There are only two places to put a projector relative to a surface, and the choice determines what the stage can do for the rest of the evening. Front projection puts the light on the audience side and buys a shadow. Rear projection puts it behind and buys a depth requirement. Neither is free, and productions routinely pick one for a reason that turns out to be about the equipment rather than about the staging.
The shadow line is a blocking constraint, not a technical detail
Anything between the lens and the surface appears on the surface. The question is therefore where a performer may stand, and the answer is fixed by the geometry: a line across the stage, downstage of which a body intercepts the beam. A high projector position pushes the line upstage and shrinks the shadow; a low one pushes it downstage until most of the acting area is unusable.
This belongs in the room when blocking is being set, not afterwards. A director who learns during the technical rehearsal that the duet cannot be played where it was rehearsed is being asked to restage under time pressure, and the alternative on offer is usually to lose the projection.
The shadow is not always a problem. Placed deliberately it is one of the cheapest effects available, and a silhouette against a lit surface costs nothing and reads from every seat. What ruins a production is an unplanned one.
Rear projection removes the shadow and takes the space
With the projector behind a translucent surface, nobody on stage can interrupt the beam. Performers may work right up against the material, which is exactly what front projection forbids.
The price is measured in metres. The lens still needs its throw distance, and that distance now sits behind the screen where scenery, crossovers and storage used to be. Short throw optics reduce it, and a mirror folds the path into a shorter run at the cost of alignment sensitivity, another surface to keep clean, and an image that has to be flipped in software before anyone can read it.
Hotspot, and why the middle of the house sees a different image
Rear projection surfaces diffuse light passing through them, and how evenly they do that is the whole specification. A material that transmits efficiently tends to transmit directionally, so a viewer whose eye is roughly in line with the lens sees a bright patch at the centre of the image while the rest of the house does not.
More diffusion evens it out and throws away light. This is the same gain against viewing angle trade described for front screens, and in a theatre, where seats are spread wide, the wide and dimmer material is usually the right choice even though it measures worse.
Ambient behaves differently on the two systems
Front projection surfaces are chosen to reflect, which means they reflect stage light as enthusiastically as they reflect the image, and every wash that touches them raises the black level. Rear projection materials can be specified to absorb a large part of the light arriving from the audience side while still transmitting from behind, which is why darker rear screens hold contrast under stage light in a way a white front surface cannot.
That advantage disappears if the surface is lit from behind by anything other than the projector, so the space behind a rear screen has to be run as a dark space, with worklight, running lights and crew torches disciplined accordingly.
The gauze case, which behaves like neither
A scrim or gauze occupies a category of its own. Lit from the front it reads as a solid surface and takes an image. Lit from behind it turns transparent and reveals whatever is upstage of it. A production can therefore project onto it, then dissolve through it into a scene, using one piece of cloth for two purposes.
The catch is that it does neither job as well as a dedicated surface. The weave scatters and the image is softer and dimmer than the same projector would deliver onto proper material, and the transparency depends on keeping light off the front, which constrains the lighting state on both sides at once. It is a genuine effect rather than a compromise to be avoided, but it should be chosen for what it does rather than as a way of avoiding a decision.
Two practical costs nobody budgets
A projector behind a screen is close to the stage, and it has fans. In a quiet passage the machine is audible to the first rows in a way it never is from a booth at the back of the house.
And a rear surface has to be flat. Wrinkles, slack and draught show as distortion in a way they do not on a front cloth carrying reflected light, so the material needs tensioning and needs to be kept out of moving air.
What we cannot verify
Gain, half angle and ambient rejection figures for screen materials are published by their makers, measured under laboratory conditions and not comparable between suppliers. We reproduce none of them. Whether a given material holds up in a given house is answered by hanging a sample and looking at it from the corner seats with the show states running, which suppliers will generally provide for exactly that purpose.
The short version
- Front projection defines a line downstage of which a body becomes a shadow.
- The shadow line is a blocking constraint and belongs in rehearsal, not the technical.
- A deliberate silhouette is cheap and effective; an accidental one is not.
- Rear projection removes the shadow and consumes the depth behind the screen.
- Efficient rear materials hotspot; even ones throw light away, and usually win in a theatre.
- Rear surfaces can absorb front-incident light, which front screens cannot.