Lighting Volumes Of Water

Is there a formula for calculating how much light is needed to light a large tank?

I imagine the old inverse square law doesn't apply in water...

Bobby Stone

Bobby Stone writes :

>Is there a formula for calculating how much light is needed to light a >large tank?

Are you referring to "columns" (tall, thin tanks) or "volumes" (any old shape)?

>I imagine the old inverse square law doesn't apply in water...

The inverse square law applies very seldom with motion-picture lighting anyway because it describes the fall-off of a theoretical point source radiating in all directions, i.e., with no reflectors, lenses, diffusers, etc. involved.

As for water, obviously the falloff with distance depends on its clarity. Presumably the exponential falloff factors for clear fresh or salt water should be known to those who do underwater photography.

Dan "There must be a manual somewhere...." Drasin
Producer/DP
Marin County, CA

Dan 'there must be a manual somewhere...." Drasin wrote:

>...The inverse square law applies very seldom with motion-picture >lighting anyway because it describes the fall-off of a theoretical point >source radiating in all directions, i.e. with no reflectors, lenses, >diffusers, etc. involved....

It's true that, strictly speaking, the inverse square law applies only to an infinitely small point source. But the many of the tungsten-halogen fixtures we have now are small enough to follow the law surprisingly well at the distances we use them.

Ex: With a Lowell Tota Light, not really a point source with its long filament and reflector, a 500 watt lamp gives f/11 at 3 feet, f/5.6 at 6 feet, and f/2.8 at 12 feet, all of these plus or minus less than a sixth of a stop (24fps, EI 500.)

With a Mickey-Mole instead, the numbers are f/18, f/9.8, and f/5, respectively, well inside one third stop difference.

Photographically, that's dead on to what the inverse square law predicts.

Larger diameter fresnels would stray farther from theory at the above distances, but if used at distances more appropriate to their greater output, they'd be as close.

An interesting sidelight (!) is that generally speaking, Fresnels are used at distances appropriate to their output (teners much farther away than 5Ks, 2Ks closer, 1Ks closer yet.) Consequently, from the subject's position, they all have roughly the same diameter and cast shadows that have approximately the same "hardness."

None of the above applies to large, diffused sources, which, depending upon their size, tend the obey an inverse direct law (twice the distance, half the light, instead of one-fourth.)

I would expect that, if the light sources are to be inside these "volumes of water", submerged, they'll be relatively compact sources and tend to obey the inverse square law, but at lower light levels due to greater absorption. But that's a guess, I don't have a stage I can flood to check it out.

Wade K. Ramsey, DP
Dept. of Cinema & Video Production
Bob Jones University
Greenville, SC 29614

Between

>Consequently, from the subject's position, they all have roughly the >same diameter and cast shadows that have approximately the same >"hardness."

and

>None of the above applies to large, diffused sources, which, >depending upon their size, tend the obey an inverse direct law (twice the >distance, half the light, instead of one-fourth.)

No wonder many rave about you as a teacher.

Art Adams, DP [film|hdtv|sdtv]
Mountain View, California - "Silicon Valley"
http://www.artadams.net/
AIM: ArtAtoms