Xenon Bulb Measurements

By David Harrison <harrisonfamily@worldnet.att.net>

Today I tested 2 Osram H4 bulbs, one a BILUX lamp that came in a Sylvania blister pack and the other a BILUX SUP that came in an Osram box claiming 30% more light. Both bulbs were new and had not been previously run prior to these tests.

The bulbs were compared by clamping them one at a time in a fixture with indexing marks to assure positioning repeatability better than 0.1 inches. The fixture was surrounded by a crude integrating sphere (a cardboard box) which did not change during the sequence. The lamps were viewed by a Tektronix J6502 radiometer about 22 inches from the lamp filaments. The lamps were positioned such that the radiometer viewed the filaments directly and was not in the low beam reflector pattern. To eliminate color differences, the radiometer was equipped with 2 different bandpass filters. One was centered at 511 nm (blue-green) and the other at 571 nm (green). I would have preferred to use a 540 nm filter, centered between photopic (day) and scotopic (night) vision peak sensitivities, but could not locate one within the bootleg constraints of doing this at break time at work (the official story). I think the filters chosen do give enough information. All measurements were repeated several times and the setup was reconfigured twice with no significant (more than 1%) variations in the measured data. I have good confidence in the data. The wiring was the same for both lamps as they were done singly.

The lamps were powered at 12.44 V and measured for current, low beam optical power and high beam optical power coming directly from the filaments. The energy measurements were in Watts/square meter, but the system was uncalibrated and the measurements are useful only as relative numbers, i.e. 0.33 is 10% brighter than 0.30. The meter scale was changed during some measurements, but relative brightnesses are preserved.

Now the data:

LOW BEAM  Standard   Super
Current    4.38 A    4.19 A
Power In   54.5 W    52.1 W
511 nm     0.29      0.27
571 nm     0.31      0.33

HIGH BEAM  Standard  Super
Current    5.20 A    4.91 A
Power In   64.7 W    61.1 W
511 nm     0.33      0.30
571 nm     0.40      0.37

The obvious conclusion is that the Super (Xenon-filled bulb) was no better than the standard bulb. If I had to guess, I would guess that the Standard was a Krypton-filled bulb and that the Super was actually filled with Argon. The literature claims that a Krypton-filled bulb should be 10% brighter than an Argon-filled bulb and that a Xenon-filled bulb should be 30% brighter. The measured data is inconsistent with a Xenon-filled bulb. The base of the Super was clearly stamped SUP, the designation Osram gives for Xenon-filled bulbs.

(A boring aside - the brightness depends upon the temperature of the filament, a hotter filament burns brighter. Normally a hotter filament has a shorter lifetime. Edison noted that an Argon backfill on lightbulbs makes them last longer. Apparently the Argon atoms 'reflect' tungsten boiloff back to the filament. Atoms with more mass (higher atomic numbers) reflect better than lower mass ones. Argon's atomic number is 39.944, Krypton's is 83.80 and Xenon's is 131.3, hence, Xenon is the best choice. This has little to do with the halogen cycle, which carries tungsten from the outer glass envelope back to the filament. A properly working halogen cycle lamp should have no darkening over its lifetime. If you have darkening, either your system voltage is too low or you wiring has too high resistance. Most modern lamps use Bromine as the halogen.)

Caveat - This test was done with just one sample of each type of bulb. Another test might be different.

My conclusion is that it does not make any difference which bulb you use, a standard H4 BILUX or an H4 BILUX SUPER. Maybe Osram has been sneaking Xenon-filled bulbs into the standard line all along and what I saw was a sample-to-sample difference. I could see no obvious differences between the filaments or construction of the 2 lamps I had, certainly no filament tightness or size differences, same length, diameter, number of turns.

Sorry that this does not make a good case for the BILUX SUP; I was as surprised as many of you may be.

Dave Harrison

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