A Projector Cannot
Project Black.

Why Black Is the Only Interesting Problem

Point a projector at a white wall in a properly dark space and play a night scene. The sky will be gray. Not dark gray, if the projector is a big one. Gray.

This surprises people, and it should, because everything about the way projectors are sold suggests the opposite. The brochure quotes a contrast ratio with four zeros in it. The demo in the hotel suite looked fine. And yet here is the night sky, sitting there like wet concrete.

The reason is simple enough to state in one line, and the rest of this guide is really just that line worked out in detail. A projector is an additive device. It can put light onto a surface and it can decline to put light onto a surface, but it cannot reach out and remove light that is already there. Black, in projection, is not something the projector produces. Black is what is left when everything that could add light has failed to.

Three things add light to a black frame, and they are worth naming now because you will meet them again:

• The projector itself. Every imager leaks. Asked for black, it still passes some light down the barrel and onto the surface. This is the leak, and only the projector can reduce it.
• The light already in your venue. Windows, house lights, exit signs, the laptop on the lectern. It lands on the surface, and the surface returns a share of it to the audience.
• The picture, coming back at itself. Light you deliberately threw at the surface bounces off it, hits the walls, ceiling and floor, and a portion of that comes straight back onto the screen. Big bright images in pale spaces feed themselves.

Add those three together and you have the black floor: the brightness of the surface when the content is showing nothing. Every shadow in every frame you ever play has to live above that floor. Detail below it does not look subtle. It does not exist.

A screen cannot make an image brighter. It can only decide how much of the light that lands on it comes back, and where it goes.

Which brings us to the thing a projection surface is actually for. A screen has no power supply. It cannot add anything. Its entire contribution is a decision about return: how much of the light hitting it comes back toward the audience, and how much is absorbed and never seen again. A white screen is generous with everything. It returns the picture beautifully, and it returns the exit sign, and the daylight from the atrium, and the bounce off the ceiling, with exactly the same enthusiasm. It cannot tell them apart.

A black surface can, at least partly. It is engineered so that light arriving from the projector direction is returned at a useful level while light arriving from everywhere else is largely absorbed. That is the whole trick, and it is why the range is described by how much it absorbs rather than by how much it reflects.

Here is the part that catches people out, and it is worth reading twice. In a bright space, a darker surface usually looks better than a lighter one. Instinct says the opposite: there is a lot of ambient light about, so surely you want a screen that pushes plenty of light back. But a lighter screen pushes back the ambient light too, and the ambient light is arriving from all directions at once. You gain some white and you lose the entire bottom of the picture. Push the projector instead, keep the surface dark, and the picture stays legible. This is the single most useful counterintuitive fact in the discipline, and it has a chart of its own later on.

If You Are Coming from LED

A lot of people arrive at projection sideways, from the direct-view LED world, because a client asked for something a wall could not do: a 30 meter image, a curved surface, a scenic piece that has to disappear when it is not lit, a budget that stopped somewhere short of the tiles. If that is you, most of your instincts transfer. Three of them will actively mislead you, and it is worth getting them out of the way before anything else.

Brightness Is Quoted at the Wrong End

An LED panel is specified in nits, which is brightness at the emitting surface. It is a property of the panel. Build a wall twice the size and every square meter is still doing the same number of nits, because you added emitters along with the area.

A projector is specified in lumens, which is total light leaving the lens. It is not brightness. It becomes brightness only when you decide how much area to spread it over. The same 20,000 lumen projector is four times brighter on a 300 inch image than on a 600 inch one, because the area went up by four while the light stayed the same. Nothing about the projector changed. You just asked it to do more.

This is why the size question and the brightness question are the same question, and why a lumen figure on its own tells you almost nothing.

Black Works Differently Here

On a wall, black is an emitter that is off. It is very nearly zero, and the only thing lifting it is ambient light falling on the panel face and reflecting off the plastic. Contrast figures are enormous and mostly honest.

In projection, as we have just covered, nothing switches off. The projector leaks, the venue contributes, and the surface returns some of both. Your black is a physical result, not a state. This has a pleasant consequence, mind: the black of a good projection surface with the projector idle is a genuinely dead, matte, non-emissive black. It does not glint. Scenically it beats a dormant wall, which always looks like switched-off electronics, because it is.

Pitch Does Not Mean What It Means to You

An LED wall has a physical gap between emitters. That dark grid is part of the picture, and it is why a 2 mm wall looks like a wall until you back away from it. Projected pixels have no gap at all. They are laid edge to edge by the imager, so a projected 2 mm pitch reads noticeably smoother than a 2 mm wall at the same distance.

The catch is that projected pitch is not something you buy. It falls out of arithmetic: imager resolution divided by image width. Want a finer pitch? Use a higher resolution projector, make the image smaller, or add projectors and give each one less area to cover.

And Two More Differences Worth Knowing

Tiles butt together; projectors overlap. There is no such thing as a seam in a blended array, only a band where two projectors are both painting and both ramping down so that the sum comes out right. That band costs you image area and demands stable rigging, but done well it is genuinely invisible, which no tile joint has ever quite been.

And the picture is manufactured somewhere else in the venue. Every projector needs a position, a sightline that people will not walk through, a throw distance, power and access. A wall needs a wall. This is the real price of projection, and it is paid in the drawing rather than in the invoice.

The Projector, Honestly Assessed

Four things about a projector matter for this exercise, and the one printed largest on the box is the least interesting of them.

Lumens

Total light out of the lens. Use the manufacturer's figure to start with, then be a little suspicious of it. Measurement standards vary, the unit measured is not always the unit that ships, and the number is quoted for a fresh machine in a laboratory with no dust in it.

The useful mental model: lumens divided by area gives you illuminance on the surface, and the surface then decides what fraction of that comes back to the audience as luminance. Everything else is detail.

Laser or Lamp

Lamps are rated at their first hour. They fade continuously from there, and a lamp machine well into its life is simply not the projector on the datasheet. If you are specifying against a lamp projector that already exists in the venue, find out how many hours are on it before you trust the number.

Lasers hold much closer to their rating for much longer, and they behave better at the dark end. A laser can be driven down toward nothing, while a lamp burns at full output all evening and relies on the imager to block whatever the picture does not want. Blocking is never perfect, which is why lamp systems generally leak more light onto a black frame.

Native Resolution

The pixel count the imager actually lays down, which is not always the highest signal the projector will accept. Plenty of projectors take a 4K input and display it on an HD imager. That is a legitimate product, but for our purposes only the imager counts, because the imager is what lands on the surface.

With image width, native resolution gives you pixel pitch, and pitch gives you the distance at which structure stops being visible.

On/Off Contrast, and Why It Is the Number to Ask for

On/off contrast is full white divided by full black from the same machine without touching anything. It sets the leak. It is the only projector specification that changes your black floor.

Ask for the native sequential figure: no dynamic iris, no laser dimming, no clever frame-by-frame tricks. Dynamic systems produce spectacular numbers on a full black frame, which is a shot that appears in almost no real content. The moment a single lamp appears in the corner of a night scene, the dynamic system has to open up and you are back to the native figure. Imager technologies differ here by more than an order of magnitude, so it is worth knowing which one you have.

The Lens Is Part of the Black Level

Glass scatters. A little of the light passing through any lens ends up somewhere it was not aimed, and some of that lands on the surface as a soft veil across the whole image. A clean prime at its design point keeps it low. A zoom worked hard at the end of its range, a very short throw squeezing the picture out at an extreme angle, or a lens that has spent two years in a dusty ceiling, all raise it. It is not a large effect next to a leaky imager, but it is real, and it is one of the few things on this list you can improve with a cloth.

The Image: Shape, Size, and the Pixels That Fall Out of It

Get the Shape Right or Pay for It in Light

The image format should match what the projector is putting out. When it does not, part of the imager is lighting nothing at all and you have paid for that part. A 16:9 projector filling a 2.39:1 scope image is using about three quarters of its chip; the remaining quarter is producing black bars that land above and below your screen. If the shapes genuinely have to disagree, an anamorphic lens or a native scope imager buys the light back, at a price.

Every Inch Costs Light

Brightness falls with area, and area goes up with the square of the diagonal. Double the diagonal and you have quartered the light per unit of surface. This is the arithmetic that quietly kills more projected images than anything else, because image size tends to grow during a project while the projector budget does not.

Pixel Pitch and the Arcminute

Divide the image width by the horizontal pixel count and you have the pitch: the distance between pixel centers on the finished surface. The human eye resolves detail down to roughly one arcminute, which works out at about 3.4 meters of viewing distance per millimeter of pitch. Inside about one meter per millimeter, the pixel grid is unmistakably part of the picture.

So a 2 mm pitch disappears at around 7 meters and is visible at 2. That is a specification, not a preference, and it is the calculation to do before anyone commits to a front row position.

Cameras are less forgiving than audiences. A camera framed on part of the surface magnifies the pitch by its crop factor, and fine structure that no viewer would ever notice turns into moiré on the plate. If the surface is going to be photographed, judge the pitch through the lens that will be pointed at it, not from the seats.

More Than One Projector

At some point the image gets big enough that one machine cannot fill it with any conviction, and you start adding projectors. This is where projection gets genuinely enjoyable, and where it gets expensive if you are careless.

Why an Array Helps so Much

Two projectors side by side across a wide image each cover half the width, which is a quarter of the area each. Each one is therefore working over far less surface than a single machine would be, and the picture gets dramatically brighter. Splitting the work is much more effective than buying one enormous projector, which is why almost every large image you have ever admired was made by several.

The cost is the overlap, plus another machine to hang, power, align and keep aligned.

What a Blend Actually Is

Where two projectors overlap, both are painting the same physical area. If you did nothing, that band would be twice as bright as everything else and would look like a stripe of daylight through the middle of your picture. So both projectors ramp their output down across the band, in complementary curves, so that the sum comes out matching a single tile. Done properly the eye cannot find the join.

Wide bands forgive alignment drift, temperature movement and slightly soft rigging, and they cost you image area. Tight bands keep more pixels and demand better mechanics. Twenty per cent is the usual compromise; twelve per cent is for people with good trusses and better nerves.

Two things about blending that get assumed wrongly, often in writing, in specifications, by people who should know:

• Blending does not add resolution. Inside the band, both projectors are painting the same physical pixels. The surface gets one tile's worth of density, never the array's pixel sum. Four 4K projectors in a 2 × 2 array do not make a 8K image; they make a brighter image of whatever density the geometry gives you.
• The dimmer projector wins the band. If two machines in a shared band have different outputs, the pair has to be governed by the weaker one, or the band will not match. One tired projector quietly sets the level for everything it touches.

Stacking

Stacking is two or more projectors on the same zone, aligned pixel on pixel. It buys light, roughly doubling white for a pair, and it buys redundancy for a show that cannot go dark. It buys no resolution at all, since every machine is painting the same pixels in the same places.

And it raises your black floor, because every projector in the stack contributes its own leak. Two stacked machines are twice as bright and twice as leaky. In a dark venue, where the leak is the dominant term, stacking can cost you as much contrast as it gains you brightness. In a bright venue, where ambient light dominates, stacking is almost pure profit. Which of those you are in is exactly the sort of thing the configurator exists to tell you.

Throw Ratio

Throw ratio is the distance from the lens to the surface divided by the width of the image that lens is covering. A 2.0 throw on a 10 ft wide zone puts the projector 20 ft back. It is the number that decides whether your lens plot fits inside the building, whether the beam crosses a walkway, and whether the projector ends up somewhere a technician can reach it.

Covering a Zone: Three Honest Choices

When a projector's native shape does not match the zone it has been given, there are three ways out and each costs something different.

• Fill and mask. Overfill the zone and blank the excess. Costs light and pixels, keeps the geometry perfect. This is what most careful installations do.
• Fit inside. Land the whole imager inside the zone and accept unlit strips where the shapes disagree. Wastes nothing, leaves gaps unless a neighbour covers them.
• Stretch. Warp the image to the zone exactly. No light wasted, no pixels masked, but the pixels are no longer square. It shows first on fine text and on near-vertical diagonals, and it shows immediately to anyone who knows to look.

Layers, Portrait, and Layouts That Are Not Grids

In a real venue the tidy grid rarely survives contact with the architecture. A pillar, a balcony, a scenic element or a lighting position moves a projector, and suddenly you have a hero machine in the middle and two wings at odd angles. That is normal. What matters is being explicit about what each projector is doing: blending with its neighbour, or stacking on top of it. Two zones on the same layer blend. Put one on a different layer and it stacks instead. Getting that distinction right in the plan is most of the battle.

Turning projectors portrait is worth remembering too. Three portrait machines across a wide image give more height per tile and more vertical pixels than three landscape ones. Tall scenic surfaces are almost always done this way.

The Venue Is Part of the Optical System

Here is a habit worth forming. When you walk into a space you are about to put an image in, do not look at where the screen goes. Look at the walls, and the floor, and where the daylight comes from. You are standing inside the second half of the projector.

Measure the Light at the Surface

The number that matters is illuminance at the surface itself, with the meter flat against the screen position and facing the projector. Not at the seats, not at the lectern, not an average of the venue. A lux meter reads it in a second and a phone app will get you close enough to choose between the steps below.

If you work in foot-candles, one foot-candle is 10.76 lux. If someone quotes you a figure without saying where they stood, the figure is decoration.

The Bounce Nobody Budgets for

Your surface returns light into the space. The space hands a share of it straight back. That share lands on the screen as lifted black, and it scales with image area, which means it bites hardest on exactly the jobs that can least afford it.

Dark walls and a dark ceiling return almost nothing. A white hall with a large bright image can put more light onto the screen by bounce alone than the projector's own leak does. It is the most commonly ignored number in the whole discipline and often the cheapest one to improve: dark drape on the side walls in the right places will do more for your contrast than a projector upgrade costing thirty times as much.

The floor deserves its own mention. It looks like it should matter less than the ceiling, and it matters more, because the screen faces it across a shorter path and the first bounce arrives almost undiminished. Pale timber and polished concrete under a large image are real contributors. Dark carpet is close to free contrast.

The Color of the Light

Ambient light does not lift the black neutrally. Warm house lamps put an amber cast into the shadows. North light and cool LED work light put a blue one. The eye is remarkably tolerant of a black that is slightly raised and remarkably intolerant of a black that is slightly colored: it reads as dirty long before it reads as bright.

This is why two setups can measure the same and only one of them looks right, and it is why the color temperature of the ambient light is worth knowing rather than guessing.

The Surface Itself

Most screens are sold on gain. Gain is a comparison against a perfect diffuser: a gain of 1.3 means the screen returns 30% more light than that reference, in the direction the manufacturer measured. It sounds like free brightness, and it is not free at all. Gain is achieved by aiming: taking light that would have gone to the sides and steering it toward the middle seats. The center gets brighter, the sides get dimmer, and if the surface is at all directional you get hot spotting, where the middle of the image is visibly brighter than the edges from every seat.

These surfaces carry no gain figure, because they are not doing that. They hold an even hemisphere: what you see from the middle is close to what you see from the side, which is why the three quarter view in the configurator looks almost like the straight on view. The trade for that evenness is that there is no free brightness on offer. What you get instead is a decision about absorption.

Reading the Ladder

Each surface carries a grade, and the grade describes how much of the light landing on it is absorbed rather than returned. OP 0.8 absorbs a great deal and returns a little. OP 0.4 returns considerably more. Every step down the ladder is the same trade in the same direction:

• Darker grades hold black beautifully and ask more of the projector. In a lit space they pull ahead of everything else. Underpower one and the picture goes dull.
• Lighter grades are easy on the projector and give your venue more opportunity to spoil the picture. In a genuinely dark, dark-walled space with a modest projector, they are the right answer.

There is no universally best grade, which is the entire reason the configurator exists. The answer is wherever your projector output and your ambient light stop fighting each other, and it moves with both.

The published grades are stops on a ladder rather than the whole ladder. When the arithmetic lands between two of them and there is a good reason not to round, a Cluster is dialed to that grade and made to order.

Opaque, Acoustically Transparent, Scrim

Three families, and the choice between them is not about picture quality. It is about what else the surface has to do.

100% opaque passes no light in either direction. Nothing behind the screen shows on the picture, and nothing on the picture leaks out behind it. This is what you want when there are windows, work light or a second image on the other side. It is not a sound barrier; sound still crosses a thin tensioned fabric, but it arrives attenuated and colored rather than intact, which is why loudspeakers belong in front of it.

Acoustically transparent lets the sound through so the loudspeakers can sit behind the picture, where the mix intends them to be, instead of underneath it. Broadband insertion loss is about 1.3 to 1.5 dB: small, smooth, and correctable with EQ, which matters far more than the raw figure. A loss that is flat across the band is a level adjustment. A loss with a notch in it is a problem you cannot fix.

The catch with AT is that light passes through as well. Everything behind the surface has to be black and treated, or it comes back through the weave as a gray haze and you have thrown away the contrast you bought.

Transparent scrim is a different instrument again: a surface that holds an image and then vanishes when you light what is behind it. Its number describes transparency rather than absorption, so that ladder runs the opposite way.

The Object That Arrives on the Truck

A surface is also a physical thing, and large ones are logistics. The material itself is 254.97 g/m², so a 30 ft wide image is a real mass before you have added anything. Reinforced edges, eyelets, pockets, bonded hems, frame hardware and packaging are all accounted for separately, which is why the weight of a finished screen system is always the material plus the build.

Two numbers matter to whoever is building the frame. Tension travel is the relaxation a rigid frame has to take up: 0.74% along the run and 0.46% across it. Permanent installations are usually re-tensioned once in the first four to six months and hold after that. Shape travel is the elastic give at working tension, and it is what lets a curve be pulled true instead of forced.

On site, hang it and let it relax for a day or two before final tension. The elastic give is what takes the fold marks out. Tension immediately and the creases will still be there. Give it the day, and they leave on their own.

Reading the Picture: Three Numbers

Once all of the above is settled, the picture can be described honestly in three figures. Everything in the configurator's readouts is one of these or a component of one.

Image White

The brightest white this projector can put on this surface, in foot-lamberts. Cinema reference is 14 fL in a dark house, which is dimmer than most people expect. A lit space wants considerably more. One foot-lambert is 3.426 nits, so multiply if you think in nits, and note that a 1,500 nit LED wall specified for daylight is about 440 fL. No projector is going to meet that on a large image. The projection answer to daylight is not to match it but to remove it, or to accept a different kind of picture.

Black Floor

How bright the surface is when the content is black: leak, plus ambient, plus bounce. It is a floor, not a setting, and no amount of grading will get you below it. When you see the three contributions broken out as a bar, the useful question is which one is largest, because that tells you what to spend money on. If it is leak, you have a projector problem. If it is ambient, you have a drape and blackout problem. If it is bounce, you have a paint problem.

On-screen Contrast

Image white divided by black floor, in your venue rather than in a laboratory. This is the number people are describing when they say a picture has depth, and it is the one the surface moves most.

It is also the number that makes datasheet contrast look silly. A projector quoting 20,000:1 was measured in a black box with no screen, no audience and no exit signs. Put the same machine in a hotel ballroom at 150 lux and the on-screen figure can land in the low hundreds. That is not a failure of the projector; it is what contrast means once there is a venue involved.

The projector sets the ceiling. The surface and the venue set the floor. Only the gap between them is visible.

Measure It on Site

Eyes adapt. Ten minutes into a demonstration, in a dark space, with someone talking beside you, your visual system has quietly rebuilt its idea of what black is. This is why demonstrations are persuasive and why they should always be checked against something that does not adapt.

The scopes in the configurator are the standard post-production instruments, pointed at the simulated screen rather than at a signal. Each answers a different question.

• Histogram. How many pixels sit at each brightness. Watch the dark end pile up against the left wall as the black floor rises. That pile is shadow detail that no longer exists.
• Parade. The same distribution split into red, green and blue. When the channels disagree at the dark end, your shadows have a color cast, which is exactly what tinted ambient light does.
• Waveform. Brightness against horizontal position, so the trace lines up with the picture left to right. An uneven array or a mismatched blend shows here as a step or a bump.
• Vectorscope. Hue around the wheel, saturation as distance from the center. As black lifts, color drains and the whole trace pulls toward the middle.
• Transfer curve. What goes in against what comes out. A perfect surface in a perfect space would be a straight diagonal. The lift at the left is your black floor; the flattening at the top is the surface running out of return.

Two of the charts are specific to this problem rather than borrowed from grading, and they are the ones worth spending time in.

Contrast against ambient light plots on-screen contrast as the light in the space rises, for your surface and for matte white together. This is the chart that shows the counterintuitive result from the first chapter, in a way that is hard to argue with: as the space gets brighter, the darker surface pulls further ahead, provided the projector has the output to keep white where it needs to be. That proviso is the whole art.

Grade sweep runs the entire ladder for your scenario at once. Flat regions mean the choice does not matter much and you should optimise for something else, like budget or lead time. Steep regions mean it matters a great deal and you should be careful.

One caution, offered honestly. These scopes measure a physical model of a simulated screen, not a calibration report from your venue. Real imagers, real optics and real venues carry tolerances. Treat a difference of a point or two as noise, and look at the shape of the change instead of the last digit.

Doing It: The Configurator, the Seed, the Order

The configurator is this guide made operational. You answer a few short questions, the picture updates as you go, and every readout described above is live while you change your mind. Rest the pointer on any control and it explains itself, in roughly the words used here.

A few things about it that are worth knowing before you start.

Everything packs into a seed. One short code carries the projector, the image, the array, the light, the venue and the surface. Copy it into an email or a project note and the whole configuration comes back on any device. It is stamped on the corner of every image and sheet you export, so a screenshot that ends up in somebody else's deck can always be traced back to the numbers behind it. Read it to us over the phone and we are looking at your picture.

Start from a scenario if you like. There are five worked examples, from a blacked-out cinema to a ballroom keynote to a broadcast backdrop. They set all four answers at once, and nothing is locked afterwards. Changing one variable on a finished scenario is the fastest way to learn what that variable does.

You can order without traveling. A demonstration is the best way to settle a specification, and the facility is in Hollywood, and not everybody can get to Hollywood. So the order path does not depend on it. Send the seed with your finished size and a quotation comes back with a drawing and a lead time, priced against the actual build: the grade, the size, the edge treatment and the fixing. Standing grades are catalog items. A dialed grade is commissioned instead.

And if you would rather see it first, ask. We will set your seed up on the real thing, with your content, and if your projector can be got into the building we will use that too.

Written by the people who make the surfaces, which means we have an interest, and also means the numbers here are the ones we work to. If something in this guide is wrong, or unclear, or missing the case you are actually facing, write and tell us. info@qualia-forge.com