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Do different watt light bulbs produce different amount of heat?

Find out if different watt light bulbs produce different amounts of heat.

Fluorescent light bulbs are more energy efficient and produce less heat. [©Jupiter Images, 2009]
©Jupiter Images, 2009
Fluorescent light bulbs are more energy efficient and produce less heat.

Different watt light bulbs produce different amounts of heat, which can affect electricity bills, the ambient temperature of a room and the safety around young children who might get burned. The amount of heat also varies depending on what type of light bulb it is -- incandescent, halogen (a type of incandescent) or fluorescent. Some types of bulbs, such as those used in brooding boxes, incubators and reptile tanks, are specifically designed to produce heat. However, light bulbs for lighting are more energy efficient if they produce less heat.

How Incandescent Light Bulbs Work

An incandescent bulb is the oldest and most widely used bulb in homes, cars, appliances and flashlights. This type of bulb contains a thin wire, called a filament, which is surrounded by a vacuum or inert gas. The filament acts as a resistor when electricity is applied to the bulb, and the temperature of the filament increases until it radiates heat at the same rate that the electricity is converted into heat. Therefore, an incandescent light bulb's heat is proportional to the power it consumes.

Lighting Efficiency of a Light Bulb

According to the U.S. Department of Energy, lighting efficiency is typically defined as the total amount of light emitted by the bulb divided by the amount of power consumed by the bulb. The standard unit of light (luminous flux) is the lumen and the standard unit of power is the watt. Lighting efficiency is therefore measured in lumens per watt (lm/W). The term "efficacy" is used instead of "efficiency" when the input and output units differ, such as when measuring the lumens of light produced by a certain number of watts of electricity.

Incandescent light bulbs are not particularly efficient and only radiate a small fraction of their energy as visible light with the majority being lost as heat. However, this efficiency does increase as the wattage of the bulb increases. For example, a 40 watt tungsten incandescent bulb has a luminous efficacy of 12.6 lm/W, a 60 watt bulb produces 14.5 lm/W and a 100 watt bulb produces 17.5 lm/W. However, even a 100 watt light bulb radiates 97.4 percent of its energy as heat.

Types of Incandescent Light Bulbs

The color of light that an incandescent light bulb produces is also an indicator of its heat -- whiter light generally indicates a hotter bulb. The yellow light of an incandescent bulb in the 75 to 100 watt range indicates a temperature of at least 2,500 C. The hottest incandescent light bulbs commonly found in the home may be up to 3,000 C and will be less yellow. Studio lights and photoflood lamps can be up to 3,125 C and are nearly white. By comparison, daylight is a true white and may range from 4,725 C to 5,725 C depending on weather conditions.

Halogen Lamps

A halogen lamp is a type of incandescent lamp in which the tungsten filament is sealed in a compact envelope and filled with an inert gas along a halogen, like bromine or iodine. This significantly increases the temperature of the bulb and subsequently the brightness of its light. The surface of a 75 watt incandescent bulb reaches about 125 C, whereas the surface of a halogen lamp ranges from 250 to 600 C. This makes halogen lamps a fire hazard and can cause a serious burn when touched. A halogen lamp may also be designed to have twice the lifetime of an incandescent bulb of the same luminous efficacy.

Fluorescent vs. Incandescent Light Bulbs

Fluorescent lamps use electricity to excite the atoms in mercury vapor. This causes the atoms to emit ultraviolet light, which causes another substance called a phosphor to produce visible light. Fluorescent lamps are considerably more efficient than incandescent bulbs and convert up to 28 percent of their power into visible white light. Fluorescent lamps also generate 65 to 75 percent less heat than an equivalent installation of incandescent lighting. However, the ideal temperature for the most common fluorescent lamps is around 25 C, and they may not turn on at temperatures below freezing.

The End of Incandescent Light Bulbs

The low efficiency of incandescent light bulbs is the primary reason the U.S. government has included them in the Independence and Security Act of 2007. According to the U.S. Department of Energy, incandescent light bulbs will not be banned, but they must be 25 percent more efficient by 2014.

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