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myth? fluorescents violate physics

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Bob Cole
myth? fluorescents violate physics
on May 20, 2009 at 8:08:54 pm

I've read that fluoros are good keys for interviews, because their light falls off more rapidly -- that they don't light the background as much as fresnels, say.

I always accepted that as true, except that yesterday it occurred to me that fluoros probably don't violate the laws of physics, i.e. "The intensity of light diminishes as the square of distance."

I actually tested this with an open-face and a Kino Flo, and all I know for sure is that my trusty old Spectra light meter probably needs to be calibrated or replaced. (Side question: is the old selenium-cell Spectra obsolete? Why has everybody gone on to meters that need batteries? more accurate in low light??)

Results of my test: light from a fluoro diminishes as the square of the distance, just like all other light.

So why the myth about fluoros throwing off magically-diminishing light? Is it just that we tend to position them closer to the subject, resulting in a greater fall-off? (2' to 16' = 8 stops, right?)

Or, was I just dreaming up that story?

Bob C


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Todd Terry
Re: myth? fluorescents violate physics
on May 20, 2009 at 8:21:03 pm

[Bob Cole] "Is it just that we tend to position them closer to the subject, resulting in a greater fall-off?"

Yup.

You'd get exactly the same falloff with a fresnel as a key if you positioned it just like you would a flo... say, two feet from your subject's head...which no one in practicality would ever do. But if you did, the intensity of the spill onto a background would be exactly the same falloff ratio as a Kino (except, you'd get harder-edged shadows in the spill which would probably lead to the illusion that the falloff is not as great).


T2

__________________________________
Todd Terry
Creative Director
Fantastic Plastic Entertainment, Inc.
fantasticplastic.com






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Michael Palmer
Re: myth? fluorescents violate physics
on May 20, 2009 at 9:10:15 pm

"So why the myth about fluoros throwing off magically-diminishing light?"

Flos and fresnel lights are completely opposite light sources.

The fresnel has a designed lens to focus the beams of light and has a rear reflector to increase these beams of light and they are directed in what I call a straight line.

The Flo is a completely diffused light to start with and outputs what I would call spinning light beams that to some seem to fall off faster.

The spill from a flo is much harder to control and needs enough space to be properly cut, and by the nature of this diffused light some people may not take issue with this spill as they would with a fresnel.

But I'm sure you're correct with each type of light has the same diminished value.


Michael Palmer






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Bill Davis
Re: myth? fluorescents violate physics
on May 20, 2009 at 10:41:51 pm

I had an interesting conversation with a lighting equipment designer on just this subject a few months ago.

One thing I found interesting was that most lamp source photometrics are measured in a 360 degree sphere around the source. The lamp is expected to output light in every direction reasonably evenly.

As videomakers, we're usually concerned with putting light out in a controlled direction. Which is why the lamp housing is such a big deal. Reflectors, barn doors, parabolic mirrors, etc all attempt to get the light constrained to a single direction so that it can be cut and controlled.

Both standard tungsten and fluorescent lamps work this way. The lamp is "omni" - and the fixture is the key to control.

However, with LED lighting that changes. A led is actually more like a tiny Leko (elipsoidal spotlight) directing most of it's photon energy in a single direction. So using them in arrays is like having an array of a lot of spotlights - rather than a single light emitting surface.

That's been my experience with my ongoing conversion to LEDs as well. Some things work like conventional lights. (color temp, gelling, great power and weight savings, etc.) others are just different. Like using a barn door to cut an LED fixture. It just doesn't work the same.

All great tools, but if like me you're interested in the convenience, portability, energy savings, and longevity of LEDs - you've got to re-learn a few things.

FWIW.



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Dennis Size
Re: myth? fluorescents violate physics
on Jun 11, 2009 at 3:27:58 am

This was a great post Bill. I missed your original posting. (probably because of our greenscreen series). This is something that has been plaguing me and other LD's.
The current solution for me is a wonderful development from the CITY THEATRICAL Company in New York City.
They've taken a Color Kinetics fixture and created a LED television fixture package. Check out the iW BLAST 12 TR Fixture Kit they're marketing. The fixture itself contains 36 hi-intensity warm AND cool LED's. Via their contoller you can vary the intensity and the color temperature (from 2700K to 6500K). To address the beam control issues which you bring up (and also drive me crazy), they've also created louvers and barndoors and MOST IMPORTANTLY a series of 12 lenses -- both Symmetric and Asymmetric. The lens system really helps shape and control the beam. You can almost turn the fixture into a fresnel. Why not use a fresnel? -- the power and heat savings!
So many of our clients are demanding that we install "green" (there's that word again) studios. Up until now the best fixture I've found has been the LITE PANEL 1'x 1' ...which unfortunately doesn't have any control or beam shaping ability. This tragic flaw was a hindrance to me when I was lighting the White House Press Room (check out the pic on the LITE PANEL website).

I'm currently designing a complete studio in which there will be no dimmers and the power is limited to 100amps. This iwBlast 12 TR fixture (which I'll need at least 50 of) is the leading contender as the instrument of choice in the facility --- even for the greenscreen! :-)

DS






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