Just How Long Is a Foot-candle?

Let me begin by stating the blindingly obvious fact that light is our stock-in-trade; something we manipulate every working day as we craft our pictures. Yet despite this, many of us don't know all that much about how to measure, specify and describe it.
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Let me begin by stating the blindingly obvious fact that light is our stock-in-trade; something we manipulate every working day as we craft our pictures. Yet despite this, many of us don't know all that much about how to measure, specify and describe it.

Your light meter may be calibrated in foot-candles, or worse, in lux. Luminaire specifications quote candela, footcandles, lux, and lumens, while lamp specifications talk about initial lumens or lumens per watt. Let's examine what these units measure.

To begin with, there's light itself: a stream of photons of various energies. Each photon is a burst of electromagnetic radiation, produced when an electron drops to a lower energy orbit around its atom. The larger the drop, the more energetic the photon, and the shorter its wavelength.

Visible light is the narrow range of photon energies that our eyes can detect. Light sources are systems that pump energy into atoms at levels that will produce visible light as the electrons give up their boosted energy.

This is equally the case whether we heat up the photosphere of a star with a fusion reaction, pass a current through the junction of an LED, create a plasma in a cocktail of metal vapors, heat a piece of tungsten wire, react a couple of organic chemicals together or burn some candle wax vapor on a wick.

LETTHEREBECANDELAS

The candela (cd) is the fundamental unit of all photometry and corresponds to the amount of light--quantity of photons--produced by a standard light source. Originally, the standard source was a real candle. Today, it's a theoretical construct, like most measurement standards. At least the candela has almost exactly the same value as an original standard candle. Every other measurement related to light is derived from the candela.

The lumen (lm) is the unit of luminous flux, luminous flux being the amount of light radiating out from a light source through a specific solid angle or cone of space. In the case of the lumen, the amount of light is one candela, and the solid angle is one steradian (which happens to be a cone with an angle of approximately 65.5 degrees). You may sleep more soundly tonight knowing that exactly 4p steradians fit into a sphere.

Lumens are the units used to measure the output of lamps and luminaires and thus can be used to evaluate the brightness of both fixtures and light sources. The efficiency of light sources is assessed by comparing the energy input in watts with the luminous output in lumens (lm/W). These numbers will haunt you soon, as laws governing energy-efficient light sources move beyond the street, home and factory and into studio and location shooting.

The more familiar lux (lx) is a unit of illuminance. It measures the amount of light reaching a surface. An intensity of one lux is produced by one lumen striking an area of one square meter. Of course, there is a much more familiar unit of illuminance: the foot-candle (fc).

Although the metric system was first recognized by the United States in 1866, and by the U.S. federal government in 1975, the United States is one of only three countries on the planet that have not implemented it. (Liberia and Myanmar--formerly Burma--are the other two.)

The foot-candle is a truly wonderful hybrid unit, using a metric measurement for luminous flux, in combination with the venerable imperial square-foot for the unit of area. As there are 10.76 square feet to the square meter, one foot-candle is the equivalent brightness to 10.76 lux.

I have always found that a conversion factor of 10 is perfectly adequate when it comes down to actually using the numbers for lighting. My guess is that there is way more than 10 percent variation between television light meters anyway.

While film cinematographers are constantly having their exposure meters recalibrated, then shooting test footage to verify the results, I've almost never seen television lighting directors send their meters off for a sanity check.

DOTHEMATH

A simple direct application of these measurements is to calculate the expected intensity of a fixture in an unknown situation, or one for which there aren't figures on the spec sheet.

A few moments with a calculator and the spec sheet will give you the beamspread of the fixture at a specific distance. A few more keystrokes will let you derive the intensity at that distance by just working out how many square feet of beamspread will be receiving the total beam lumens from the fixture.

It ain't rocket science, in fact it's barely even eighth grade trigonometry.

If you would like to learn a little more about any of the units or concepts mentioned in this article, almost every introductory college physics book will cover the topic, but more conveniently, there's good coverage in the 21st century's replacement for the home encyclopedia: www.wikipedia.org.

Andy Ciddor has been involved in lighting for more than three decades as a practitioner, teacher and writer. You can reach him via e-mail c/o TV Technology.