A projector cannot make black. It can only decline to add light, and whatever the room is already putting on the surface stays there. That single fact governs every projection decision taken in a theatre, where the room is not dark and cannot be made dark, because the stage is lit and the stage is usually the thing next to the screen.
Contrast is set by the floor, not by the ceiling
An image is legible because its brightest and darkest parts differ. The bright end is the projector's business. The dark end is the room's, and it sits wherever ambient light puts it. Add spill to the surface and the floor rises; the peak does not move; the distance between them closes.
This is why contrast ratios printed on a specification sheet describe a condition no theatre has. They are measured in a blacked out room with nothing else emitting, and a machine claiming an enormous ratio delivers a small one the moment a wash lands on the cloth.
The audiovisual standards bodies handle this properly by specifying contrast at the viewing position rather than at the machine. The relevant standard sets out categories of use, from material that is merely being looked at through to full motion video, with the required ratio rising as the task gets more demanding. It is published by a professional body rather than by a manufacturer, and it asks the useful question: what ratio arrives where people are sitting.
Where the ambient actually comes from
In a lecture room it is daylight and downlights. In a theatre it is almost always the production's own lighting. A backlight with a wide enough field, a wash that overshoots the acting area, a practical on the set: each of them lands some quantity on the projection surface and raises its black level.
The consequence is a scheduling one. Projection content is normally assessed in a dark room, lighting is assessed with the projection off, and the two meet for the first time in the technical rehearsal, at which point the available remedies are moving a fixture, moving a surface, or accepting a washed out image.
Unlike sound, more power does help, up to a point
The sound section of this publication makes much of the fact that raising the level does not improve the ratio of direct to reverberant energy, because both rise together. Projection is not analogous, and it is worth saying so plainly. Ambient light on the surface is fixed by the lighting state, so raising the projector's output does raise the top of the range while the floor stays where it is. Brightness genuinely buys contrast here.
What it buys it with is money, heat, fan noise in a quiet house, and a machine large enough to become a rigging problem. The economics turn over quickly: removing a few hundred lux of spill from the surface usually costs a barn door and ten minutes, and buys more than the next size of projector.
Screens that reject light, and what they take in exchange
Surfaces exist that reflect light arriving from the projector's direction while rejecting light arriving from elsewhere, whether by angular structure or by selecting wavelengths. In the right geometry they work well.
The exchange is the viewing cone. Rejection is directional, so a surface that looks excellent from the centre block can be dim, tinted or uneven from the sides, and a theatre seats people at angles a conference room does not. This is the same trade described for high gain screens in the article on throw ratio, arriving by a different route.
Masking is cheaper than any of it
Before anything is bought, there is usually light landing on the surface that nobody wants there. Shutters and barn doors on the fixtures that overshoot, a steeper angle on the wash, a border that stops spill from the wings, and in touring venues the exit signage and worklight that nobody has ever switched off during a performance because nobody knew it was on.
None of that is glamorous and all of it is free. The measurement worth taking early is a meter reading on the surface with the show states up and the projector off, which tells you what floor the room is imposing before anybody argues about lumens.
The emissive alternative
A light emitting wall does not reflect ambient at all in the way a screen does, so its black level is far less affected by the room. That is a structural advantage rather than a marginal one, and it is why so much work that used to be projected is now emitted.
It is paid for in pixel pitch, which sets how close an audience may sit before seeing the structure; in weight and power, which are rigging and distribution questions; in the panel's own refresh behaviour when a camera is present, which is the subject of the article on flicker; and in cost per square metre that is not comparable to a projector and a piece of cloth.
What we cannot verify
Lumen output, contrast ratio and ambient rejection figures are published by the manufacturers of projectors and screens, under conditions that are usually unstated and are not comparable between products. We reproduce none of them. The standard referred to above is named rather than quoted, because its categories carry conditions that a single number extracted from them would misrepresent. The only assessment worth acting on is a measurement of the surface under the actual lighting states, from the actual seats.
The short version
- A projector cannot make black; ambient light sets the floor of the image.
- Specification contrast ratios describe a blacked out room that no theatre is.
- Standards bodies specify contrast at the seat rather than at the machine.
- In a theatre the ambient is usually the production's own lighting.
- Unlike sound, more output does buy contrast here, but badly per unit of money.
- Rejecting screens and emissive walls both trade the problem for a different one.