How do you do that Spotlight/Light Beam effect

Started 4 months ago | Discussions thread
Rico Tudor Contributing Member • Posts: 833
Re: How do you do that Spotlight/Light Beam effect

Inverse square law does not apply to focused or collimated light!

Unless you deal with point sources (not available in this reality) the light cannot be collimated. Laser light is, of course, collimated but not by refractive means, and is rather impractical for head shots! With a projector, the "spotlight" is the imaged stop of the optical train, and where you might place a gobo or iris. As distance increases, the diameter of the projected optical stop enlarges linearly—simple geometry. With increased distance, a particular spotlight framing around the subject can be maintained by closing down the iris but flux density has nonetheless been reduced by the inverse-square law. This flux reduction can be eliminated by using a longer projector lens with same f-stop (iris open).

For a mathematical treatment of inverse-square, search for "flux as a surface integral" on Wikipedia. Salient concepts are "orientable" and "divergence".

For pedagogical insight, shine a Maglite on the wall. The central blob is the (imaged) bulb. Yes, the back reflector is a crude paraboloid! With increasing distance from the wall, the blob enlarges. If you remove the Maglite reflector by unscrewing the head, you can project the naked bulb with a photographic lens onto a far wall and make out the tungsten filaments: they will be much larger than life-sized and dim.

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