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Photometrics 101

Patrick J. Immel • Light On The Subject • August 7, 2007

Whether you are a new lighting designer, or a seasoned veteran working in a new space, you might run into the challenge of selecting the correct type of lighting fixture for your plot. The following method will allow you to quickly and accurately make the right choice when working on either a hand-drafted or CAD light plot. You might ask, “Don’t most modern CAD packages have the capability to provide photometric feedback?” Yes, but they also sometimes require sub-stantial setup. This method is as easy as inserting a symbol into your plot and rotating it into place. The paper version of this method may be printed either onto card-stock or transparency film and laid over the plot.

A few words of warning regarding this method: First, although this method is fairly accurate, it is still only an educated guess, and you might find some adjustments are in order once you get onsite. Second, this method requires some math…now don’t you wish you’d paid attention in math class!?

Image 2

1. The first thing we have to do is create a photometric symbol for each lighting unit in your in-ventory. For this exercise, I am using ETC Source Four fixtures. Each fixture has very specific photometric data associated with it. This data may be found on the Web site of the lighting manufacturer or in the book, Photometrics Handbook by Robert C. Mumm. The first photometric symbol I am going to create is an ETC Source Four 36° fixture. Here is the photometric data from the ETC Web site, (See Image 1.)

Image 3

2. Although I am creating these symbols in a CAD program, you can create symbols just as accu-rately using paper and pencil. Before we create the first symbol, there are a few photometric terms you should know:
a.Distance — This is more often noted as “Throw Distance” and is the distance from the lighting unit to the object on stage being lit.
b. Beam Angle — That point where the light emitted by an instrument is diminished by 50 per-cent when compared with the output at the center of the beam.
c. Field Angle — That point where the light output diminishes to 10 percent of the output at the center of the beam. (See Image 2.)

Image 4

3. Image 3 is a photometric symbol for an ETC Source Four 36° fixture created in a CAD pro-gram. This is a scaled representation of the beam of light produced by the fixture with the data coming from the ETC Web page. Items of note for this symbol and things to consider for your symbols:
a. The beam angle is indicated by a dashed line; the field angle by a solid line.
b. You see quite a few numbers on the symbol. For example, 25’ and D16’, which means, “At a throw distance of 25 feet, the field has a diameter of 16 feet.”
c.  Included, at appropriate places in the symbol, are notations of the unit type and actual beam/field angles.
d. Notice the small dashes on the symbol center line. These are used to provide a quick meas-urement tool.
e. When using a CAD program, I create the “insertion point” for my photometric symbols in the center of lighting unit (yellow symbol at left of image). This approximates the C-clamp position on the fixture so I can use my program’s snap features to “hang” it on a pipe.

4. I am assuming you have your plot started with focus areas determined. Laying out focus areas includes determining what size (pool) of light for each focus area will work best for your design. For my example, I have decided that approximately 12 feet is a good size. Let’s begin by choos-ing a lighting unit for area “I”. (See Image 4.)

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5. Now we must determine the throw distance. This is the distance the beam of light travels from the lighting unit to the object it strikes on stage. From the plot and center section, we can deter-mine the vertical and horizontal measurements. The vertical distance is 34 feet and the horizontal distance is 36 feet. (See Image 5.)

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6. Now for the math! We will be using the Pythagorean Theorem (a2+b2=c2). To avoid scaring off the anti-math crowd, I am adjusting the formula to common names. (See Image 6.) Plug the vertical and horizontal distances into the formula as shown in Image 7.

Image 8

You will notice in the second line of the formula, (34 – 6). Unless lighting an object lying on the stage floor, we always remove six feet from the vertical dimension to provide the maximum amount of light on our sub-ject’s face. Six feet is just an average amount. Adjust if lighting small children or a basketball team!

Notice that our throw distance is 45.6 feet. If you have access to the aforementioned Photomet-rics Handbook, there are charts that will easily answer the throw distance question.


Image 9

7. It’s time to start choosing lighting units. I’ll start with a 36° Source Four fixture. (See Image 8.) At this point, insert and rotate your 36° symbol (that you created earlier), using whatever method your CAD system requires. If you’re doing it by hand, just lay your symbol on top of the existing drawing to accomplish the same thing.
When inserting this symbol, you are only taking into account the 36 feet horizontal distance, where the “throw” distance is actually closer to 46 feet! Just slide the photometric symbol back 10 feet from the pipe to give a more accurate representation of its beam of light. (See Image 9.)

Image 10

Even before you move the fixture, notice that the fixture creates a beam of light that is already way too big (field diameter > 23 feet) for this purpose.

Let’s try a 19° fixture. It creates a beam of light (approx. 16 feet) that is closer to what we need. (See Image 10.)

Image 11

At this point, an educated guess tells me that a 19° fixture will work pretty well for any units on this position, throwing to approximately the plaster line area of the stage. If you have doubts about any unit, at any position, shooting to any place onstage, just try another photometric sym-bol. It is easier to change units here on the plot than to change actual fixtures after they are hung.

This method will allow you to test any unit that is more or less an upstage and/or downstage fo-cusing unit. Also, straight side lighting can use this method.

Image 12

What happens when you need to determine the correct fixture for a top/back light or sidelight? For that, we can use many of the same techniques discussed above, but now we’ll need to use a center section. Do you notice the red Popsicle-looking guy? That is our six-foot-tall figure we will be focusing on. (See Image 11.)

Image 13

8. The first fixture I want to try is an ETC Source Four PAR with a WFL lens. (See Image 12.) In this instance, I believe that this fixture has too wide a beam…it’s too big. Let’s try an ETC Source Four PAR with a MFL lens. (See Image 13.) {mosimage}This fixture looks like it will be a bit more manageable!

One note regarding top-, side- or high-side lighting positions. If your units focus straight down or upstage, this method should be very accurate. If they turn off to the side one way or another, your calculations will become more inaccurate. Also, a flat 90-degree sidelight should be done in a front elevation.

This method:
— will allow you to quickly choose the correct lighting fixture for your particular design needs.
— is not 100 percent accurate, but will get you quite close.
— Can be used in either a hand-drafted or a CAD environment.

Using this method, you don’t necessarily need to be an engineer or a math whiz. With a basic understanding of stage lighting and some common sense, even you can create a relatively accu-rate light plot. Let’s get out there and start lighting.   

Patrick Immel is an Assistant Professor of Theatre and Scenic/Lighting Designer at Northwest Missouri State University in Maryville, Missouri. He is also a member of the United States Insti-tute for Theater Technology.

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