Mercury calibration lamps are small, rugged, and easy to use. Because the lamps operate at low pressure, well spaced, intrinsically narrow and bright Mercury emission lines dominate the output spectrum. Background between the lines is very low. The output wavelengths are known exactly and are ideal for spectral calibration. Single lines are easily isolated with optical filters for alignment or other applications. Closely spaced lines are very useful for testing the resolution of dispersive instruments. Mercury spectral calibration lamps make excellent high intensity short wave UV sources too. About 90% of the total output is in the 253.65nm emission line.