Blended mercury vapour lamps are an interesting technological development to see. The so called "blended" mercury lamp is an example of a hybrid of two very different lighting technologies. Mercury vapour lamps have a number of advantages, especially compared to the high wattage incandescent lamps they were intended to replace at their launch. They were not however perfect; the first obvious thing to anyone who encounters a "traditional" mercury vapour lamp is that the colour rendering is rather awful, and that the light produced is far from a pure white. Medium pressure (MA) lamps in particular having a distinctly greenish tint to the light. Also, the lamp needs a ballast to operate. This equipment is relatively cheap and simple, but it's still money that has to be spent when setting up a new lighting system.
Later lamps attacked the colour rendering problem by the use of phosphor coatings on the outer bulb, which can provide a good deal of colour correction, and make use of the naturally produced UV radiation from the discharge, increasing the efficacy of the lamp.
Another method of colour correction was thought up though, its original form being seen long, long before phosphors were thought of for this use. The Cooper-Hewitt Electric Company experimented with connecting carbon filament lamps in series with their mercury vapour discharge lamps. These had the effect of both ballasting the arc due to the positive impedance characteristic of incandescent lamps, and also produced a dim light - rich in the red wavelengths which the mercury spectrum is almost entirely devoid of.
In the late 1930s, the first lamps were released by Philips which featured a tungsten filament built into the outer jacket of a mercury vapour lamp. Other lighting companies also adopted the idea quickly. The letter 'T' was added after the MA or MB prefix for the lamps, indicating the tungsten filament. Later lamps made use of a combination of both the incandescent filament and phosphor coatings, allowing lamps with quite acceptable colour characteristics to be created. However, all was not perfect.
In a conventional mercury vapour lamp, the outer jacket contains a hard vacuum. This provides significant thermal insulation for the discharge vessel, allowing the vapour pressure to increase, and the efficacy of the lamp to be high. Unfortunately, this arrangement is unsuitable for the incandescent filament in a blended lamp, which ideally requires a relatively high gas filling pressure to prevent the filament material from evaporating excessively rapidly. Additionally, nitrogen must be used as a fill gas as it is one of very few which will not break down in the areas of the lamp where strong electrical fields are present. While this works well in terms of making a lamp which operates, and has a long lifetime, the lower amount of thermal isolation offered for the arc tube does have a negative effect on the efficacy of the lamp. This compromise leads to efficacies of 15-18Lm/W - worse than can be achieved by many modern incandescent lamps!
Problems are there to be solved though, and the compromise seen in the gas fill used in blended mercury lamps is not an impossible one to solve. In the early 1960s, Philips successfully demonstrated a blended lamp which made use of a linear halogen incandescent lamp as the ballast. Because this lamp has the filament sealed in an envelope of its own, it allows the outer of the main lamp to be set up in a way most suitable for the arc tube, without having any degenerative effect on the filament. This allows both the efficacy of the lamp as a whole to be increased (this design capable of 25-30Lm/W), and the colour rendering to be further improved. Unfortunately, the idea was brushed aside at the time, as halogen lamps were a relatively new invention, and as a result a lamp making use of such a technology would have been prohibitively expensive. The idea has however started to be taken up again by a number of Chinese manufacturers. An example of one of these new lamps can be found at the excellent Lamptech website.
As an interesting side note, the clock faces of Big Ben in London until the last few years, were lit using blended mercury lamps for many years. Recently however they have been replaced by induction lamps (Philips QL series lamps to be precise).
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