Beam angles and Lighting theory
“One Buffalo….Plus Two Buffalo………Three Buffalo!”
Whether we like it or not (or know it or not) a lot of math goes into lighting design. While it is not necessary to think about it all the time, it is important to have a basic understanding of it. This chapter aims to get you to a basic understanding of math in stage lighting (as painlessly as possible.)
Note that some lighting instruments are long-throw instruments and others are short-throw instruments. That just means the beams of long-throw instruments tend to reach farther than the beams of short-throw instruments. The beam is the directional projection of light: the actual light coming from the lighting instrument. Beams in stage lighting instruments are almost always conic.
The above diagram shows a conic beam. One of the most important things on this diagram (and in this lesson) is the capital ‘B’. The capital ‘B’ is the beam angle. The intensity of the light beam is most intense at its center, and fades out from there. The beam angle is the angle between the two planes of light (the hypotenuse on the right triangle is one plane of light) where the intensity of the edge of the beam is at least 50% of the intensity of the center beam (the most intense part of the beam). This area of the beam where intensity is greater than half of the maximum intensity (i > .5) is what is considered the usable part of a light beam. The field angle is the area of the beam where i > .1 sometimes called the spill or ghosting. Some of the field angle is not considered usable light.
Lighting instruments are identified by their beam angles. The beam angle is the degree of the light that you see on the light’s body. For instance, most ERS Source Fours have a beam angle of 26͒, though some have 36 degrees, 19 degrees, etc. Fresnels and other short throw instruments of beam angles of about 25 degrees to 45 degrees. Long throw instruments have smaller beam angles than short-throw instruments.
On the above diagram, the line ‘d’ is the distance from the lighting instrument to the surface it is hitting, and the line ‘r’ is half of the beam spread. The angle B is coming from the inside of the lighting instrument, and the circle face of the cone is what is hitting the stage. The points and lines that go through the cone represent the diameter of other places in the beam where i > .5
As you can imagine, you could go into a lot more math using these basic principles (you could even write an entire Maths paper on it -_-) but that’s pretty much all you need to know for now. See? Nearly painless.
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