I have used a light curtain before. It is a very thin fan of light coming from an upstage source, and visible because of a heavy
haze (or well dispersed
fog). Of course, lasers are sometimes used this way, but as soon as you start aiming powerful lasers where people can go, the rules and laws get understandably tougher.
The
laser will allow the swirls of
smoke to be outlined, but you don't need a super thin beam for the "light curtain"
effect. Just what a "derby" disco light puts out is fine, and perhaps puts out more total light than the lasers used for this type of
effect. I have an old non-moving fanned "derby" light that has a 5inch
bulb. It looks very much like a
laser type beam. Other, more contemporary, but non-led versions can have their motors disabled for the same
effect.
I should mention, that the above is assuming a very dark theater, and a black
stage. But, but I have not tried this with white clad actors. If they show up, you can hide them by using black-clad ninja-crew holding up a black cloth between their hands. They simply
drop the cloth when the actors need to step forward of the beam.
When the actors step in front of the light-curtain, they will be seen in silhouette. But, it may be possible to have a
bit of side lighting on that area of the
stage. Note that it would need to be well controlled to avoid putting light upstage of the light-curtain before their
reveal.
Visible light sources around the
stage, as long as they don't cast much light on the hidden areas, will also help make it harder to see into the shadows.
Regarding the
monochromatic light approach, I do recall reading about it. The idea is that an actor dressed in red, standing in front of a white background will somewhat disappear if they are illuminated with a
monochromatic red light, since both they and the background will be the same color. But, if you
switch to a white light, you will see red actors on a white background. If you
switch to a
monochromatic blue light, then the actors will be black, against a blue background.
If my recollection is correct, this all was some very early experimentation in the development of theatrical lighting. The whole
monochromatic thing is unnecessary for the
effect if you can light the actors and the
backdrop separately. Something we take for granted nowadays. Though, I imagine you still need your actors dressed in a single color.
Joe Dunfee