Light bulbs are rated in terms of their luminous efficacy, which describes how much illumination a source provides in visible light from a given amount of electricity. It is visible light output per unit of electrical power measured in lumens per watt.

Measuring light intensity is a complicated issue for two reasons. One is because the human eye is not equally sensitive to all frequencies, and almost all light that we see consists of different frequencies of light of different intensities.

The other complication is that the intensity of light decreases away from its source, so that distance must be taken into account.

The lumen is a unit that incorporates the distance factor in comparing light output. A single candle emits light equivalent to a little more than 12 1/2 lumens.

Incandescent bulbs rely upon heating of a filament of tightly coiled tungsten wire by electric current. Fluorescent bulbs rely upon an energized mercury plasma that emits UV radiation that strikes a fluorescent coating and re-emits the energy as visible light. A certain amount of heat is produced along with the light but in much smaller amounts than with incandescent bulbs, so the output of light is higher per unit of power consumed.

Luminous efficacy of fluorescent bulbs is four to eight times higher than incandescent because of the thermal properties of matter.

Even at ideal efficiency, incandescent light has a luminous efficacy of 95 lumens per watt. A typical light bulb is only 10 percent to 20 percent of this because the maximum occurs at a temperature of more than 11,000 degrees Fahrenheit. No substance yet discovered remains solid at such high temperatures. Tungsten, which is used for the filament in incandescent light bulbs, melts at just over 5,600 degrees Fahrenheit.

Compact fluorescent lights (CFLs) contain 5 miligrams of mercury, and there is concern for the environmental impact of their disposal. Each bulb contains less than 1 percent of that contained in old mercury thermometers.

Although there are arguably more CFLs per household than there were mercury thermometers, it is doubtful if there are 100 times as many.

The main objection to fluorescent lights in the past was their odd color balance, which lacked red and blue frequencies and gave everything a greenish hue. Advances in design and manufacture of phospors of different colors have made this problem disappear.

The other problem with fluorescent lights that kept them from wide dissemination before the 21st century was the need for an inductive baffle to increase the voltage necessary to operate them, and which created a distracting hum and often made the bulbs hard to start up in humid weather.

CFLs overcome these shortcomings by using an electronic baffle that accomplishes the same starting effect in a smaller space that allows it to be enclosed inside the bulb housing.