A Process for Designing Lighting for the Stage: Determining Lighting Areas (Part 2)


It is important for the designer to understand the characteristics of the beam output for the instrumentation that is available, so as to select the best fixture for the intended purpose.  Remembering that one of the goals is to render an 8′ to a 14′ diameter lighting area on the stage, the Lighting Designer must be able to specify the most suitable fixture.  Often, when a designer is working in a familiar space or with familiar equipment, the designer can work almost from instinct.

However, an inexperienced designer, or a designer working in an unfamiliar space, or with unfamiliar fixtures, would be well advised to do some “homework”.  Below is a table comparing 9 different ETC Source Four fixtures and their related Beam and Field angles.  The diagrams indicate the throw distance necessary to produce a 10′ pool of light (using the Field)

Comparing Field and Beam Angles of ETC Source Four Fixtures

The beam and field angle information comes from the individual online fixture cut sheets available at the ETC web site Source Four Downloads site.  Multiplication factors for beam spread come from the Source Four Beam Spread Table on the same downloads site.  Multiplicate foacors marked with an * are not included on the available beam spread table and have been calculated by the author.  The designer may find information on many lighting fixtures available in the U.S. in the Photometrics Handbook (published by Broadway Press) or in other texts.  However, most manufacturers also offer the information online (as in the case of ETC).

One of the handiest tools available to the Lighting Designer is the “Multiplication Factor” .  This is a number assigned to each instrument that provides a quick aproximation of the size of the pool of light that that instrument will render from a known distance using the following formula:  (Throw Distance) X (Multiplication Factor) = (Lighting Area Diameter).

For example, suppose you know that you have a 20′ throw distance to the ligting area from a lighting position that you intend to use and you want to have about a 12′ lighting area.  You can test each available fixture to determine its lighting area from that distance by plugging the Multiplication Factors for Field into the formula:

Source Four 26 —  (20′) X (0.42) = (8.4′)

Source Four 36 — (20′) X (0.58) = (11.6′)

Source Four 50 — (20′) X (0.93) = (18.6′)

This would suggest that although the 36 degree fixture would yield a slightly smaller diameter lighting area than we hoped for, it is the best (as in closest) choice.  Note that while it is 6″ in diameter narrower than the target size, the next fixture (the Source Four 50) will produce a much larger diameter than desired.

If you didn’t want to run trial and error calculations, you could use the following formula to learn the desired multiplication factor, and then choose the closest best match:

(Desired Area Diameter) / (Throw Distance) = (Desired Multiplication Factor) Thus, with the 20′ available throw distance and the 12′ desired lighting area:  The formula (12′) / (20′) = (0.60) indicates that we are looking for a fixture that has close as possible  to a Multiplication factor of 0.60.  By comparing that value to the values of the Field Multiplication Factor  of the available fixtures, you learn that the 36 degree fixture is the closest.  You would have to run the actual Field Multiplication factor through the first formula again to determine the exact Lighting Area diameter (so that you could plan your areas accordingly)

If the reader is like most of my students, it is at this point that you will realize that all that math from High School actually did count for something!

That’s enough for now! Have fun!  Be safe!








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