DIY Frequently Asked Questions / Incandescent Bulbs')">
Produce light by passing current through a thin coil of wire called a filament. As the wire heats, it becomes white hot and emits visible light. Vacuum filled.
Use for general and task lighting around the house.
Wattage indicates the amount of electric power used by a bulb to produce light. Generally, the greater the wattage, the greater the light output. Some bulbs produce more light per watt than others.
Standard household bulbs have an average life of 750 to 1,000 hours, which can be lengthened or shortened by the treatment they receive. For example, a 120V bulb operating on a 125V circuit may produce more light but won’t last as long as one on a 120V circuit.
Underwriters Laboratories (UL) tags on fixtures designate the maximum allowable wattage. A mercury or fluorescent system is typically limited to a single bulb size.
Long-life bulbs may last longer than ordinary bulbs, because they have heavier filaments that do not burn out as quickly. However, these bulbs do not produce as much light as standard bulbs.
Manufacturers disclose average light output (in lumens) and average bulb life (in hours) on package labeling.
The filament vaporizes as a result of current flowing through it, and generally, the bulb “burns out.” Three-way bulbs lose two light levels when one filament burns out because two filaments are used in the bulb separately for two of the levels and together for the third.
Bulb designations denote size and shape. The figure following the bulb shape letter designation is the bulb’s maximum diameter in eighths of an inch. Thus, A-19 would mean an A-shaped bulb with a diameter of 19 x 1/8″, or 2-3/8″.
Most standard bulb envelopes are made of lime glass, but bulbs that must withstand greater heat are made of harder, heat-resistant glass. Hard-glass envelopes are used in many high-wattage bulbs and in bulbs recommended for outdoor use where there is danger of thermal shock from condensation.