A black body is a theoretical solid that, when cold, absorbs all electromagnetic radiation (including light) that falls on it. If you heat up this body, it begins to radiate electromagnetic waves, or it begins to shine. Think of a piece of cold black iron, a pretty good approximation of a black body. Now begin heating the iron up by pumping energy into the material with a flame or an electric current. Pretty soon the iron begins to glow red-hot emitting visible light having an abundance of the longer, lazier wavelengths that give the glow a red color. Keep on adding heat and the iron soon gets very excited and turns white hot as it emits more of the energetic blue wavelengths. We could keep on going making the material hotter and the radiated light whiter, but unfortunately the iron soon melts and begins to evaporate. Of course the theoretical black body won’t melt at extremely high temperatures like our piece of iron.

The color temperature of a light source is therefore directly related to the temperature of a theoretical black body radiator heated to a certain temperature. This black body temperature is usually stated I degree Kelvin or simply Kelvin’s. (Remember that 0 degrees Kelvin is absolute zero, the temperature at which all molecular motion stops and no more heat is in the material.) For example, a 20 watt light bulb has a filament that is heated to a relatively low temperature around 2000K. It emits a dim reddish yellow light. But it takes a gas like xenon, and gasses can be good black bodies too, and heat it up to the temperature of the sun with an electric arc and it emits snow-white light with a color temperature or 6000K. Our ubiquitous tungsten halogen light source that is so commonly used in theatrical fixtures has a maximum color temperature of 3200K that is, incidentally, the melting point of tungsten.

Efficiency is usually a ratio of energy (or work) output by a device compared to the energy required to run that device. The ratio is always less than one unless you believe in perpetual motion machines. Focusing on optical efficiency, we typically ratio the total number lumens output by a luminaire to the total number of lumens produced by the light source inside that luminaire. Since there are losses in all optical systems, the ratio is again, less than one. We usually multiply the fraction by 100 and express efficiency in terms of percent.

Generally, the more complex an optical system is the more losses occur, so the efficiency is usually lower for spot luminaires than for wash fixtures. For typical, state of the art automated spot fixtures, efficiencies run from 15% to around 25%. A popular automated spot uses a 1200 watt arc lamp that emits around 110,000 lumens. This fixture can put about 18,000 lumens on the wall. Every aperture inside the fixture, every surface of each lens, each filter eats away at the total amount of light that passes. These losses can be minimized by taking extreme care in the optical system design and by employing highly efficient optical coatings on all optical surfaces to minimize the losses there. With careful attention to detail, optical efficiencies of very complex automated spotlights can be as high as 40%.

PRG has a long history of developing innovative proprietary products for the entertainment production industry. Beginning with the revolutionary Stage Command rigging control system in PRG’s Scenic Technologies group and continuing with Icon and Icon M automated luminaires and control console from Lighting and Sound Design and the original Vari-Lite luminaires and Virtuoso lighting control system from VLPS Lighting Services. All of this equipment was specifically designed to serve the needs of the professional rental markets in the entertainment industry.

PRG continues with this long tradition by employing engineering personnel in Dallas, TX and New Windsor, NY in the US and Birmingham, UK to develop professional rental equipment such as the S400 Power and Data Distribution system, the MBox media server and the Virtuoso control system. Developing proprietary equipment for rental allows PRG engineers to design equipment that is targeted to the needs of top industry professionals with very sophisticated features, rugged and road-worthy packaging, modular construction for efficient shop and road maintenance and a long market life with product upgrades and enhancements.

For example, S400 is the first product to offer a power distribution system that combines power and an Ethernet network in a single cable and distribution system. Any DMX universe can easily be routed to any DMX output in the system and S400 supports 120V or 208V circuits and employs many safety features. The MBox media server is based on the revolutionary Icon M digital luminaire software and it has many sophisticated video playback features and the innovative I/O module provides theatrical style dimming and integrated SDI and HDSDI outputs. The Virtuoso console has been a mainstay in the lighting industry for years and PRG continues to support and improve the performance of Virtuoso with software upgrades such as the recent V6.0 release that added many new features.

PRG will be introducing several exciting new proprietary products this year that are targeted to the needs of the professional entertainment production markets.

I hesitate to use analogies but . . . think of lumens as a flow of photons, like so many gallons per minute of water coming out of a garden hose. Visualize footcandles as the local pressure caused when this stream is directed onto something, like a concrete driveway, or in the case of light, on your scrim or on the sensor in your footcandle meter. If you put a nozzle on your hose you can narrow the stream, producing a large impact. Or, you can spread it out into a fine spray over a large area that has little force. The same stream (lumens) can produce a large difference in impact (footcandles) depending on how the stream (beam of light) is shaped.

The relationship between lumens and footcandles varies with the area illuminated. If a source of light produces one lumen of light flux that evenly illuminates an area of one square foot, the illuminance or intensity produced over that square foot will be one foot candle. If that lumen is evenly spread out over one square meter, the illuminance is one lux in the metric system. Since one square meter is equivalent to about ten square feet, one lux has about one-tenth the intensity or brightness of one footcandle.

It must be noted that the lumen, as well as the footcandle which is derived from the lumen, is a photometric unit of energy or a unit designed to describe visible light, the light we as human beings see. Therefore, the spectrum of the energy that is collected by the instrument measuring the lumen (or footcandle) is filtered to the color response of the human eye. The photopic response curve for the eye in daylight conditions is a highly peaked curve that reaches its maximum sensitivity in the yellow greens, near the wavelength of 556 nanometers. Your footcandle meter has a strong yellow green filter placed over its sensor and is much more sensitive to greens than it is to reds or blues.