Plywood in being flammable won't make a very good
donut as is described by SketchyCroftPpl. Also given the nature of a
PAR lamp, you can't really add a
donut with ensuring the
spill will be proper.
A say 100w
PAR 38 NSP lamp will have a beam width of 2.62' at it's
beam angle - that
point where the
field angle of the beam drops down to 50% given a beam perpendicular to the surface. This thanks to the free GE Light Beams program. Differentuate the
spill field angel of light at less than 50%
intensity from the
field angle that's down to 10% light. That less than optimal light you can clean up.
The
PAR 38 spot in most cases for a say 100w lamp, will have the
foot candles on the surface of 75.6fc. Remember however you are not lighting the floor, you are lighting the talent that is about five feet above the
deck. This means that the
intensity at a now shorter distance will be a smaller angle. The
beam angle of 10 degrees on a
PAR 38 will only be about 1.75 feet at 10'.
Remember however that perpendicular distance is different than hang angle distance to the
stage in being a trigonomitry factor of A(2) + B (2) = C (2) in being the diagional angle to the surface in not even accounting for the second use of this figure due to the angle to the
center line of the
stage.
Given these two factors, your distance will probably end up being about your requested three
foot in dia. given the
fixture type.
Most ray lights work on a four instead of the spot's 10 degree
beam angle. It's
field angle is even more narrow in no chance of spreading at this distance sufficiently. Such lights won't work well at this distance. Most ray lights will be set up for a
PAR 36 instead of
PAR 38 anyway which while it might fit within the
fixture once you cut a slot for a
PAR 36 lamps' keyway, it might also not. Wiring on such a ray light
fixture is also dependant upon the manufacturer but expect it won't just screw in.
Also there is AR-111 lamps in the size of a
PAR 36 that are like ray light lamps only they have both lamp and
reflector plus a exterior center
filament shield included in them. Such lamps also would conform to about a
PAR 36 iin being just 1/4" less in dia. but for
fixture size would have a better output than a
PAR 38. There is a 8 degree versions of AR-lll available that would get similar to a
beam angle on the
PAR 38 spot.
Say on a 8 degree AR-111 from Philips or Osram having 48,000
Candlepower at 100 watts/12 volts - (you provide the
transformer), verses say 40,000
Candlepower for a 250w
PAR 38 lamp as the largest
PAR 38 available and something that might melt down the wiring to the
PAR 38
fixture. Sorry but I don't have the figures for a ray light output given it's normally too small of a
beam angle anyway.
Beyond these, if your light
fixture can accept a
PAR 36 lamp, one might install say a FBE lamp into it that has a 5,000K
color temperature and 36,000
Candlepower, or a FBO that has 3,400K and 67,000 candle
power for a 25 x 15 degree beam of light. You can expect the normal
color temperature to be about 3,200K. Granted you would have to
barn door or mask the beam in some way due to it's slightly larger than wished for width. Black Tack Tape - a adhesive backed black aluminum foil tape works well for this in creating barn doors, otherwise
City Theatrical should have some
PAR 38 barn doors on the market.
The barn doors or
top hat in lengthing
spill light clean up of the beam of light will clean up a
PAR beam as opposed to a
donut that encloses the light beam but relies upon a
Leko's control of the beam to control beam angles not at the
gate it's limitating.
In other words, a few small coffie or large soup cans will form a better control of your light beam than a
donut/plywood circle on a
PAR lamp. Also when in addition to a
donut such a
top hat will help with a
Leko.
Why go with a spot lamp instead of a MFL? While you can cut off the
beam angle by way of
barn door or
top hat, the
intensity across the area at a certain distance will be much less than the
intensity of a lamp with a smaller coverage in area due to a set amount of light given off distributed across a narrower angle.
An alternative to these lamps might be a #Q4631
PAR 36 sealed beam lamp. It has a
beam angle of 13x12 degrees which is right in your 15' figure, and a candle
power of 80,000
candlepower. Granted you have to get a
transformer that will run a 250w/13v lamp.