When was the incandescent light bulb invented?
The first electric lamps were demonstrated in 1801 by Sir Humphrey Davy. But it was not until 1879 that Thomas Edison patented the incandescent electric light. The Edison bulb allowed electric lighting to become a commercial success.

How does an incandescent light bulb work?
Incandescent lamps produce light by heating a filament. As the temperature of the filament increases, more visible light is produced. However, high temperatures also have an impact on the life of the filament. Filament materials are chosen for high melting point, low vapor pressure and high strength among other characteristics. Early incandescent lamps used carbon, osmium and tantalum filaments. Eventually, the industry discovered that tungsten provided the optimal set of characteristics for generating visible light and sustaining a reasonable life.

How efficient is an incandescent light bulb?
Most of the electric energy going into an incandescent light bulb generates heat, not visible light. The efficacy of an incandescent light bulb is less than 20 lumens per watt. By comparison, the efficacy of a compact fluroescent light bulb is over 80 lumens per watt.

What is the difference between an incandescent and a halogen light bulb?
Halogen bulbs are technically incandescent light bulbs - illumination is produced in both when a tungsten filament is heated sufficiently to emit light or "incandescence." The difference between the two is in the composition of the glass envelope and the gas inside the envelope. A standard incandescent bulb has a heat sensitive glass envelope that contains an inert gas mixture, usually nitrogen-argon. When the tungsten filament is heated it evaporates and deposits metal on the cooler glass envelope (this is why incandescent bulbs appear black at the end of life). This process requires incandescent bulb filaments to be heated less than optimally to give the bulb a reasonable life. The lower filament temperature gives incandescent bulbs their typical orange-yellow, warm appearing light.

Halogen light bulbs utilize a fused quartz envelope ("capsule") allowing for higher temperatures. Inside the quartz envelope is a vapor, originally iodine, now usually bromine. The tungsten filament evaporates as usual but the higher temperatures are sufficient to cause the tungsten to mix with the vapor instead of depositing on the envelope. Some of the evaporated tungsten is re-deposited on the filament. The combination of this "regenerative cycle" and higher filament temperature results in a bulb that has a longer life and slightly higher efficiency than standard incandescent bulbs. The higher temperature filament also produces the "white" light often associated with halogen bulbs.