I've search the forum and read it quite a lot but never get to answer this question: Why do the same Watt incan bulbs have different lumen output for different voltages? 230V 120V
Actually there is something to this.
An incandescent filament will have a different thickness and total length, depending on the wattage and intended life time. Of course, voltage will change these specifications.
Wattage, for any given voltage, is determined by resistance. A thicker shorter filament with have lower resistance. If the filament is designed to be thicker, to keep the wattage constant it must also be a longer length. This increases the lifetime for two reasons. The filament is thicker so the evaporation of the tungsten takes a longer time to cause a defect, and the filament also operates at a slightly lower temperature because the heat is being emitted over a wider area. The lower temperature also makes the light more orangish. Obviously, for any given wattage, a higher temperature filament will give off more light.
If we look at Ohm's law, current = voltage divided by resistance (in ohms).
And voltage multiplied by current = watts.
So another formula to express this is W = V
2 / R
(wattage equals voltage
squared divided by ohms)
You can use an ohm meter on a common incandescent light bulb and make a close approximation of the bulb's wattage, if you factor in the voltage it will run on when turned on. We also have to remember that a hot glowing tungsten filament has a resistance value about 15 times higher than a cold filament.
Filaments that run on lower voltages can be thicker, so they can have longer lifetimes, or stated in another way, they can operate at a higher temperature for any given lifetime.
This is why the higher color temperature halogen bulbs in museum galleries usually run on low voltage.
(there is of course a practical limit to how low the voltage can go, at some point, 6-12V, there will be considerable resistance loses in the wires used to bring the current to the filament, not to mention
voltage drop, and also with very short filaments there will begin to be significant conductive heat loss to the filament scaffold)
That being said, the design of the bulb has much to do with it also. In Europe, the 230 volt medium base replacement halogen bulbs are generally rated for 2000 hours, whereas in America the 120 volt replacement halogens are mostly rated for 1000 hours. Of course, the 1000 hour American bulbs put out significantly more light and are much more whitish.
