Mercury (Hg)

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Low pressure Mercury

We all know them – love them or hate them, they have been around for decades, first appearing in England in the late 1930s-early 1940’s illuminating the London underground tube system –the Fluorescent Lamp. Named because of their phosphor coating applied to the inside of the discharge tube that transforms the energy emitted by the ionised Mercury vapour to an array of shades, colours and even different wave lengths of ultra-violet radiation ( e.g. blacklight lamps), depending on the composite of the phosphor coating. There are generally two classes of phosphor –halophosphate and modern tri-phosphor.Used in our homes, offices, schools, hospitals, factories, retail and just about anywhere else that requires bright, long lasting, low cost and uniform light. It’s easy to understand their popularity.Fluorescent lamps are not the only low pressure mercury lamps in existence. The thing with mercury is that it’s a rich source of ultra-violet radiation and this can be invaluable in some applications. The commonest form is the UVC germicidal lamp that is in essence a regular fluorescent lamp fabricated from quartz to allow the UVC to pass easily and lacking an internal phosphor. These lamps are used more that what you would first think for the simple reason, the dangerous short wave radiation produced when these lamps are energised means they have to be screened from direct view. They are found in water purifiers for ponds and aquariums, water treatment plants, air purifiers and conditioners.  They even find home in such places as laboratories, hospitals, mortuaries, vetinaries and beauty salons to sterilize surfaces where chemical solutions would not be viable and in special appliances to sterilize sponges, medical instruments and specimen containers.  They even find a place in geology and stamp collecting where they are used to fluoresce dyes and elements that long wave radiation produced by the safer ‘BLB’ lamp doesn’t excite.

fluorescent lamps

Compact Fluorescent

UVC Low Pressure Mercury

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Medium pressure mercury.

Also known as the MA lamp. These lamps really found their home in the factory lighting bracket where their high efficiency was more desirable to that of high wattage incandescent lamps and the cold greenish-blue light emitted from these lamps was of no great concern. In this application lamps could be opperated vertically with the lamp cap above the discharge vessel utilizing large dome reflectors to push the light downwards to where it was required. These lamps were also employed in street lighting –but there was one major problem, the discharge vessel of these lamps is fabricated from aluminosilicate glass. When lamps are operated horizontally the arc path naturally bows upward due to thermal convection leading to the glass envelope overheating, softening and eventually breaking. To combat this problem an electro-magnetic coil was installed in the luminaire above the arc path to prevent the arc from bowing to an unsafe state. The advent of the high pressure quartz lamp that had the benefits of being able to operate in any orientation, whiter light (although still with a strong greenish-blue cast) and greater efficiency meant the MA lamp was under threat. They remained popular for some time owing to the fact glass is cheaper than quartz and easier to manufacture. When the price of quartz fell and manufacturing processes advanced its production ceased in favour of the high pressure lamp.


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High pressure mercury.

Once one of the most popular forms of high intensity discharge lamp. Low wattage lamps were first created typically 125w then later 80 and 50 watt lamps were launched on the market. Once the cost of quartz dropped lamps started to be manufactured in the higher wattages 250, 400, 700 and 1000w replacing the medium pressure lamps as seen in the last chapter. Now high pressure mercury lamps lighting roads and streets have almost totally been replaced with the more efficient high pressure sodium and metal halide lamps. Their cool white light, simple control gear comprising of a ballast and capacitor for power factor correction and long service life means that high pressure mercury lamps still hold place for lighting in parks, sign lighting and residential lighting. These lamps are also frequently use to light planted freshwater aquaria (MBF-R lamps for example in the osram ‘floraset’ luminaire) and in zoo’s for reptile care owing to the UVA and UVB output. It’s also interesting to note that in Asia, Europe and America mercury lighting is very common – even in home lighting for illuminating gardens and yards.


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