Lamberts My Foot

by Richard Cadena

“I will prepare and someday my chance will come.” – Abraham Lincoln

I witnessed it. Right in front of God and everybody, LD Jeff Ravitz said the “f” word – “footlamberts.” It was during a seminar at Pro Production in San Diego right in the middle of a very interesting discussion about a Bruce Springsteen production for MTV. It really gave me pause.

If you’re like me, you’re probably familiar with the concept of illumination, which is a measure of how much light is projected onto a surface. Illumination is measured in footcandles in the English systems or in lux in the metric system. It is often used by lighting manufacturers who want to convey how incredibly bright their fixtures really are. Most lighting spec sheets have information in one form or another about the illumination produced by that particular fixture at a given throw distance. It’s also used by lighting consultants and designers to specify how much light they want at a certain point. For example, you might specify that your target illumination at the podium is 100 or 150 footcandles.

But I had to stop and think about the “footlamberts” word when it came out of Jeff’s mouth. What exactly is a footlambert? I’ve heard the word before, but it’s not a term you often hear in the entertainment lighting business. I wasn’t sure exactly what it was. From the context of the discussion, I could tell that it was a unit of measure that was related to the use of a television camera.

I once took a really good course from the Illuminating Engineering Society called IESNA ED-100. They offer a number of courses that are administered on the national level and taught on the local level. The ED-100 course is all about the fundamentals of lighting. In it, they talk about the five lighting metrics; luminous flux, illuminance, luminous intensity, luminance, and luminous exitance. These are all ways to measure and quantify light. When I heard Jeff talking about footlamberts, I made a mental note of it and looked it up later on. I found that it is a way of expressing luminance, one of the five lighting metrics that I had learned about in the course. One footlambert is the same as 3.426 candelas per square meter.

One of the most useful things I’ve ever learned in my lifetime is that when you don’t understand mathematical expression or term, if you look at the units you can often get clues about its meaning. The units of measure of a footlambert are candelas per square meter. I know from ED-100 that a candela is a measure of luminous intensity and a square meter is a measure of area. So I can infer that this metric has to do with luminous intensity coming from a surface area. Looking up the definition of luminance, I found that it is indeed the luminous intensity in candelas divided by the area from which the light is emitted times the cosine of the viewing angle.

L (Luminance) = I (Luminous Intensity) / Area x Cos α (Projected Angle),

Where α is the viewing angle relative to a perpendicular from the surface area

The cosine α is a factor that decreases the luminance as the viewer gets farther off axis and away from the perpendicular. At a 90° viewing angle, the luminance drops off to zero.

So what does all this have to do with lighting and why do we care? Illuminance is a measure of how much light reaches a surface area, like the stage or set, where as luminance is a measure of how much light is reflected off of a surface back to the viewer or a camera. It is important because different materials reflect different amounts of light. Even if a set is illuminated perfectly uniformly, different objects on the stage or set will reflect different amounts of light and they create a range of contrast. For the human eye, this presents little problem because the eye, being the incredibly flexible instrument that it is, makes adjustments compensates for any flare or glare, as long as the contrast ratio is not too great.

The camera, on the other hand, is a cold, unforgiving instrument that can only deal with a limited range of contrast. If one object reflects a lot of light relative to the rest of the set, it will produce flare in the camera. If another object reflects little light relative to the rest of the set, it will be dark and unintelligible. The ratio of the two is the contrast ratio and it is very important to television and video lighting designers.

I recently came across a copy of the 11^th edition of “A Syllabus of Stage Lighting” written by Stanley McCandless in 1964. McCandless was a professor of lighting at the Department of Drama – School of Fine Arts in Yale University, and he pretty much single-handedly invented the stage lighting method. This syllabus was what he used to teach lighting classes at Yale. On page 16, he talks about coefficients of reflection: “It is necessary to differentiate somewhat between selective and ordinary reflection, and the following table gives the list of coefficients of reflection for atypical color under white light.” (Formatting added for clarity)

White	75 to 90%	Dark Green	17 to 20%
Light Blue	35 to 50%	Dark Red	10 to 15%
Gray	15 to 60%	Dark Blue	3 to 10%
		Black Paint	1 to 5%

I’ve read many books on lighting design and seldom do you find any information about this, but it is very important, especially since the recent proliferation of video.

By measuring the luminance, the lighting designer can get an idea of what the camera sees and of the contrast ratio. He works to keep the lighting levels and the contrast ratio within the tolerance of the camera. According to the 8^th Edition of the IES Lighting Handbook, in video the contrast ratio should not exceed 20:1 and for television film production it should not exceed 25:1.

Then, once the lighting and set are well lit, when the talent walks on stage dressed from head to toe in white, all the lighting designer can do is to throw up his hands.

Let your light shine by e-mailing the author at rcadena<at>swamicandela.com.

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