He-Cd laser
5-200mW power usually at 325 nm (UV) or 441 nm (blue)
Wall power, air cooled
Uses cadium vapor as the lasing medium
Major problem is noise (plasma noise between 300-400 kHz)
RMS noise mostly about 1.5%
Good for ratio measurements (pH or calcium) because power fluctuations dont matter here these lasers do have power fluctuation problems eventually.