Incandescent light bulb: Encyclopedia II - Incandescent light bulb - Voltage light output and lifetime

Incandescent light bulb - Voltage light output and lifetime

Incandescent lamps are very sensitive to changes in the supply voltage. These characteristics are of great practical and economic importance. For a supply voltage V,

• Light output is approximately proportional to V^3.4
• Power consumption is approximately proportional to V^1.6
• Lifetime is approximately inversely proportional to V¹⁶
• Color temperature is approximately proportional to V^0.42

This means that 5% reduction in operating voltage will double the life of the bulb, at the expense of reducing its light output by 20%. This may be a very acceptable tradeoff for a light bulb that is a difficult-to-access location (for example, traffic lights or fixtures hung from high ceilings). So-called "long-life" bulbs are simply bulbs that take advantage of this tradeoff.

According to the relationships above (which are probably not accurate for such extreme departures from nominal ratings), operating a 100 watt, 1000 hour, 1700 lumen bulb at half voltage would extend its life to about 65,000,000 hours or over 7000 years – while reducing light output to 160 lumens, about the equivalent of a normal 15 watt bulb. The Guinness Book of World Records states that a fire station in Livermore, California has a light bulb that is said to have been burning continuously for over a century since 1901 (presumably apart from power outages). However, the bulb is powered by only 4 watts. A similar story can be told of a 40 watt bulb in Texas which has been illuminated since September 21, 1908. It once resided in an opera house where notable celebrities stopped to take in its glow, but is now in an area museum [2].

In photoflood bulbs used for photographic lighting, the tradeoff is made in the other direction. Compared to general service bulbs, for the same power, these bulbs produce far more light, and (more importantly) light at a higher colour temperature, at the expense of greatly reduced life (which may be as short as 2 hours for a type P1 lamp). The upper limit to the temperature at which metal incandescent bulbs can operate is the melting point of the metal. Tungsten is the metal with the highest melting point. A 50 hour life projection bulb, for instance, is designed to operate only 50 °C (90 °F) below that melting point.

Lamps also vary in the number of support wires used for the tungsten filament. Each additional support wire makes the filament mechanically stronger, but removes heat from the filament, creating another tradeoff between efficiency and long life. Many modern 120 volt lamps use no additional support wires, but lamps designed for "rough service" often have several support wires and lamps designed for "vibration service" may have as many as five. Lamps designed for low voltages (for example, 12 volts) generally have filaments made of much heavier wire and do not require any additional support wires.

Adapted from the Wikipedia article "Voltage light output and lifetime", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki