Field Guide: A Projector Cannot Project Black

Everything worth knowing before you specify a projected image, in plain language. No prior projection experience assumed.

This is a condensed version. The full field guide is a 20-minute read with charts and worked examples.

The One Idea Everything Else Follows from

A projector can add light to a surface, but it cannot remove light that is already there. Black, in projection, is not something the projector produces. Black is what is left over.

Three things add unwanted light to a black frame:

The projector itself. Every projector leaks a little light even when showing black.
The light already in your venue. Windows, house lights, exit signs.
The picture bouncing back at itself. Light from the image reflects off walls, ceiling and floor, and some returns to the screen.

Added together, these make the black floor: how bright the screen is when the content is black. Detail below the floor does not look subtle. It does not exist.

What the Surface Does

A screen has no power supply; it can only decide how much of the light landing on it comes back. A white screen returns the picture and the ambient light with equal enthusiasm. A black absorptive surface returns the projector\u2019s image at a useful level while absorbing light from everywhere else.

The most useful counterintuitive fact in the discipline: in a bright space, a darker surface usually looks better than a lighter one. A lighter screen pushes back the ambient light too, so you gain some white and lose the entire bottom of the picture. Keep the surface dark and push the projector instead.

If You Are Coming from LED

Brightness is quoted at the wrong end. LED panels are specified in nits (brightness at the surface). Projectors are specified in lumens (total light from the lens). Lumens only become brightness once you choose an image size: the same projector is four times brighter on an image half as wide.
Black is earned, not switched off. On an LED wall, black is an emitter that is off. In projection, black is a physical result of leak, ambient, and bounce.
Pixel pitch works differently. Projected pixels have no gap between them, so a projected 2 mm pitch reads smoother than a 2 mm LED wall at the same distance.

The Projector, Honestly Assessed

Lumens: total light out of the lens. Divide by image area to get brightness on the surface.
Laser or lamp: lasers hold their rating far longer and leak less on dark scenes. Lamps fade from their first hour.
Native resolution: only the imager counts, not the highest signal the projector accepts.
On/off contrast: the one specification that changes your black floor. Ask for the native figure, with no dynamic tricks.

Size Is the Brutal Number

Brightness falls with area, and area grows with the square of the diagonal. Double the diagonal and you have quartered the brightness. A 20% larger diagonal is about a third of your brightness gone. Every "could we make it a bit bigger" has a brightness cost.

For sharpness: divide image width by pixel count to get the pitch. As a rule of thumb, structure disappears at about 3.4 meters of viewing distance per millimeter of pitch.

Using More Than One Projector

Arrays make pictures dramatically brighter. Two projectors side by side each cover a quarter of the area each would alone. Splitting the work beats buying one enormous machine.
A blend is not a seam. Where projectors overlap, both ramp down so the sum matches. Done well it is invisible.
Blending does not add resolution, and the dimmest projector sets the level for every zone it touches.
Stacking (two projectors on the same zone) roughly doubles brightness and adds redundancy, but every machine adds its own leak.

The Venue Is Part of the Optical System

Measure the light at the surface, flat against the screen position, facing the projector. A reading taken anywhere else is decoration.
Budget for bounce. Pale walls, ceilings and floors return the picture\u2019s own light onto the screen. Dark drape and dark carpet are the cheapest contrast upgrades there are.
Mind the color of the ambient light. The eye tolerates a slightly raised black and hates a tinted one.

Reading the Picture: Three Numbers

Image white: the brightest white this projector puts on this surface. Cinema reference is 14 foot-lamberts in a dark house; a lit space wants considerably more.
Black floor: leak + ambient + bounce. Whichever part is largest tells you what to spend money on.
On-screen contrast: image white divided by black floor, in your venue. This is the number people mean when they say a picture has depth.

Doing It

The Configurator on the full site makes this guide operational: answer four questions and every number above is calculated live. It produces a short seed code that carries your whole configuration; send it with your finished size and a quotation comes back with a drawing and a lead time.

Questions the guide does not answer: info@qualia-forge.com.

Request a demo