Rear projection or front projection

Draper’s TecVision XH1200X is a front-projection surface. Its page publishes “Gain: 1.2” and a “Recommended viewing cone: 150°.”

Draper’s CineFlex CH1200V is a rear-projection surface. Its page publishes the same “Gain: 1.2”, a “Half-gain angle: 30°”, and describes a “Controlled viewing cone.”

Draper’s own classification sets the scale those sit on: a viewing cone marked X is “Extra-wide. Half gain is greater than 60°”, M is “Moderate. Half gain is 30-60°”, and C is “Controlled.” The rear surface is the controlled one.

Half-gain angle is the angle off centre at which brightness falls to half the on-axis figure. Seat a general session 50 feet back from the screen and the outside seat of a 60-foot-wide block sits about 31 degrees off axis — at the edge of a 30° cone, and well inside a 150° one.

CineFlex White XT700V publishes “Rear projection gain: 0.7. Front projection gain: 0.5. Half-gain angle: none”, with an “Extra wide viewing cone with typical contrast.”

So inside one manufacturer’s rear-projection line the choice is already set out twice: 1.2 gain with brightness halving by 30° off axis, or 0.7 gain with no half-gain falloff published at all.

Going from 1.2 to 0.7 leaves on-axis brightness at 58% of what the brighter surface returns. Widening the cone costs about 42%, and that comes back out of the projector’s lumen budget.

Both CineFlex surfaces publish “Maximum height without seams: 136” (345 cm).”

At 16:9 that height carries 241.8 inches of width, a little over 20 feet, in a single unseamed piece.

Wider than that and the surface is seamed or it becomes two surfaces. Either answer gets settled at the drawing stage rather than on the load-in.

Christie publishes the bands. An ultra short throw projector “has a throw ratio between 0.25-0.5:1”, and short throw projectors “have a throw ratio between 0.5-1:1.”

Draper’s XH1200X page asks for a “throw distance ratio for best brightness uniformity” of “1.3:1 or longer.”

Run the 20.1-foot image from section 03 through those numbers: a standard lens at 1.3:1 wants about 26 feet of depth behind the screen, a short throw at 0.5:1 about 10 feet, an ultra short throw at 0.25:1 about 5.

That depth is masked depth. Whatever stands behind the screen gets drapes, or the audience sees it lit by the projector’s own spill.

In front projection the light path crosses the audience space. Anybody who steps into it puts a shadow on the screen and wears the slide while they do it. On a stage where an award changes hands in front of the image, that is routine rather than occasional.

Rear projection takes that away, and pays for it in the masked depth from section 04 and the viewing cone from section 01.

Christie states the same trade from the other side: “In applications where rear projection isn’t possible, a UST projector is a great choice.” A short-throw lens shortens the stretch of audience space the beam has to cross, and needs no depth behind the screen at all.

So a survey takes two distances and one width: how deep the space behind the screen position runs, how far back the last seat sits, and how wide the seating block spreads. The first number decides whether rear projection is available. The other two decide whether a 30-degree cone covers the house.