Nobody is choosing a display. They are choosing a contrast ratio.
There is a published standard for how much contrast a viewing task needs, and it is the thing the decision is actually about.
A projected image and an LED wall are two ways of putting the same picture in front of the same audience. What separates them is not resolution and it is not really brightness either. It is what each one does with the light that is already in the space before anything is switched on.
AVIXA, the audiovisual industry body, publishes a standard for this. It sets four contrast ratios according to what the audience has to do with the image: 7:1 for passive viewing, where a viewer needs to recognize an image and read text but not study it; 15:1 for basic decision making, where simple decisions are made from the content without depending on fine detail; 50:1 for analytical decision making, the medical and engineering case; and 80:1 for full-motion video, where the point is to follow what is happening on screen.
Those four numbers are the useful frame, because they replace a matter of taste with a target. A keynote with slides is a basic decision making image. A sizzle reel, a live camera feed of the stage, or an awards package cut to music is full-motion video. Those are different jobs and the standard says so in numbers.
Ambient light does not dim the picture. It raises the black.
The arithmetic, with every step shown, using a published formula and a published lighting level.
The luminance of a projected image is the projector output divided by the screen area, multiplied by the screen gain — foot-lamberts from lumens, square feet and gain. Take a general session screen 16 feet wide. At 16:9 that is 9 feet tall and 144 square feet of surface. Put a 20,000-lumen projector on it, on a plain unity-gain screen: 20,000 divided by 144 is about 139 foot-lamberts of peak white. By cinema standards that is very bright — the SMPTE figure for a light-controlled theater is 16 foot-lamberts, and digital cinema is specified at 14.
Now turn the lights on. A conference space is lit to roughly 30 foot-candles. Light landing on a matte white screen comes back off it at approximately the same number in foot-lamberts, and it comes back off every part of the screen equally. It lands on the white and it lands on the black, which is the half of this that gets missed.
The projector’s own black is nothing by comparison. Even at a native contrast of 2,000:1, black is 139 divided by 2,000, or about 0.07 foot-lamberts. So in the lit space the image measures about 169 foot-lamberts at white (139 plus 30) and about 30 at black (0.07 plus 30). That is a contrast ratio of 169 to 30, or 5.6:1.
5.6:1 does not reach 7:1. A 20,000-lumen projector on a 16-foot screen, in a normally lit conference space, does not clear the lowest category the standard defines — the one for an audience that only has to recognize what it is looking at.
The honest caveat is that 30 foot-candles is the level measured flat on the table, and rather less of it reaches a vertical screen, so the real figure is lower and has to be measured rather than assumed. Take a third of it. At 10 foot-lamberts of ambient the same screen measures 149 at white and 10.07 at black: 14.8:1. That clears passive viewing comfortably and lands just under basic decision making. It is nowhere near the 80:1 that full-motion video asks for.
The instinct at this point is to order a bigger projector, and the arithmetic is unkind about it. To reach 80:1 against 30 foot-lamberts of ambient, peak white has to be 2,370 foot-lamberts, which across 144 square feet is roughly 341,000 lumens. To reach even 15:1 takes about 56,000. The ambient light sets the floor, and no projector removes a floor.
The difference is what the black is made of.
Same space, same white level, and an answer that differs by more than an order of magnitude.
A projection screen is a reflector. Its job is to send back as much as possible of whatever hits it, and it cannot distinguish between light from the projector and light from the chandeliers. An LED wall emits. Its black is an unlit panel, and what the room does to that black is limited to whatever reflects off the panel face.
A panel face is dark and largely matte, so call its reflectance 5 percent — a deliberately conservative figure, and the assumption worth arguing with if you want to argue with the conclusion. Thirty foot-candles falling on it comes back as about 1.5 foot-lamberts rather than 30.
Hold everything else equal. Give the wall the same 139 foot-lamberts of white the projector managed, which is an undemanding brightness for LED. White measures about 140 and black about 1.6, and the contrast ratio is roughly 87:1 — past the threshold for full-motion video, in the same lit space where projection came out at 5.6:1.
Move the assumption and the conclusion holds. At 10 percent panel reflectance the wall still measures about 46:1. At 20 percent, which no rental panel has, it is 24:1 — still better than projection managed at a third of the ambient. The gap is structural rather than a matter of degree, and it does not close by buying a brighter projector.
None of which makes projection the wrong answer. It makes it the right answer in a space where the light can be controlled, at sizes where covering the same area in panels is disproportionate, and for content that is mostly static and mostly light — a slide deck on a white background in a room that can go dark is a job projection does well and has always done well.
Four questions, and none of them is which technology.
Answer these and the choice has already been made. Every one is knowable weeks ahead of the event.
- What does a light meter read at the screen position?
Held vertically, facing the audience, with the lights as they will actually be during the session. Not the level in the specification and not the level with the house lights up for load-in. This single number does most of the deciding, and it takes a minute to take.
- What is the audience doing with the image?
Reading a slide, making a decision from a chart, or watching video. Those are the standard’s categories and they ask for 7:1, 15:1 and 80:1 respectively. An event that ends on a sizzle reel is a full-motion video event however many slides preceded it.
- Can the lights come down, and will anyone allow it?
This is the question that most often settles it, and it is rarely a technical one. Cameras need faces lit. Servers need to see plates. An audience taking notes needs to see paper. A space that cannot go dark has ruled on the technology whether or not anyone framed it that way.
- How far back is the last row?
Distance sets the image size before anything else, and image size is where the two technologies separate on cost rather than on physics. It is a separate calculation from this one and it runs the other way — the larger the surface, the more often projection is the proportionate answer.
The technology is the last thing decided.
Measure the light at the screen position, name what the audience has to do with the image, and find out whether the lights are allowed to come down. Those three answers pick the technology between them, and they can all be had before anyone quotes anything. What they cannot be replaced by is a bigger projector, because ambient light raises the black and a raised black is not something more lumens can reach past.
Where the figures come from
- Foot-lamberts = (lumens ÷ screen area in ft²) × screen gain — the standard relationship between projector output, screen size and screen material, as published by Acoustic Frontiers. Every luminance figure in §02 is this formula with the numbers stated beside it.
- 16 and 14 foot-lamberts — SMPTE 196M for a light-controlled theater, and the Digital Cinema System Specification at 14 ±3, both as cited by Acoustic Frontiers. Given in §02 only to show the size of the number, not as a target for an event.
- 7:1, 15:1, 50:1 and 80:1 — the four viewing categories — passive viewing, basic decision making, analytical decision making and full-motion video — from the AVIXA/ANSI image system contrast ratio standard. Category names per AVIXA’s own release; the ratios as published by Dataton.
- 30 foot-candles — the average maintained horizontal level for a conference room in the Lighting Design Lab footcandle guide. §02 states plainly that this is measured flat on the table and that less reaches a vertical screen, and works the case at a third of it as well.
- 5 percent panel-face reflectance — an assumption, not a measurement, and named as one in §03. The section works the same arithmetic at 10 and 20 percent so a reader can see how much of the conclusion depends on it. The answer is: not much.