Incandescent
Incandescent bulbs create light by passing electricity through a
tiny wire called a filament. As the filament heats up, it begins to
glow, giving off the 'warm' orange-colored light we have all become
familiar with. The higher the wattage passed across the filament, the
brighter the light becomes.
An incandescent bulb gets hot because the tungsten filament heats to
about 2500° C. At that temperature, the thermal radiation
that the filament emits includes a substantial amount of visible light.
But the filament also emits a great deal of infrared light (heat light)
and it also transfers heat via conduction and convection to the glass
bulb around it. When you put your hand near the bulb, you feel both the
infrared light and the heat that has worked its way to the surface of
the bulb.
Manufacturers can change the color of light coming from an incandescent
bulb by surrounding the filament with different gases, as is the case
with halogen bulbs, or by coating the inside of the globe surrounding
the filament with a variety of phosphors that produce different shades
of light. Reading lamps or 'soft-white' bulbs are examples of light
bulbs that use different phosphors to create different shades of
light.
- One 100-watt incandescent light bulb operating 6 hours a day costs
$1.30 per month in electricity.
- An 18-watt compact fluorescent lamp operating for the same time costs 22¢ a month.
