Choosing Lighting Fixtures Through Photometrics

Last month I discussed the aesthetic side of how cinematographers and lighting directors go about choosing specific fixtures for the job.
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LOS ANGELES—Last month I discussed the aesthetic side of how cinematographers and lighting directors go about choosing specific fixtures for the job. This month we’re going to take a look at the technical side and how to interpret manufacturer photometrics.

The basic unit of measurement of light is the footcandle (fc). A footcandle is the theoretical light emitted from one candle, one foot away from a one-foot square “target.” In many other countries, they refer to the basic unit of light measurement as the lux, which could also be thought of as the “meter-candle” (1 fc is equal to 10.764 lux).

HOW MANY FOOTCANDLES?
Footcandles are merely a method of measuring intensity, or brightness, of light. Many lighting manufacturers present photometrics in their literature, which are charts listing the various intensities (most often in footcandles) of light that will be output from a specific fixture at a given distance. With an understanding of how footcandles relate to exposure, you can evaluate how well a particular lighting fixture will serve your needs by simply reading the literature before you buy or rent that fixture.

There’s a great rule of thumb for being able to calculate how many footcandles of illumination you’ll need for any given exposure assuming you’re shooting at 24fps with a 180 shutter:

100 fc at 100 ISO is an f/2.8.

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Table 1: 24fps 180[o] shutter or [1/48] sec shutter speed

From there, you can calculate pretty much anything, always remembering that each doubling or halving of light is equal to one stop. Each doubling or halving of ISO is also equal to one stop. Table 1 may also be handy for determining how many footcandles are necessary at a given ISO to achieve a specific f-stop.

MAKING CALCULATIONS
Now we can look at photometrics for a fixture and get an idea how that fixture will perform. Somewhat randomly, I elected to look at a Mole-Richardson Studio Junior LED fixture. Most of us have a pretty solid idea what we’ll get from a traditional incandescent bulb Studio Junior, but you may not have had a chance to work with the LED version yet.

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Table 2: Photometric information from Mole-Richardson

On the Mole-Richardson website (www.mole.com) you can find the photometric information (Table 2). As the fixture comes in two different colors (Daylite and Tungsten), they provide data for each color temperature.

Looking at this simple chart we can learn quite a bit. We see, first off, that the tungsten fixtures are slightly dimmer than the Daylite fixtures—this tracks through to all LED fixtures, the phosphors necessary to achieve the lower color temperature limit the output of the LED.

Looking at the numbers and remembering the footcandle/exposure axiom (100 fc at 100 ISO is a 2.8) we can see that at 10 feet, at full-flood, the Daylite fixture outputs 397 footcandles and the tungsten outputs a slightly less 380 footcandles. This is about an f/8 and 2/3 at 400 ISO. That’s quite a bit of light at 10 feet! Plenty to put this fixture through some diffusion and soften it up and still get a healthy stop out of it.

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Table 3: Photometric information for the KinoFlo 401 Celeb LED fixture

Let’s take a look at the KinoFlo (www.kinoflo.com) 401 Celeb LED fixture (Table 3). So, at four feet, the Celeb gives us 279 footcandles of light. That’s about a 4 1/3 at 100 ISO or an 8 1/3 at 400 ISO. The Celeb is giving us nearly as much output at four feet as the Mole Junior LED at 10 feet.

Now, if you need that kind of stop, which fixture is right for you?

The Mole has more output at a greater distance, which gives you room to put large diffusion in front of it. Also, you can spot in the Mole Junior a bit to get some more intensity out of it if you’re only using a 4x4 diffusion frame. If you imagine the actor will be four feet from the light source, you can use the KinoFlo Celeb bare, or you can use the Mole LED through larger diffusion to get a softer source.

That isn’t to say that either one of these is better or worse than the other—just two different choices. It might be that if you’re working in a confined space and you need less of a lighting footprint, then the Kino-Flo Celeb is going to win over the Mole. If you’re looking for a larger, softer source and you have the room, the Mole will win out over the Kino.

Being able to understand photometrics will help you considerably in planning your lighting, placing your equipment order and choosing the right fixtures. Happy shooting!

Jay Holben is the author of the book “A Shot in the Dark: A Creative DIY Guide to Digital Video Lighting on (Almost) No Budget.”