson Morgan, where the symbols U, B and V are used for the ultraviolet, blue and the visual (yellow-green) filters. The R and I filters were later added to this for read and infrared to create the UBVRI system. The transmission efficiency of the filters are shown below as a function of frequency.

{{:un:ubvri.webp?nolink&600|}}

As evident from the figure, the U filter only allows ultraviolet light, the B filter blue light and V yellow-green light. In the Johnson-Cousins-Glass (JCG) system more filters are added. In the following table, we show the effective wavelength $\lambda_e$, effective frequency $\nu_e$, the FWHM passbands $\Delta\lambda$ and $\Delta\nu$, and the spectral flux density at magnitude $m=0$ at $\nu_e$ in units of jansky (Jy) for each of the filters in the JCG system (Bradt, AM).

^ Filter ^ $\lambda_e$ (nm) ^ $\Delta\lambda$ (nm) ^ $\nu_e$ (THz) ^ $\Delta\nu$ (THz) ^ $S_0$ (Jy) ^
| U | 367 | 66 | 817 | 147 | 1780 |
| B | 436 | 94 | 688 | 148 | 4000 |
| V | 545 | 88 | 550 | 89 | 3600 |
| R | 638 | 138 | 470 | 102 | 3060 |
| I | 797 | 149 | 376 | 71 | 2420 |
| J | 1220 | 213 | 246 | 43 | 1570 |
| H | 1630 | 307 | 184 | 35 | 1020 |
| K | 2190 | 390 | 137 | 24.4 | 636 |
| L | 3450 | 472 | 87 | 11.9 | 281 |
| M | 4750 | 460 | 63 | 6.1 | 154 |

===== - Radio bands =====