I recently added two helium neon lasers into the collection, thank to Mr. Gebarowský. These are the types NL0100 and NL05 made by TESLA. I was able to get the beam from NL0100, but the NL05 so far appears to be faulty (it has a discharge, but there is no beam). NL05 parameters: current 5.9 mA, 1.1 mW power. For the second laser I could not find data, but probably it is similar. Lasers were probably made in the 1970s or 1980s. It is now rarely used, In most applications have been replaced by solid-state lasers (laser diodes). They are filled with a mixture of helium and neon gases that lights typically orange-pink color. The wavelength of laser light is 633nm (orange-red). Discharge runs through a thin glass tube, which is located between two mirrors. The front semi-reflecting mirror is planar, the fully reflecting rear mirror is hollow (concave). The HeNe laser needs current limited high voltage power supply.