Does a 12v bulb rated 100 watt give the same amount of light as a 120v bulb rated 100 watt?
In first order: yes. Differences in efficiency can spoil that a bit.
If you want the amount of light, you should be looking at lumens, not watts,
I would say much more than 'a bit'. A tungsten filament with the appropriate resistance for 12V operation is much more chunky and basic 12V filament lamps can be run much hotter for a given Power dissipation when a (skinny) 230V filament wouldn't last long enough at that temperature. A few degrees higher operating temperature makes a significant difference to the amount of visible light. The 25W 12V lamps that were used in motor cars before the Halogen versions used to put out a lot more than the feeble 25W mains bulbs. Since halogen lamps came along, things are a bit different because high voltage filaments last better when run at higher temperature.
I'm with Vanadium 50. There is no conversion between watts and lumens. Any load of 1.44 ohms will dissipate 100 watts with a 12 volt source whether it is a resistor on a circuit board, a 12 volt heater or a 100 watt bulb.
Yes the light emitted depends upon the temperature, which can be anything from Room temperature to 6000K.
It's certainly not true for halogen down light bulbs like these..
Here in the UK a 12V 50W halogen (MR16 base) can emit 900-1000 Lumens whereas a 230V 50W Halogen (GU10 base) is more like 500-600 Lumens. Almost half as bright. This is why 230V halogen down lights should be installed closer together than 12V halogen down lights.
This has caused issues when people have switched to LEDs. Some advertisers claimed their LED down lights were "equivalent" to a 50W down light without specifying that they mean a 230V 50W Halogen not a 12V 50W Halogen. This has led to some people being disappointed when they replace their 12V 50W Halogen bulbs with LEDs.
Fortunately the situation has improved a lot in the last two years as LEDs have got brighter. My house was wired for 230V down lights and I've fitted LED bulbs throughout. I look for LEDs that produce at least 450 Lumens. Typically these will consume anywhere from 4 to 6 Watts depending on the technology. Obviously I go for the lower wattage version if the both produce the same Lumen output. Personally I prefer "warm white" over "cool white" but each to their own.
PS Beam angle may also be important. If you have a matrix of these down lights in your kitchen then narrow angle bulbs tend to create more obvious shadows but less glare than bulbs with a wider beam angle. I prefer the wider beam angle.
This is true between 230V and 120V bulbs - ref:
However, I think this hits diminishing returns at or near 12 V. The filament becomes so short, that a larger portion of it is close to the metal supports/conductors. These draw heat away from the filament, reducing lumens/watt. Have not found a source yet that illustrates at what voltage we would see a cross-over. Will keep looking, or maybe someone will beat me to it.
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