Magical blue headlight, sans light
Trick bulb from ebay tested
Last year you wrote this fascinating article about the Tungsram lab and about lumens, which is why I am writing to you now.
The 'Angel Eyes' positioning lights on my BMW 1 feature H8 (yellowish tint) light bulbs. Newer models use LEDs which I like a lot more. For long now I have been thinking whether I should go for LED's or replace the factory installed H8's with highly cool tinted (8500K) incandescent bulbs. Since forum posts have indicated various errors and faults with the former I opted for incan which I managed to pick up from eBay for significantly less than an LED set would have cost. However I only re-read your aforementioned article after making the purchase.
Because increasing the lux value may result in higher incandescent temperatures I am wary to actually install the new bulbs in my headlamps lest I should fry something – these BMW headlamps cost a fortune.
Could you please recommend me someone who could determine the incandescent temperature of these two bulbs? I will send or deliver them, uninstalled, to anywhere you tell me to.
Looking forward to hearing from you
Well we did receive the bulbs – two of them in fact. One was the original one and the other the newly purchased ' GP Thunder' product, allowing us to do a direct comparison of them. The eBay-sourced light has lots of labelling: it says HID, is says Plasma, and also promises a colour temperature of 8500K. As we were to find out, none of this was true. HID means High Intensity Discharge, today known as Xenon. The word 'Plasma' again refers to something the bulb isn't: H8 bulbs use a glowing piece of tungsten (filament) instead of ionised gas (a.k.a. plasma) as a light source. As for the color temperature we need to measure that later on.
But before we do that let us define colour temperature. First of all, this is not the same as the temperature of the filament. Tungsten evaporates at around 3400-3500 degrees so it cannot possibly reach 8-10 000 degrees. The actual temperature of the filament in warmer tintet bulbs is around 2700 degrees.
Colour temperature refers to the composition (wavelength distribution) of the light. What the human eye perceives as white light is in fact a combination of multiple colours. We will still see it white even if there is a certain variation within these colours. Colour temperature refers to the approximate temperature a pitch black body would have to glow at in order to emit a light of similar wavelength composition. This is all theoretical of course since we rarely have such an object close at hand. The point is, daytime sunshine measures in at 5500-5600K, anything colder would nudge the light towards red, anything hotter towards blue.
For some reason customers have begun to favor high colour temperature lights, even though the colour temperature of contemporary xenon (HID) lights is very close to actual daylight. Those however who cannot afford xenon seem to have a fix on these 8-10 000K halogen lights. One reason can be that cooler tints (higher colour temperature) are perceived more powerful than warmer tints of the same output. This is pure psychology.
However you cannot generate such high colour temperatures by heating tungsten, as it is far beyond the melting point of the metal. Which is why manufacturers usually resort to applying a blue coating (colour filter) on the housing in order to achieve the desired high colour temperature.
This is exactly what happened in this case. Experts at the Tungsram lab were enthusiastic about the romantic find. Lacking the necessary E markings, this light cannot be legally used in public traffic – although police or even MOT personnel do not usually check to see if bulbs are E-approved or not.
But there are more serious issues here than that of legality. Tungsram's experts have found that the blue coating absorbs about two thirds of the light emitted, meaning the GP Thunder is nowhere near the legally prescribed level of 680-920 lumens for H8 bulbs. This, again, is irrelevant here since our reader wanted to use this bulb in his BMW Angel Eyes, but H4 and H7 lights are also manufactured using the same technology. Either way, the amount of light coming out of the GP is only about one third of a regular Philips lamp.
Here is the chart we received after the testing:
The light is significantly overdriven, just as development engineer Jozsef Gallai had predicted. Power uptake is almost 43W, as oposed to 35W of the originak Philips bulb, obviously a result of the manufacturer trying to compensate for the lumen loss. Imagine the output if it was'nt overdriven.
Looking at the chart it is obvious that the colour temperature of the bulb was far from the 8500K promised on the packaging. In fact it had more of a warm tint (lower colour temperature) instead of being blueish. Yet the colour temperature of the light easily fit within the ECE standard for white light.
And so we have come to answering our reader's question. Measuring the temperature of the filament is impossible because of the blue coating but it does not matter anyway. Jozsef Gallai says light developers know that the surface of the bulb can reach 800°C during use. This value varies from product to product. Tungsram's laboratory performed a comparative test, checking the temperature of the standalone H8 with a contact thermometer. The GP yileded 180°C at the centre of the bulb, and 330°C on the tip; while the Philips light measured 140 and 285°C. Higher tungsten temperature means lower life expectancy, although Mr Gallai says there values are far from dramatic. The main problem with the GP Thunder H8 it that is is awefully low on lumens. This does not mean you must never purchase bulbs with blue coating – but you do need to look for high quality products, preferably from major manufacturers because they are certain be correspond to legal standards.