Re: How do you do that Spotlight/Light Beam effect

2

Rico Tudor wrote:

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 linearlyâ€”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.

I think you my be confusing terms.

Collimated means the light is parallel - for all practical purposes sunlight is collimated and can also be achieved with lenses, and approximated to by paras etc.

Laser light is in phase or coherent - all the photons have a similar wavelength and the wave peaks and troughs coincide - which is what gives laser light its intensity.

The inverse square law is all about radiated energy (light in our case) spreading out over spherical space - the area of that sphere increases with square of distance (4 Pi r^2), thus the amount of energy per unit area falls off by the inverse amount. Parallel light does not spread out into a sphere, so (its energy) does not fall off in the same way.

Of course strictly speaking the inverse square law ONLY applies to the theoretical point light source, but is a good rule of thumb for most photographic light modifiers, but not so good for focused or parallel light.

I happened to read physics for my degree, and although we never covered the inverse square law's application to photographic lighting, we did in relation to other types of radiated energy