Philips HPI Plus

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Standard Metal Hallide Lamp for indoor and outdoor lighting with 250W and 400W (E40 base), coated bulb


  • Diffuse internal elliptical coated bulb
  • Internal high pressure quartz discharge tube
  • E40 base
  • Color rendering index Ra of 69
  • Typical lifetime of 20.000 hours
  • Luminous efficacy of 72 to 82 lumens/watt
  • Cool white or daylight colour designation (4500 or 6700 K colour temperature)
  • Vertical or horizontal operating position

Lamp control gear

  • Requires external gear (similar to mercury vapour lamps) and appropriate wiring:
    • Philips BHL or compatible reactive inductive ballast with matching power rating
    • Philips SI or compatible parallel ignitor for 5000 Volt pulses (except "S" designated lamps)
    • May require power factor correction capacitor for balancing line load

Philips HPI Plus 250W

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 coll 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.

Exterior lighting is a good application as most gardens and outside areas are well showcased. The lamp themselves do release some heat meaning that closed fixtures are not well suited if not specifically built for this purpose. Care should be taken when using these metal-hallide lamps as they are sensitive to the operating position especially when inside enclosed fixtures. Improper heating of lamp can destroy it prematurely and present a safety hazard so special precaution should be exercised. While lamp warm up time is low, with the lamp achieving high output quite quickly, they are slow to restart, 10 minutes pauses not being uncommon.

While both high pressure mercury vapour and high pressure sodium gear and fixtures can be used, the lamp works closer to designed parameters when used with inductive ballasts for mercury vapour lamps and a suitable ignitor added on the circuit as required. Compact, boxy symmetric and asymmetric fixtures were popularly. Clear lamps are well suited to high efficiency reflector fixtures while coated lamps work better in open luminaries or classic mercury vapour fixtures.

With adequate care and good ventilation, HPI metal hallides can be used vertically, in a freestanding position as mercury vapour lamps but lifetime will be reduced. HPI-T lamps are much more sensitive to operating position. Lamps age differently and this means that a series of lamps may exhibit different colour intensity and light intensity depending on wear and group replacement should be considered to maintain uniformity. Just as well, lamps should be replaced as soon as light and colour uniformity is affected, as old lamps present a higher risk of catastrophic failure.

While products that have an internal ignitor are easier to operate, they are quite rare. Also, these lamps may be less reliable than standard ones that do not include internal ignitors since the ones being supplied are built to a lower quality standard and made to a set cost and space constraint.

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.

Technical details

Designation Base Lamp wattage Luminous flux Colour appearance Colour Temperature Colour Rendering Index (Ra8) Life (to 50% failures)
MASTER HPI Plus 250W/745 BU E40 CRP/SLV E40 250 W 18000 lm Cool White 4500 K 69 20000 hours
MASTER HPI Plus 250W/767 BU E40 CRP/SLV E40 250 W 18000 lm Daylight 6700 K 69 20000 hours
MASTER HPI Plus 250W/745 BU-P E40 CRP/SLV E40 250 W 18000 lm Cool White 4500 K 69 20000 hours
MASTER HPI Plus 250W/767 BU-P E40 CRP/SLV E40 250 W 18000 lm Daylight 6700 K 69 20000 hours
MASTER HPI Plus 400W/745 BU-P E40 CRP/SLV E40 400 W 32500 lm Cool White 4500 K 69 20000 hours
MASTER HPI Plus 400W/745 HOR E40 CRP E40 400 W 30000 lm Cool White 4500 K 69 20000 hours
MASTER HPI Plus 400W/745 BUS E40 CRP/SLV E40 400 W 30000 lm Cool White 4500 K 69 20000 hours
MASTER HPI Plus 400W/745 BUS-P E40 CRP E40 400 W 30000 lm Cool White 4500 K 69 20000 hours
MASTER HPI Plus 400W/767 BU E40 CRP/SLV E40 400 W 32500 lm Daylight 6700 K 69 20000 hours


  • "S" designation indicates lamps with integrated internal starter that do not require an external ignitor;
  • "P" means protected lamp, suitable for open air use;
  • "BU" means base up operation while "HOR" means horizontal operating position.

Durability and Repair-ability

Generally, sodium vapour lamps were very durable and most modern products were released with even longer typical lifetimes. This is mainly due to the fact that the main market of street lighting pressured producers to cot servicing costs which means that reliability improvements were mandatory. Unfortunately, due to the way that almost all lamps are designed, they age in a different way compared with other light sources. Sodium slowly permeates the ceramic arc tube meaning that the lamp voltage raises as the lamp ages, making it harder to remain in stable operation after startup. This creates the well known on-and-off lamp cylcles that repeats every couple of minutes, accelerating as the lamps is close to demise. Eventually, the lamp does not start but this happens after a long cycling and gear stress that might span over months or even a whole year.

Compared with modern LED lamps, sodium lamps compare favourably but they have a much poorer light output and require careful wiring. In most cases operational cost is lower as less powerful LED luminaries can be used since all useful light is directed downwards compared with the inefficient fixtures used on most sodium vapour lamps. Ballast and ignitors can survive tens of years if properly protected against direct rain or snow exposure. A complex circuitry means that lamp servicing is difficult and there are major risks due to improper wiring on servicing as large voltages may be present. 70, 100 and 150 watt lamps are very well built and they were highly popular. Ignitors can be potentially repaired but the lack of schematics means that they can be repaired only by experienced persons and are, thus, mostly discarded when not working.

Greatest features & flaws

Features Flaws
High light output in very compact package No lower wattage lamps available
High efficiency, long life, average colour rendering Quite expensive lamps
Mostly omni-directional light Sensitive to operating position and unbalanced wearing
Reduced lamp glare, quick lamp start Slow restart, requires bulky, heavy, complex lamp gear


These high pressure sodium vapour lamps are compact light sources that are viable for wide area lighting. Their poor colour reproduction and large output means that they are entirely suited to outdoor lighting or greenhouses as grow light. Compared with fluorescent lamps they were very compact, having great performance at low temperatures. A very long lifetime, that was around 16 to 25 times higher than tungsten lamps, and almost double that of some fluorescent lamps, meant that these lamps are well suited for use in places where lamp replacement was costly, as in high bay lighting, street lighting, tunnel lighting.

High pressure sodium lamps had higher light output, almost double, of mercury vapour lamps while having very low quality colour reproduction. This means that mercury vapour lamps were still suited for areas where colour reproduction was moderately important and there was still an advantage from compact light sources. However, street lighting was a very important market as the lamp's efficiency ensured cost savings in labour and electricity use, after the 1970s oil crisis. There were some advances in colour reproduction with different lamp technology but standard lamps were not favourite of industrial, workshop lighting commercial premises. The light spectra was, however, highly beneficial for plant growth.

The inherent flicker of mains voltage lamps is very low even with conventional gear due to the fact that a large amount of light happens due to thermal emission and balance of the temperature of internal arc tube. There were some electronic medium frequency gear similar with ones used on fluorescent lamps but they were very costly. However, sodium and metal hallide gears are generally compatible, meaning that most sodium lamps can be easily replaced with compatible wattage metal hallide lamps that have a much better colour appearance, sacrificing some energy efficiency due to different lamp technology. The requirement for ignitors along ballasts while it raises total cost and increases complexity, as well as raising the risk due to passing thousands of volts through some wires, means that fixtures have a clear advantage of maximum useful light and restart in case of power failure in almost one minute.

Fixtures are generally compact, considering the light output made available, but are generally heavy due to the bulky electromagnetic gear being used. There are directional and omni-directional fixtures that are suited to various purposes. In street lighting, the well known cobra-head style lamppost was used, and they may still be available for purchase. They were only moderately effective as light covered a large area but a considerable amount of light was lost due to the fixture directing some of it upwards, with no control. Omni-directional fixtures were used in residential areas and parks, where ambience was more important than efficiency. Indoor fixtures were centered on the typical high-bay or low-bay types due to their application in greenhouses where light has to be directed entirely downwards. Other fixtures were compact, having a boxy design, with a symmetrical or asymmetrical lamp placement that was suited for flood lighting. These type of lamps were used outdoor, for floodlighting. There were no home designed fixtures as these lamps were never used in home environments.

Nowadays, sodium vapour lamps are still very widely used as outdoor lighting. As the number of available fixtures is massive, availability is high and will diminish no early than 2030s, meaning they are economically effective to purchase except and they can be repurposed for mercury vapour of metal hallide lamps if proper lamp gear is used. Free standing lamps are quite capable on their own so if you need a compact light source that is highly durable and colour output is not important. They have the additional advantage of attracting less insects during the summer, if used as outdoor light. They are especially well suited for security lighting. Of special interest is greenhouse lighting, as was mentioned before, even if specialized lamps have also been designed for these purposes, the standard lamp is also appropriate.


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.


HPI Plus lamps are the best choice since they do not require specific fixtures for operation, their ovoid outer bulb allowing simple operation and less glare with possible operation as free standing, hanging, lamps. HPI-T lamps are much more effective for flood lighting but they require specific fixtures and have higher surface temperature, making them more susceptible to damage when improperly handled. Chinese branded lamps are much worse in all performance and quality metrics with shorter useful life, worse colour reproduction and and efficiency as a proper, stable mix of hallides required huge research investment that was too costly for Chinese producers.

The best choice are, probably, the 250 watt lamps as larger powered lamps are too powerful for most purposes. This lamp wattage was also cheaper as they were highly popular. Electro-magnetic ballasts or conventional gears are plentiful and can be used interchangeably with all manufactured lamps for a specific wattage and ignitors' performance is less critical as on high pressure sodium lamps. 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 market for large lamps was important as there was an interest in using more energy efficient lamps when the early 1970s oil crises brought new challenges. High pressure mercury vapour lamps were efficient but their light was not white while improvements in high pressure mercury vapour lamps were not satisfactory. While metal hallide lamps were more expensive than mercury vapour lamps, the energy savings that were around 25 to 50% meant that this technology was attractive enough considering much improved colour reproduction despite shorter lifetime. The ability to run on mercury vapour gear and the ability to retrofit these fixtures was critical to market success while the ability to run on high pressure sodium lamps ensured improvements on other installations. These are the most important factors that ensured market success as metal hallide lamps required careful installation, operation and restocking.

Of course, metal hallide lamps were always less successful than fluorescent lamps, especially when larger fixtures, especially indoors, were not an issue. Fluorescent lamps were easy to stock, replace, had a longer useful life and they offered custom light reproduction with great prospects for energy efficiency improvements. Lower power rating metal hallides were developed and marketed but they required specific fixtures that were able to withstand the possibility of a blown old arc tube and the high heat output and complex operating gear, along with worse flicker than fluorescent lamps. All this, along with a large price justified by complex manufacturing quality control, meant that fluorescent lamps were always more popular in most commercial and all residential applications.

LED lamps affected the most this expensive lamp and gear technology, as it quickly surpassed it in ease of use, much longer useful lifetimes, better colour reproduction and versatility in both indoor and outdoor applications. This means that the market for metal hallide lamps was quickly diminishing even quicker than high pressure sodium vapour or even the older mercury vapour lamps, both surviving longer due to lower lamp replacement costs.

Manufacturing specificities

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. Lamps produced in the 70s and early 80s are probably not commonly found today. In the 90s and early 2000 Philips was manufacturing lamps in Holland and other countries, while in the last years they were doing the same thing in China. Quality was not severely affected as manufacturing quality was maintained and applications were demanding, used in wealthier countries. It should be noted that there are larger differences between compatible lamps released by Osram, General Electric or other major manufacturers as each of them settled on slightly different chemistry with different reliability and performance.

Products manufactured by Philips were similar to General Electric and Osram, with a quality and consistency much above some East-European manufacturers in terms of colour rendering, light output and life. However, Philips was manufacturing lamps with shorter lifetimes and poorer colour reproduction than typical Osram ones, especially on standard metal hallide lamps, as Philips was focusing on cost and marketing. On the other side, Philips lamps were much more popular and are easier to find.