This figure, from Pound & Goodman 1997, shows an "integrated" spectral-line map in the left panel. This map is constructed from measurements of the emission from gas atoms, using a radio telescope. The "beam size," or area on the sky, over which photons are collected in a single observation is shown by the small black filled circle labeled "HPBW." In each observation, a spectrum like the ones shown in the right panels is measured. The full map is made by observing a grid of positions, along columns and rows spaced by just one "beam size," thus taking spectra for the entire region of interest. The grey-scale plot shows the result of "integrating," or measuring the area, under the spectrum at each position and then displaying the numerical result as proportional to a level of grey. The strongest emission is color-coded to be black.