LED wall or projection: how the room settles it

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: V201.01:2021, Image System Contrast Ratio, a revision and redesignation of ANSI/INFOCOMM 3M-2011, Projected Image System Contrast Ratio. 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.

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.

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.

Screen luminance in foot-lamberts equals ANSI lumens multiplied by screen gain, divided by the image area in square feet. One foot-lambert is 3.4263 candela per square meter, which is to say 3.4263 nits. That is the same formula §02 used, applied now to named hardware and to a different screen, so the two sets of numbers complement each other rather than conflict. Everything below is at gain 1.0, on a matte white surface.

A projector’s weight and amperage are flat with image area, while its brightness falls in proportion to it. An LED wall’s brightness is flat with image area, while its weight and amperage rise in proportion to it. Quadruple the picture and the projector is unchanged and a quarter as bright. The wall is as bright as it was, and four times the weight and four times the current.

So projection wins on weight, on amperage, on image area per kilogram, and on landing a picture on something that is not a flat rectangle. LED wins on brightness that does not fall as the picture grows, on a black that is an unlit pixel, and on needing no clear light path. The first list is the case for projection in a space that can go dark, and nothing above weakens it.

None of this makes a projector the wrong tool outdoors. After dark, under cover, or in a built enclosure, the weight and amperage figures in §05 are the argument, and they favor projection. What the sheets settle is narrower: where the picture is exposed to the weather, one product publishes an ingress rating and a wider envelope, and the other publishes neither.

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. Outdoors, add what each product publishes for weather. 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.