Radium MRL 160W/235/E27 Blended Mercury Lamp

My first blended mercury lamp!  Woohoo!  Okay, I guess that means that I'm a really strange person in that I was actually really rather excited when I got back and found the box which I knew contained this in my room.  Then proceeded to get polystyrene packing everywhere in my zeal to get it out of said box.  Had been waiting to get hold of one of these for a long while, and needless to say, all but jumped out of my seat when I spotted one at a sensible price.

Mercury blended lamps are an interesting little category all their own.  And I really didn't know whether to categorise this under incandescent or mercury vapour, but decided on the latter.  A blended mercury lamp is effectively a cross between these two types of lamp, taking advantage of the very different characteristics of these two technologies.

High pressure mercury lamps have a hugely long lifetime and relative to incandescent lamps are pretty efficient, especially when used in conjunction with a phosphor on the outer bulb.  However, the emission spectrum is highly deficient in red, and as with all discharge lamps, needs a ballast to limit current to the arc due to the fact that it has a negative impedance characteristic (as more current flows through it, it gets hotter, therefore conducts more).

Incandescent lamps are horribly inefficient compared to just about any other type of light source out there, but they have an output which is rich in red, and have an impedance which increases with filament temperature...meaning that they are self current limiting devices.  At some point in the past, someone had the bright idea to connect an incandescent filament in series with a mercury vapour lamp.  In this situation, the incandescent filament both adds to the red output of the lamp, and at the same time acts as a ballast for the mercury vapour lamp.  This idea can actually be traced back as far as 1902, where the Cooper-Hewitt Electric Company experimented with using carbon filament lamps in series with their low pressure mercury vapour lamp for these very reasons.

Modern blended lamps are somewhat more compact than this, having both the arc tube and the incandescent filament in the same outer envelope.  This means that the whole system is highly compact, as no external ballast unit or power factor correction capacitors are required.  The lamp is the same size as you would expect for any high pressure mercury (MB) lamp of the same wattage - hence can be used effectively with common types of optical systems.  Unfortunately, the relatively complex construction of these lamps have always meant that their price was relatively high, and in recent years, their production has been gradually scaling down, with several of the major companies having all but ceased in many cases producing these lamps.  Sufficient market volumes remain though, that they are fairly readily available from a number of smaller manufacturers though.

Part of the reason for their fall from favour was that high pressure mercury vapour lamps are relatively inefficient compared to other types of discharge lamps, which have continued to evolve, developing higher efficacies and higher CRI ratings, whereas these figures have all remained relatively stable for this type of lamp for a few years now.  The inclusion of an incandescent filament in the lamp also further decreases the efficacy, for two reasons.  Firstly is that the filament operates at relatively low temperatures when the lamp is up to temperature - therefore a huge percentage of its radiation is invisible infrared.  Secondly is that the gas filling of the outer bulb is less than ideal for the mercury arc tube.  The arc tube will operate at its best when operated in a vessel with a very low gas fill pressure, as this affords the best degree of thermal isolation.  Unfortunately, this is incompatible with the needs of the incandescent filament, which in this situation would evaporate extremely rapidly, leading to blackening of the outer bulb, and premature lamp failure.  The filament requires a considerably higher gas filling pressure, ideally with a heavy buffer gas such as argon (which is also incompatible with the arc tube, which requires nitrogen to be used, as it can withstand the strong electrical fields present without breaking down) has to be used.  As a result, nitrogen at a fairly high pressure is generally used in the outer jacket, this causes the efficacy of the arc tube to be reduced, but the filament to live a relatively long life.

Another way in which blended lamps have an advantage over their conventional ballast driven equivalents is that they give out a considerable amount of light as soon as power is applied, and continue to do so throughout their warm-up time.  There is however no hot re-strike facility, so you must wait for the lamp to cool down before it can be lit again if the power is interrupted during operation.

When the lamp is initially powered up, the vapour pressure in the arc tube is low, giving out very little light, and has a correspondingly low arc voltage.  As a result of the low voltage across the arc, it conducts a higher than normal current, nearly all of the light at this point being produced by the incandescent filament, which due to the high current it is passing at this stage is brightly lit - the lamp actually appearing just as a fairly bright incandescent lamp at this point.  In the case of the example shown here, the lamp is actually BRIGHTER when first turned on than it is when fully run up! (though it is also pulling considerably more power when first started up than when it is at full operating temperature as well).  As the temperature in the arc tube increases, the mercury vapour pressure increases, increasing both the voltage dropped across it, and the amount of light produced by the arc.  Due to the increasing voltage across the arc, the current flowing through it decreases as it warms up, causing the light output from the incandescent filament to decrease - this causes both a slight decrease in total light output (in this case), and a significant increase in colour temperature (from a starting point of around 2500K, increasing to 3600K when it has run up).

Radium MRL 160W/235/E27 Blended Mercury Lamp - General overview of lamp

Radium MRL 160W/235/E27 Blended Mercury Lamp - Detail of lamp cap

Radium MRL 160W/235/E27 Blended Mercury Lamp illuminating my workstation from a distance of approximately two metres

Radium MRL 160W/235/E27 Blended Mercury Lamp - Overview of lamp while alight

Radium MRL 160W/235/E27 Blended Mercury Lamp - Overview of lamp packaging

Radium MRL 160W/235/E27 Blended Mercury Lamp - Displayed with ruler to show size of the lamp

Radium MRL 160W/235/E27 Blended Mercury Lamp - Detail of lamp etch

This Lamp added to the Virtual Display Shelf on the 21st October 2005 at 23:42.

References: Datasheets from Manufacturer's Website.  Fellow Lighting collector James Hooker's Website - Lamptech.

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16th June 2023: Page formatting changes to improve readability on mobile devices, also made some background code changes to improve search engine behaviour.