Mercury Vapour Lamps
Mercury-Halide Lamps
Mercury became popular in discharge tubes early on as it produces a strong blue-green light. They also emit Ultra-Violet; this can be used to fluoresce compound adding other colours to the mercury discharge and creating a whiter light, as in the fluorescent tubes. Fluorecents use mercury at low pressure and were in common use in offices and home for many years. The first picture is an example of a germicidal UV light. The tube wall is uncoated and made form quartz. Quartz transmits UV light which is harmful but doesn't pass through the glass in a normal fluorescent.
The pictures on the left shows an early MA/V 250W medium-pressure mercury lamp. These lamps were succeeded with the high-pressure version once quartz arc tubes could be manufactured. Quartz was required due to the high running temperature of the arc. The higher vapour pressures broaden the spectrum and improve the light quality slightly. It also reduces the arc length for a given wattage.
The MB/U 250W lamps is electrically identical to the MA/V above and could be fitted into older fitting. Both these lamps have been obsolescent for quite a few years due to their poor colour rendering properties.
The addition of a fluorescent coating to the outer bulb forming the MBF/U lamp further improved the light output and reduce glare. The outer coating would convert UV to a red/orange to improve the colour. These became superseded with the introduction of metal Halide which offer improvements in both efficiency and colour.
Mercury Halide Lamps
By adding halides to the mercury arc the colour-temperature and colour can be controlled. This results is light output that can be close to that of tungsten. Although still a line spectrum, the red, blue and green activity can be balanced to trick the eye. This technique is so versatile coloured flood lighting can be produced.
The halide lamps shown are a 400W GE multi-vapor MRV400/U/40 white halide lamp and a 250W BLV Blue halide lamp below.
The spectral images below are for high pressure mercury lamps (MB/U) and (MBF/U) note the red spectral lines introduced by the fluorescent coated outer bulb. The lower spectra belong to a white metal halide lamp and a blue metal halide lamp. Notice many other lines are still present in the blue halide but the blue predominates greatly.
