“I believe in the imagination. What I cannot see is infinitely more important than what I can see.” — Duane Michals
I was surprised to learn that three out of five people think I can’t count, and the other three have no opinion. At least that was what I gathered from the e-mail after the article in this column about the five lighting metrics. Apparently the three lighting metrics that were covered in the article don’t account for all five.
There really are five lighting metrics, or five ways to measure light. The three that we talked about were luminous intensity, luminous flux and illuminance. Those are the most common metrics that we deal with as lighting designers. The last two, luminance and luminous exitance, are less frequently used in our day-to-day activities. Why? Because they have to do with light coming from a surface area, whether it’s reflected light or generated light.
Luminance is the measure of the intensity of light coming from a surface in a particular direction. If, for example, a Fresnel produces 1,000,000 candelas and it has a 12-inch lens, then the luminance, looking directly into the fixture, is 8,846 candelas per square inch. If it was a soft box instead and it still produced 1,000,000 candelas but it had a two-foot square surface, then the luminance would be 250,000 candelas per square foot looking directly at the soft box at a 90º angle.
If we then stepped off to the side of the fixture and looked at it from an angle, then we would expect the intensity to drop. In fact, it drops in relation to the cosine of the angle so that the luminance is the intensity of the source or surface divided by product of the surface area and the cosine of the angle.
Luminance (candelas per square foot) = luminous intensity (candelas) ÷
[area (square feet) × cosine θ]
The symbol θ is the angle between a line drawn perpendicular to the source or surface and the observer. As the angle increases to 90º, the luminance drops off to zero.
The fifth metric, luminous exitance, is a measure of the total amount of light leaving a surface area. It’s different than luminance because it includes all of the light, in every direction, that leaves the surface while luminance is only the light in a particular direction. This metric is similar to luminous flux except it’s a measure of the flux divided by the surface area.
Luminous exitance (lumens per square foot) = luminous flux (lumens) ÷ area (square feet)
Luminance is an important concept in this age of video-heavy production. The amount of light being reflected from a surface area dramatically affects how the camera reacts to it. If we have a large, highly reflective surface on the set, then the reflected light can cause the camera iris to close, rendering all of the other areas of the set very dark. As a result, the contrast ratio will be very high with areas of the picture that are too bright and other areas of the picture that are too dark. That’s why your elementary school teacher used to tell you not to wear bright yellow or white clothes on picture day; your shirt or blouse would reflect too much light and your face would be dim by comparison.
The luminous intensity of reflected light depends greatly on the coefficient of reflection of the surface from which the light is reflected. The coefficient of reflection, in turn, depends on the type of material, its color, and the spectral distribution of the incident light. A shiny or specular surface, like a mirror, will reflect almost all of the incident light while a diffuse surface, like velour or painted drywall will reflect much less. The light that is not reflected is absorbed, and colored surfaces absorb every wavelength except those around the color of the surface. The type of light heavily influences the luminance of colored objects. For example, an HMI with a discontinuous spectrum and very little red content will produce a very different luminance value with a red object than will an incandescent source.
In Stanley McCandless’ book, A Syllabus of Stage Lighting, he provides a brief table of coefficients of reflection for various colors under white light:
White 75-90%
Light Blue 35-50%
Gray 15-60%
Dark Green 17-20%
Dark Red 10-15%
Dark Blue 3-10%
Black Paint 1-5%
It turns out that the human eye evaluates reflected light differently than we might expect. If half of the light is reflected, then it appears very bright. What we might evaluate as the “halfway” point between total reflected light and total absorption is really about 20 percent reflectance. The Munsell system was developed to show equal visual steps, all things being equal, of reflected light. This is how it compares to the amount of reflected light:
Munsell Value Luminous Reflectance Factor (%)
0 0
1 2.0
2 3.0
3 6.4
4 11.7
5 19.3
6 9.3
7 2.0
8 57.6
9 76.7
10 100
The zone system is similar to the Munsell system, except the zone system was developed by Ansel Adams and Fred Archer in 1941 to link film response to the human eye response. In the zone system, there are 11 zones ranging from 0, which represents black, to the Roman numeral X, which represents pure white. The middle value of gray is represented by the Roman numeral V. As in the Munsell system, the middle value represents about 18% reflectance.
Luminance can be measured with a spot meter, which is a reflected light meter with a very narrow field of acceptance, typically about 1-5%. It allows you to evaluate individual components of a set and figure out the exposure value and contrast ratio. Spot meters can usually read in f-stops, footcandles, lux, candelas per square feet or meters, foot-lamberts or exposure values.
Perhaps the best way to evaluate the lighting, reflectance, and contrast ratio on a set is to view it with a video monitor. But if you don’t have one, a quick and dirty way of making rough evaluations of luminance is to use a contrast viewing glass or neutral density filter to view the set. You’ve seen the DP or the occasional gaffer use the monocle-like contrast glass to view a set or to focus lights.
In the absence of a real contrast glass, a pair of cheap sunglasses with dark (not tinted) lenses will help. Just be careful not to use geeky looking sunglasses or you might be mistaken for one of the audio crew.
