Best retro purpose
Metal Hallide lamps are still widely used on commercial, industrial and architectural lighting but in these applications LED lamps are quickly replacing them due to much better performance, lower energy consumption and easy servicing. Light appearance is cool white or daylight, depending on lamp model, with good colour reproduction and low ultraviolet emission. The lamp can be used indoors as well as outdoors, when intense lighting is required as most surrounding look quite pleasing being light with new lamps. With the most common gear, an electromagnetic ballast along with an ignitor, flicker is noticeable.
This type of single-ended lamps were used sometimes for lighting exterior areas, especially for higher powered lamps but the most frequent application was indoor accent lighting, especially for lower wattages. The lamp was relatively affordable (for a metal hallide lamp) and very compact, having a successful niche until LED lamps appeared. Since they are very compact light sources it should be noted that heat output is very high, making the lamp unsuitable for open-air operation, seriously limiting flexibility. The universal operating position is a major benefit even if the lamp life is relatively short. While warm up time is low, with the lamp achieving high output quite quickly, they are slow to restart, 15 minutes pauses not being uncommon. Only sodium vapour ballasts can be used if provided with a suitable ignitor.
Overall, CDM-T lamps are great for retro purposes as the lamp has an easy to use base and very good colour rendering. Although fixtures must be designed to house this type of lamp, the lamp can be free-standing due to the bi-pin base. Spare lamps should be considered if long-term use is required.
Understanding manufacturer data
Lamp light output is always measured in lumens. This is a way of measuring by averaging light output at a distance of 1 meter in an integrating sphere. This was carried out in lab environments and this information was mentioned in lamp datasheets and catalogues. While this information is accurate it should be considered only across similar lamp types.
Lamp life is presented in thousands of hours. It does not point at a specific moment when an installed lamp will not operate anymore but a statistical point at which some of the lamps may not operate, giving a rough estimate of useful life.
Ra8 colour rendering index, or simply colour rendering index is a way of expressing the typical colour rendering capability of a lamp. While it may be non-intuitive this is a computed average of brightness of certain coloured samples that are light by the lamp. The brighter they are, the more efficient is the lamp in this task. Most lamps do not have a continuous colour spectra so only some specific colours might look very bright and others look very dull. Colour samples are not intense reds, greens and blues but intermediately vivid colours that are focused on human skin colours and some fabrics or surfaces, meaning that a only very high colour rendering indexes are desirable for indoor home lighting. Sun light renders colours almost perfectly, having a value of 100 while typical lamps have a rendering index between 50 and 80, a good value being above 80 and a poor one below 50.
Colour temperature is another important detail. The value is presented in Kelvins and follows a theory that boils down to the fact that light can be produced by heating a metal up to a specific temperature. A camp fire releases light as the flame reaches around 1500-2000 degrees Celsius and a slightly higher value expressed in Kelvin. The designation is warm white for a value of around 2700 Kelvin, natural white with a value of around 3200-3500 Kelvin, cool white for a value of around 4000-4500 Kelvin, daylight for a value between 5000 and 5500 Kelvin, and cool daylight for a value above 6000-6500 Kelvin. There are cultural preferences that make some warm colour temperatures preferred in colder climates and cool colour temperatures in warmer climates. The most popular home lighting worldwide is mostly warm white, due to a comfortable, pleasant atmosphere that is close to the old incandescent lamp.
|Designation||Base||Lamp wattage||Luminous flux||Colour appearance||Colour Temperature||Colour Rendering Index (Ra8)||Life (to 50% failures)|
|CDM-TD 35/830 RX7s 1CT||G12||35 W||3300 lm||Warm White||3000 K||81||12000 hours|
|CDM-TD 35W/942 RX7s 1CT||G12||35 W||3300 lm||Cool White||4200 K||90||12000 hours|
|CDM-TD 70W/830 RX7s 1CT||G12||70 W||6600 lm||Warm White||3000 K||84||12000 hours|
|CDM-TD 70W/942 RX7s 1CT||G12||70 W||6600 lm||Cool White||4200 K||92||12000 hours|
|CDM-TD 150W/830 RX7s 1CT||G12||150 W||14000 lm||Warm White||3000 K||85||12000 hours|
|CDM-TD 150W/942 RX7s 1CT||G12||150 W||12700 lm||Cool White||4200 K||96||12000 hours|
|CDM-TD 250W/830 RX7s 1CT||G12||250 W||23000 lm||Warm White||3000 K||89||12000 hours|
|CDM-TD 250W/942 RX7s 1CT||G12||250 W||23000 lm||Cool White||4200 K||94||12000 hours|
Durability and Repair-ability
Generally, metal hallide lamps fare quite poorly in terms of reliability. The aggressiveness of metal hallides and their reliance on a specific pressure and temperature requirement for vaporization and light output with a specific output, means that lamp to lamp colour shifts are quite noticeable. Aging amplifies the colour drift and light output differences. As the lamp is very compact and has high temperature hot-spots means that fixtures are noticeably degraded and especially affects gaskets and the front frame of the reflector housing. This means that waterproofing is frequently degraded and lamps are rarely suited to operate outdoors unless old fixtures are carefully maintained.
Due to the lamp being compact as well as frequent indoor applications heat degradation of the lamp gear is quite common and some design constraints may limit replacement of some ballasts and ignitors without modifying mounting points. Electromagnetic and electronic gears have been available during manufacturing but electromagnetic ballasts and ignitors were the most common ones due to lower cost. This means that special care should be taken for proper insulation and wiring and that the lamp life is slightly reduced.
Greatest features & flaws
|High light output in very compact package||Hot operating fixtures, maintenance intensive|
|High efficiency, very good colour rendering||Poor lamp life|
|Directional light||Sensitive to unbalanced wearing|
|Quick lamp start||Slow restart, requires complex lamp gear|
These metal hallide lamps are well suited for indoor lighting due to their moderately high power rating and good colour reproduction. Although outdoor lighting is entirely possible, especially for higher power rated lamps, it is quite difficult to achieve. Large wattage fixtures were never as popular as those for the typical E40 base lamps, meaning that finding and using the 150 and 250W lamps is not attractive. The lamp itself fares well in terms of light output but the point light source nature of the lamp relegates it to accent or directional rather than general lighting.
The lamp's performance is mostly mediocre, if we consider price and technology. Much better products having longer life and better colour reproduction were manufactured, such as the ceramic arc tube Philips CDM range, meaning that MHN-TD lamps are a poor choice.
These lamps are somewhat better suited for frequent switching but starting is still a major stress. They need less than two minutes to reach maximum light output and less than that to restart after being turned off. The light colour and output changes from start-up to normal operation from white to orange. White light at startup and then greenish light midway to warm-up suggests mercury is also used on the lamp. This slightly improves colour rendering but affects slightly reduces lamp life while the ones that do no change colour are either new or do not use mercury at all have a yellower light appearance. Lamps without mercury are more difficult to start but they have longer lifetimes and slightly higher efficiency.
The lamp requires a conventional or electro-magnetic gear of the same power output as the lamp, connected in series with a lamp and an ignitor that must be connected as is required in the circuit. Connecting the lamp directly to mains damages it instantly as there is no current limitation. Improper connection of the ignitor is also dangerous and proper care must be exerted during installation and servicing. A power factor capacitor that has a specific value might be used to correct the power factor and current requirements on the circuit. The lamp itself is not influenced by the presence or absence of this capacitor. When used sparingly in homes or some premises where not a lot of these lamps are located, power factor correction is not important but large industrial or commercial clients were required to maintain a specific power factor due to electricity distribution contracting.
MHN-TD lamps are a good choice for directional lamp fixtures. If a fixture in good enough condition is found, the lamp offers good light output that could make it useful on interior as well as exterior applications. The most popular lamp wattage seems to be the 70W type, as it was powerful and compact enough to justify the purchase of a fixture that has an elliptical light beam. Garden lighting is not the best application choice as the lamps are not as suited as the 250W and 400W types offer much higher outputs. Indoor lighting is also limited since use in living rooms is not entirely attractive as flicker is present with conventional lamp gears and the lack of hot or quick restart and average lamp performance does not justify the much costlier electronic lamp gear.
Lower rated power output gears can be used, but it is not recommended as colour reproduction and lamp lifetime will be reduced while higher powered ones damage the lamp due to over-stressing and is highly dangerous. Used lamp gears might work but old lamps are never desirable as they degrade much quicker than mercury vapour and sodium lamps. Rusted ballast may still operate but are a potential hazard while old, used lamps, may not have much useful life left and they may already exhibit some colour light emission issues. New old-stock lamps are the best choice, if available.
The lamp technology was an improvement of previously developed mercury vapour lamps. The potential for fluorescent powders was mostly exhausted by early 70s which meant that research concentrated on releasing better lamps that produced light directly from the discharge tube. This research was also carried previously but results were modest, as stable compounds with good colour reproduction were found to be quite aggressive and lead to short service life. Sodium, Scandium, Indium, Thallium were attempted as main elements that can offer a balanced light output but they required a specific balance of temperature and pressure to achieve best results. One major limitation was that certain compounds were costly or difficult to manufacture or mix in specific amounts, which meant that suitable results could not be found for many decades. In the late 70s and early 80s most major manufactures were able to center on specific technologies that reached good enough results for general lighting, with predictable performance.
The MHN-TD range was the result of research aimed at creating lower powered metal hallide lamps that had better colour reproduction and energy efficiency compared with high pressure mercury vapour lamps. The most important limiting factor was that the arc tube had to be formed more evenly at the ends and be reasonably well insulated and heat resistant to ensure reasonable performance. More advanced manufacturing tools and processes had to be developed. As soon as they were available and the arc tube was well built to resist the long term corrosive nature of hallides, products were ready for release. It was also important to ensure less variations in arc tube geometry as well as temperature profile. This meant that heat reflecting coatings were mandatory for the arc tube. As soon as all these conditions were met, the lamp was released. Products were quite successful on the market until the advent of the much improved ceramic tube metal hallide lamps, around mid 1990s. Although Philips CDM lamps were much more expensive, their better colour reproduction, lamp life and efficiency meant that there was no reason other than cost to choose MHN lamps.
The only improvement on MHN-TD lamps focused on slightly improved colour reproduction which required higher operating pressure and slightly changed chemistry to ensure stability in this environment to reach color rendering indexes Ra8 of more than 80. Philips was not interested to invest more in quartz metal hallide technology so other world manufacturers like General Electric, Osram and others quickly surpassed it with better performing products so Philips competed only mostly on cost and ensured its market-share with advertising. Even before the advent of LEDs MHN-TD lamp installations were pointless so sales were mostly based on the large install base created in the 1990s.
Most manufacturers have been releasing lamps for many decades and constantly improved technology so that lamps had a stabler, predictable lifetime and slightly improved colour stability due to manufacturing progress. This lamp technology was almost unchanged since introduction with the only exception of moderate colour rendering improvements so other lamps, even compatible with it, from Osram, General Electric and others, were a much better choice. As Philips was focusing on cost and marketing MHN-TD lamps were much more popular and are easier to find.