Mercury-vapor lamp
Mercury-vapor lights operate at an internal pressure of around one atmosphere and require special fixtures, as well as an electrical ballast.
In 1860, John Thomas Way used arc lamps operated in a mixture of air and mercury vapor at atmospheric pressure for lighting.
[6] The first mercury-vapor lamp to achieve widespread success was invented in 1901 by American engineer Peter Cooper Hewitt.
[8] In 1903, Hewitt created an improved version that possessed more satisfactory color qualities which eventually found widespread industrial use.
Continued vaporization of the liquid mercury increases the arc tube pressure to between 2 and 18 bar, depending on lamp size.
So if the lamp is connected directly to a constant-voltage source like the power lines, the current through it will increase until it destroys itself.
Self-ballasted mercury-vapor lamps can be screwed into a standard incandescent light socket supplied with the proper voltage.
As the main arc strikes and the gas heats up and increases in pressure, the light shifts into the visible range and the high gas pressure causes the mercury emission bands to broaden somewhat, producing a light that appears more white to the human eye, although it is still not a continuous spectrum.
[12] To correct the bluish tinge, many mercury-vapor lamps are coated on the inside of the outer bulb with a phosphor that converts some portion of the ultraviolet emissions into red light.
One of the original complaints against mercury-lights was they tended to make people look like "bloodless corpses" because of the lack of light from the red end of the spectrum.
There is also an increase in red color (e.g., due to the continuous radiation) in ultra-high-pressure mercury-vapor lamps (usually greater than 200 atm.
Low-pressure mercury-vapor lamps[16] usually have a quartz bulb in order to allow the transmission of short wavelength light.
[17] The 185 nm line will create ozone in an oxygen containing atmosphere, which helps in the cleaning process, but is also a health hazard.
For placements where light pollution is of prime importance (for example, an observatory parking lot), low-pressure sodium is preferred.
The US Department of Energy determined in 2015 that regulations proposed in 2010 for the mercury vapor type of HID lamps would not be implemented, because they would not yield substantial savings.
[20] The arctube of mercury lamps produces large amount of short wave UV-C radiation which can cause eye and skin burns.
However, care should be taken if the outer jacket of the lamp breaks, because the arctube would continue to operate, presenting a safety hazard.
As a result of the said documented cases, some American manufacturers made "safety" lamps that will deliberately burn out if the outer glass is broken.
They are enclosed and have protections to prevent human exposure as well as specialised exhaust systems to remove the ozone generated.
High-pressure mercury-vapor (and some specially-designed metal-halide) lamps find application in molecular spectroscopy due to providing useful broadband continuum ("noise") energy at millimeter and terahertz wavelengths, owing to the high electron temperature of the arc plasma; the main UV emission line of ionized mercury (254 nm) correlates to a blackbody of T= 11,500 K. This property makes them among the very few simple, inexpensive sources available for generating such frequencies.
As with the ultraviolet output, the glass outer bulb is largely opaque at these frequencies and thus for this purpose needs to be removed (or omitted in purpose-made lamps).
