I was looking for data about halogen reflectors I just bought. They are 50W, 230V.

On the package I read 320 lumen and that looked ridiculous to me, then I found this document by GE: Halogen Lamps Spectrum Catalog.

It states on page 100 that MR16 Start (normal reflectors, 12V) at 50W emit about 680 lumen, while MR16 GU10 (mains voltage) emit at 50W about 340 lumen.

Seriously? 7 lumen per watt?

Now I wonder why this big difference in the emitted light. Maybe because mains are sinusoidal and the lamp absorbs power even when the voltage is low, but maybe the temperature of the filament gets too low to emit properly. Still, this explanation seems odd to me, because thermal inertia should be enough to smooth the \$P=I^2\$ enough.

  • \$\begingroup\$ Because they built them so? There is always conflicting parameters like lifetime, power consumption and light output that have to be optimized and they found that the optimum is just there. \$\endgroup\$
    – PlasmaHH
    Oct 28, 2015 at 21:12
  • \$\begingroup\$ I think that the constraints of current and therefore filament thickness cannot be chosen freely, therefore 12V ones are intrinsically more efficient. See answer with findings. \$\endgroup\$
    – FarO
    Oct 28, 2015 at 21:19

1 Answer 1


I should have researched more before asking.

On this page it is explained: http://www.osram.com/osram_com/news-and-knowledge/halogen-lamps/professional-knowledge/high-voltage-and-low-voltage-lamps/index.jsp

The key factors are the physical properties determined by the laws of electricity. The coiled wire of a low voltage lamp is about five times as thick as that of high voltage lamp (mains voltage lamp) of the same intensity. However, the length of the low voltage coiled wire is just one fifth of the length of a comparable high voltage coil. The thicker, shorter coiled wire produced a considerably high thermal load of the low voltage coil. This leads to an increased luminous efficiency and a considerably longer service life.

Some more side effects of low voltage halogen are given here: http://wiki.diyfaq.org.uk/index.php/Halogen_Lighting#Low_Voltage_or_Mains_Halogen

The light will be whiter and have better colour rendition (note that some may find high power LV lamps too clinical in some circumstances)

The filament in the LV lamps is also thicker (since it runs at a higher current) and hence more robust. This gives the bulbs longer life and also makes the bulbs less vulnerable to vibration (which will quickly cause a short life for any bulb, but particularly mains halogens).

The transformers and control gear used for LV lamps also typically includes a "soft start" capability that increases lamp life further by reducing the thermal shock the bulb experiences at switch on from cold.

The transformer will also typically prevent the high current surges that often occur when a bulb blows from causing the circuit fuse or MCB to also trip (a frequent problem with mains halogens)

LV lamps are also available in dichroic versions which are rarely found in mains voltage halogens.


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