What Is the Difference Between Lumens and Footcandles?

Physicists measure light in footcandles.

Both lumens and footcandles measure the illuminating power of a light source. They measure not the power of the light source itself, but the resulting illumination of an object a certain distance from it. People in many occupations must quantify illumination to successfully complete projects. Interior decorators, photographers, architects and film makers are just some of those who must quantify illumination to achieve desired lighting effects.

Function

A lumen quantifies the brightness of a light in metric measurement. The amount of light energy reflecting from the surface of a sphere one meter in diameter surrounding a candle---now the standardized "candela" international unit--equals one lumen. A low-light source, such as a nightlight, might measure 12 lumens, while a 100-watt light bulb measures 1200 lumens.

In contrast, one footcandle of light is the measurement of the illumination which one candela provides on a surface one foot away from the source. A footcandle is a measurement of energy in non-metric terms. By converting lumens (one square meter worth of illumination) to footcandles (the illumination one foot away from the source), you are essentially measuring the same thing. The conversion formula is footcandles x 10.76=lumens.

Significance

Photographers use a light meter to determine lumens.

Footcandles are used in the photography, film, television and lighting industries. Americans who design lighting for museums and galleries calculate sufficient quantities of light for exhibits in terms of footcandles; Europeans use the metric-based lumen in their calculations.
Similarly, photographers use light meters which express illumination in these terms, allowing photographers to verify whether the available light will sufficiently illuminate their subjects. Film makers and television camera operators must determine optimum intensity of scene lighting for the effects they wish to create, so use tools which measure the footcandles or lumens of light. For example, a nightclub scene might call for 20 footcandles of illumination, while a courtroom scene might require 100. These measurements also help in setting up precise lighting and contrast ratios.

History

The word "lumen" came into use as a term in physics before 1900. It was officially adopted in 1919 by French scientists as a term for the measurement of illumination, and accepted as such internationally by 1946.
The term "footcandle" is being eclipsed by the term lumen. Many people working in physics and industries which quantify illumination consider the term "footcandle" obsolete, just as much of the rest of the world considers our system of foot-based measurement outmoded.

Considerations

The right amount of light creates the right mood.

Physicists term the expression of light density which illuminates an object "lux." Lux describes a metric unit equal to one lumen per square meter. If you need to determine the correct quantity of lux illuminating an object or area, keep in mind that the farther the object is from the source of light, the less lux it will reflect. Therefore, to illuminate a painting three meters from a light source requires more lux than illuminating a painting one meter from a light source.

Fun Fact

An eclipsed sun delivers fewer lumens.

A full moon in a cloud-free night sky emits .25 lux, while the sun at high noon in a clear sky emits 100,000 lux.

References

• PC Magazine: Lumen
• The LED Light: Lumens
• Absolute Astronomy: Foot-candle