Osram HWL

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Standard Mixed Light Incandescent Tungsten and High Pressure Mercury Lamp with 100W, 160W, 250W (E27 base) and 250 and 500W (E40 base)


  • Yttrium Vanadate internal coated bulb
  • High pressure mercury vapour lamp with an internal quartz discharge tube
  • Integrated internal ballast featuring an incandescent coil (filament)
  • E27 base or E40 base
  • Soft glass (160 and 250W) or Hard glass (250 and 500W) bulb
  • Color rendering index Ra of 49 to 62 (highest on lowest wattage lamps)
  • 230V operating voltage (highly sensitive to voltage)
  • Typical lifetime of 10.000 hours
  • Luminous efficacy of 18 to 26 lumens/watt (highest on highest wattage lamps)
  • Neutral to cool white colour designation (3600 to 4100K colour temperature)
  • Vertical operating position (highly sensitive)

Lamp control gear

  • No lamp gear required, can be directly used on mains (230V standard on Europe):

Osram HWL 250W

Best retro purpose

Mercury vapour lamps were traditionally used on industrial and street lighting and they can be used on these areas although LED lamps offer much better performance with lower energy consumption. There are still some specific uses that are specific to this technology. Light appearance is cool white with a bias towards reds as well as blues, and, especially ultraviolet.

The HWL lamps is highly similar to standard HQL high pressure mercury vapour lamps. It can be used on indoor as well as outdoor lighting. However, there are some differences. Due to the way the lamp operates it is very well suited for indoor lighting, being quite well suited for living room lighting. The warm colour appearance and good colour reproduction does not detract too much from a typical fluorescent lamp. While it does highlight greens and blues, the light is very well balanced due to good orange and red radiation. Flickering is present but much reduced compared with high pressure discharge lamps. Garden and animal lighting is also possible with very good results, as well as indoor plant showcasing. In these applications the lamp's heat output and spectra can be used with good results although present LED lamp technology is much more energy efficient. The lamp is highly useful also during renovation work or as general lighting in attics, due to the light output and pleasant appearance.

Osram HWL mixed-light is highly user friendly as the lamp is very easy to use if you have the mains voltage required by the lamp. The product does not require any gear to work, being usable directly connected to the mains as any tungsten, compact fluorescent or led lamp with screw base. The only major precaution is avoiding direct water splashes and mounting positions that deviate too much from vertical base up or down positioning. An inappropriate lamp mounting is not dangerous but makes the lamp cycle stopping and restarting, especially in warm areas. Closed fixtures are not optimal as the lamp generates releases quite a lot of heat. Care should also be taken as the lamp's life is even more affected by frequent starts and strong vibrations, so continuous use is advised.

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)
ML 160W E27 225-235V SLV E27 160 W 3100 lm Neutral White 3600 K 62 10000 hours
ML 250W E27 225-235V SG SLV E27 250 W 5600 lm Cool White 3800 K 58 10000 hours
ML 250W E40 225-235V HG SLV E40 250 W 5600 lm Cool White 3800 K 58 10000 hours
ML 500W E40 225-235V HG SLV E40 500 W 14000 lm Cool White 4100 K 49 10000 hours

Note: "HG" means hard glass, "SG" means soft glass;

Durability and Repair-ability

Generally, mercury vapour lamps were very durable but mixed lights, mercury vapour and tungsten, are quite sensitive. Strong, sudden shocks can damage the internal tungsten filament, permanently affecting the lamp. On each start-up the lamp is highly stressed as the tungsten filament operates at almost double the normal rated current which means that service life is strongly affected by switch on frequency. The lamp is impossible to service if any rupture of the filament occurs.

All the above particular operating issues make the lamp slightly inconvenient to operate, even if the lamp itself is highly attractive to use and install and not an expensive purchase. Failing lamps can be easily determined by the lengthened time of restart in case of power being cut off and then resumed. A longer than typical time and less definite starting after cool-down, meaning more failed attempts at restarting than 1 or 2, point to a worn of lamp that will soon be dead. Of course, as mentioned before, this should be tested with the same straight vertical position. However, as lamps age, they will be more sensitive to voltage, accumulated heat nearby the lamp than newer ones so actual service life can be severely reduced compared with what the datasheet may suggest.

Greatest features & flaws

Features Flaws
Moderate light output, high quality white light Sensitive to switching
Very good for living room and garden lighting Not very powerful
Mostly omni-directional light Major heat output, low efficiency
Can be directly plugged to the mains Sensitive to operating voltage


These high pressure mercury vapour lamps are compact light sources that are viable for indoor as well as outdoor use. Good colour reproduction and a warm to neutral white colour appearance make these lamps suited for most applications except when efficiency or positioning limitations are taken into account. The lamp does not require any additional component, making the lamp very easy to install and operate.

Slightly major drawbacks are the lamp's sensitivity to positioning and operating voltage, along with slightly lower than average service life, especially compared with modern lighting technology. The lamp's low efficiency is caused by the compromise of its internal construction. The warm light generated by the internal filament also comes with massive heat radiation, which is similar to typical tungsten lamps that were previously popular. It should be noted that compared with Philips ML lamps, light output is uniform, as the tungsten filament is optimally placed around the mercury vapour arc tube.

The pleasant light is suited both as general lighting in homes as well as gardens or workshops. A good selection of lamps can be used if power consumption is not critical and a specific scene or effect is desirable. Unfortunately, especially in small spaces, the lamp can contribute to heating, which may be advantageous as well as undesirable, depending on application. Special care should be taken when cleaning the lamp as vibrations, dirt accumulation and improper handling can easily crack the bulb, especially on soft glass lamps. Any dirt present on the lamp may be trapped longer due to melting and subsequent adhesion to glass. This means that lamp cleaning has to be as thorough and careful as on old tungsten lamps. While the lamp has a very high output when started and slowly reduces on warm-up, with a corresponding change in light colour, it is not unpleasant. At the beginning, most light is output by the internal incandescent filament rather than the mercury discharge. Lamp restart in case of failure occurs in less than 2 minutes and is less unpleasant than on other high pressure discharge lamps.

This type of mercury vapour lamps were popular mostly before the oil crisis that occurred in the 1970s. Many years have passed since use of this lamp was basically non existent, meaning that stocks are not very large. Although the lamp could directly replace even less efficient tungsten or incandescent lamps, this market very rapidly diminished as soon as compact fluorescent lamps with screw bases were made available. These had a much longer life, had no restriction on mounting, were highly energy efficient and had even better colour reproduction. There were few fixtures specifically designed for Osram HWL lamps. They could operate on typical high pressure mercury vapour fixtures but this was uncommon as they most likely replaced incandescent lamps in their fixtures. There was a market for street-lighting in France, many decades ago, as the lamp offered a pleasant light and energy was quite cheap. Due to the lamp operating restrictions as well as simple use, free standing or hanging mounting were quite common, even if lighting efficiency was diminished.


These lamps are not well suited for frequent switching. They need a couple of minutes to reach maximum light output and they require a couple of minutes to restart after being turned off. The light colour and output changes from start-up to normal operation from a pink to white. A good lamp should have a strong pink light at startup, suggesting good internal coating performance. As the lamp ages, the coating degrades and light turns greenish.

The lamp requires a conventional or electro-magnetic gear of the same power output as the lamp, connected in series with a lamp. Connecting the lamp directly to mains damages it instantly as there is no current limitation. 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 influence 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.


The OSRAM HWL lamps have the best in class best useful lifetime, colour reproduction, light output with slightly better results than competing Philips ML lamps and other compatible lamps from major manufacturers. Chinese branded lamps are much worse in all performance and quality metrics with a greener light output and poor reproduction of oranges and reds. Lifetime is also reduced. It should be noted that the lamp's coating quality was not affected by the switch of manufacturing to China, maintaining consistent performance.

The best choice of wattage are the 160 and 250 watt lamps. Higher power rated lamps, such as the 500W were more efficient but the lamp's size clearly was a major disadvantage. In such situations a typical standard high pressure mercury vapour lamp with half power rating offered much better efficiency despite requiring a bulky electromagnetic ballast.


ML lamps are slightly adapted high pressure mercury vapour lamps. They were originally designed to address the requirement of better colour reproduction compared with a bare mercury vapour lamp. While they did severely improve red output of the installation, efficiency was also reduced, since this additional light was created less efficiently with the thermal radiation of the internal filament. Until the advent of suitable internal coatings on standard mercury vapour lamps, this mixed light lamp was also easy to use as there was no requirement for complex wiring, mounting and servicing of ballasts as well as lamps. As most light fixtures were using large wattage incandescent lamps, the Philips ML lamps were very easy to retrofit and for many decades this was a simple light upgrade. Of course, there were many improvements on lamp technology but the same benefits were kept along usability and lower maintenance due to less frequent lamp changing.

Internal coatings were introduced on the entire lamp range in the 1980s although they were present even as early as 1950s on these mixed light lamp types. Coating were useful to make the beam pattern more uniform and there were many benefits to having a coating as the two internal light sources had slightly different patterns. Of course, coatings were useful if flourescent powders were used, but this increased the lamp cost and provided small improvements as light absorption, wear had to be very low. Also, light improvement was much reduced compared with a typically sized mercury vapour lamp due to less UV light being generated by the discharge tube. The advent of improved discharge tubes that were made of quartz instead of glass improved efficiency but results were not as large as on standard mercury vapour lamps. Major efficiency increases were, however, centered on lower powered lamps, as they could not be have the required balance of pressure and temperature to achieve optimal results.

Although important, the introduction of internal coatings did not have the same impact as on standard mercury vapour lamps. For many years the lamp had to be specifically designed to accommodate filaments as well as the heat being released. The coating itself does not contribute much to the light output as the mercury vapour discharge tube is smaller and has less UV output than a typically sized lamp. As a matter of fact, for a typical power rating, around half the wattage is used by the filament and the other half by the mercury vapour discharge tube. This is mostly due to the compromise of achieving a better colour reproduction than a typical standard mercury vapour lamp and this required more useful light released by the incandescent coil. Of course, better efficiency can be achieved with filaments that do not run as hot and with more light output from the mercury vapour discharge but most longtime manufactured lamps, including the Philips ML, aimed at better comfort and had to accept less efficiency.

Manufacturing specificities

Philips has been releasing lamps for many decades. 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 severely affected as, realistically speaking, there were some occasional lamp defects even in their Holland produced lamps due to average manufacturing quality. It should be noted that mercury vapour lamps were mass manufactured and considered low-cost products, meaning that quality was not paramount.

Light uniformity on Philips ML lamps was also reduced by the preference for mounting the filament at the opposite end of the lamp base, rather than centrally around the discharge tube. This created a brownish, dirty lower appearance of the lamp, with pronounced yellowing and degradation of internal coating and less useful light directed downwards. The lack of uniformity also translated to lack of colour uniformity as the lamp's output was highly different at the center of the lamp compared with the tip. Overall, the Philips lamps were built very poorly with the only exception of the internal coating that was relatively decent, although far from the best.

Until the manufacturing plants move to China, there were small differences in this product line as the 90s and early 2000s was using the same technology. Chinese built lamps have some slight efficiency improvements that trade off on light quality. The internal coating is thinner and has a different grain size with a potentially different powder mix, improving colour rendering more than what the datasheet suggests. However, the light contribution from the internal filament is reduced, reducing consumption but also slightly increasing the perceived light appearance. The colour temperature seems higher and the light has a slightly greenish appearance, making the lamp much less pleasing than competing Osram HWL lamps.

Lamp lumen maintenance and uniformity is potentially improved due to a thinner coating and less heat being released inside the bulb, reducing coating degradation. However, due to the lamp even higher operating voltage, despite what the datasheet might suggest, the lamp is much quickly rendered useless, potentially prematurely, after only a few thousand hours, with unpleasant endless on-off cycles when powered. This behaviour is very common on the 160W Philips ML lamp as they were considered to be used in less critical installations as the higher rated 250 and 500W lamps. This means that the 160W lamp is clearly a very bad purchase, unless balanced by a very low purchasing cost. Template:Footnotes