Fluorescent lights, relative output?

[Scott Davis]

Anyone know a conversion formula for fluorescent lights? I know that if a fluorescent light is rated at, say 20w, while it draws 20w of power, it puts out a comparable amount of light to a much brighter light. I believe that also there is a law of diminishing returns on power output - ie, if a 20w light puts out the equivalent of a 100w incandescent, a 55w does not put out 270w, but rather some amount less. Does anyone know a formula for either of these ratios? I'm looking at getting some fluorescent lights to use as a constant light source for doing wet-plate in the studio, and I'm trying to figure out how much power various fixtures put out, and without a standard of comparison, looking at fluorescents is almost as confusing as trying to compare strobes across various manufacturers.

[Henry Ambrose]

I don't think you can figure this out by watts. There are so many differences in how the light is projected from the fixture. For comparison, I'd go by a manufacturer's rating or "1/125th at f11 at iso 100 at 10 feet".

I've built several fluorescent lights (and owned several commercial made units) to use with scanning cameras. Its hard to get lots of light. One good part is that you can place the lights very close to the subject. But they'll be big and heavy and kind of oppressive or intimidating to some folks.

[BarryS]

Most manufacturers have photometric data (in foot candles or lux) available if you're looking to get a ready made fixture. I have a Kino Flo Diva 400 with 4 X 55w lamps and the output is much lower than normal studio strobes. You have to get it pretty close because the light drops off very fast. I use it primarily for video work, and if you're looking to avoid time exposures, I'm not sure it would be a good match for large format.