The hydrogen lamp is a long glass tube filled with hydrogen gas which is stimulated electrically. As a voltage is applied between the cathode and the anode, an electrical current will flow through the gas, and excite the electrons of the hydrogen atoms.
Wavelength check in the UV range can be performed with a hydrogen lamp and also used a source of continuous radiation in UV part of the spectrum.
The hydrogen lamp does not produce a continuous visible spectrum, but discreet lines. Four of these spectral are visible. And the 656.3nm is the brightest line, which make the lamp red to the naked eye. The rest lie in the ultraviolet and infrared regions.