A Process for Designing Lighting for the Stage: Determining Lighting Areas (Part 5)

Earlier I described how to figure out the throw distance from a lighting fixture at a known position to a lighting area by creating a section drawing.  Depending upon the complexity of the plot, this might be a daunting task to perform for as many of the fixtures as might require this determination.  It is entirely possible to calculate the throw by calulating it mathmatically from measurements taken from the plan and the center-line section.  This is because, with a quick application of the Pythagorean Theorum (the sum of the squares of two sides of a right triangle equals the square root of the hypotenuse).

In the last post, I demonstrated that a 14 degree Source Four fixture hung on the SR FOH Boom position at 21′ above stage level would throw a roughly 11′  lighting are just upstage of the proscenium and on the centerline of our example stage.  

In plan the throw distance is about 38′.  The elevation of the fixture is approximately 15′ above the face-high lighting area.

The square of 38′ is  1,444′.  The square of 15′ is  225′

The sum of the squares of 38′ and 15′ is 1,669.

The square root of 1,669 is almost 41′

With a  41′ throw, we know from the 19 degree  Source Four fixture multiplication factor of 0.31 from an earlier post that a 19 degree  fixture would render a pool of light with a diameter of aproximately 12 3/4′ (much larger than the diameter of the initial 45 degree SL front-light .  Meanwhile at 41′ throw, the 14 degree Source Four fixture with a multiplication factor of 0.26   would render a pool of light approximately 10 1/2′ in diameter.

Comparison of different fixtures from different axis and angles

While the diameter of the 14 degree fixture  from 41′ is aproximately 12″ narrower than that of the 19 degree fixture from  the 45 degree SL front-light position, the shallower axis creates an elongated ellliptical pool of light quite a bit larger than the first.  Therefore, the 14 degree fixture is probably a better choice than the 19 degree fixture for the shallower axis.

The Square Root of the sum of the

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

SJM

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