Projecting Scenery

Projecting scenery has become extremely popular, especially in situations when flying backdrops in and out is not feasible.  Projecting scenery is tricky, because there are a lot of factors that can cause grief if not planned for in advance.

Projector Output

The output of the video projector as similar to that of a lighting fixture, but one over which you have incredible control of the shape and color of output.

The output of the projector is a rectangular cone and follows the same rules as any light fixture.  Think of it as an ellipsoidal with the shutters in to form a rectangle.  Output and placement of the projector is critical to the success and effectiveness of your video produced scenery.

Similar to a lighting fixture, we project white and colors, however, we do not project black, black is the absence of light.  With that in mind, the darkest you can make the projection surface, is the blackest black is going to be in the projected image.  The difference between the whitest projected white, and the blackest black is your Contrast Ratio.  Depending on the type of projector, you may have a very high Contrast Ratio, which is beneficial for this application, or a Contrast Ratio that is not so great, which results in projecting a dark gray rectangle, even when the signal is all black.

How large do I want the projected image?

The first design decision you need to make is ‘How large do I want the projected image?’  Most would choose the entire area typically yielded by a rented backdrop, or the entire cyclorama.  Unfortunately, projector placement options and budget often prevent that from being a viable option.

Here are a couple of considerations:

In a dark movie theater, 27 lumens per square foot are considered minimum for viewing.  On a stage with lighting, I strive for a minimum of 50 lumens per square foot, if not more.  I know that doesn’t sound like a lot, however, 40’ wide by 20’ tall is 800 square feet and require 40,000 lumens of projection.  Yes, they make projectors with that output, yes, they are expensive.

The second consideration is projection distance, that ‘cone’ requires some room to develop, and with the ‘standard’ projector lens, for each foot of width, you need almost a foot and a half of throw distance.  There are Short Throw, and Ultra Short Throw lenses available, however, they too are expensive, and ‘Ultra’ expensive, but they can cut throw distance to approximately 9” per foot of projected width, and even 5” or less per foot of width.  We will come back to how to calculate lenses later.

What is the minimum size of a projected image ?

First critical design decision; what is the minimum size of a projected image required to achieve the desired results?  What is the most important dimension; height or width?  Once you have determined which, there are a couple of mathematical formulas that will give you the other dimension, and provided that is also acceptable, we can move on to placement.

Let’s begin with an HD projector, this affords an Aspect Ratio of 16:9, and is a very common video format.  Practically every ‘big screen’ TV or monitor produced today are 16:9 Aspect Ratio which is the Width as compared to the Height.  16 units wide by 9 units tall.

For the purpose of easy math, let’s say you have determined you want a 16’ wide projected image, and you need to calculate the height of that projected image: take the width, divide by 16, which leaves you 1, and multiply that by 9.  In HD, a 16’ wide image, yields a 9’ tall image.  32’ wide is 18’ tall, 48’ wide would be 27’ tall, etcetera.  Once you know the desired width, you can easily calculate height.

Let’s say that 32’ wide by 18’ tall works in your space, and gets you the desired design results, however, your budget doesn’t allow for the 25,000 to 30,000 lumens projector necessary to properly cover the 576 square feet of image area in your design.

Upon further introspection, you determine you could get by with a 20’ wide image on your stage.  Let’s do the numbers, 20’ wide divided by 16 units equals 1.25’, then multiply that by 9 units, give you 11.25’ tall, or 11’-3”.

20’ wide works, however, at 11’3”, my actors heads could be at the top of the projected image from the perspective of some of the audience members, if only I could stretch that image a bit taller.  Enter Wide Format, 16:10, WXGA or WUXGA format projectors.  With the same 20’ wide projection, you will now get a taller image.  Let’s do the numbers, 20’ wide divided by 16 units equals the same 1.25’, however, when you multiply that by 10 units, it gives you 12.5’ tall, or 12’-6”.  If this is still not tall enough, you will need more throw distance to achieve a larger image, which will increase in both width and height.

Quick takeaway, if width is the most critical dimension, HD projectors are desired, if height is more critical, select a wide format projector.

Reality check, 20’ x 11.25’ is 225 square feet and 20’ x 12.5’ is 250 square feet, so at 50 lumens per square foot, you are looking for a projector in the 11,500 lumens to 12,500 lumens projector.

Projector Placement

Most projectors with appropriate output for projecting scenery have interchangeable lenses, so you can select the lens that best serves your application.

Projector manufacturers typically offer five to six lenses of different focal lengths for their projector models.  Typically, all but the shortest throw lenses have zoom capability, and where one lens maxes out, the next lens pick up.  The short throw lenses are ‘Fixed’, and do not zoom, meaning distance of the projector to the surface determines the size of the image.

There is no ‘standard’ lens, however, 1.5-2.5:1 Zoom might be a typical offering.  Again, for easy math, let’s pick from the middle of this zoom range at 2.0:1.  The ‘2’ means that you need 2’ of throw distance for every ‘1’ foot of projected width.  To achieve our 20’ wide image, you would need 40’ of throw distance from lens to surface.  Because it is a zoom lens, that same 20’ width could be achieved from a 30’ throw distance when the lens is zoomed out to maximum angle, or 50’ when zoomed in to the tightest angle. 

This would be great if you are projecting your image on an empty stage, with no actors, or set pieces to interfere with the projection. 

Of course, you are working with both, and this is where a short throw lens is extremely useful.  The goal is to mount the projector above stage and project the image downward at a relatively steep angle, and over the heads of performers. 

Blocking becomes extremely critical.  There will be a No-Man’s-Land upstage near the projection surface, and anyone who enters will be illuminated by the projection and cast an unwanted shadow on the projection surface.

Lens Selection

Short throw lenses are typically ‘Fixed’, meaning there is no zoom functionality, the placement of the projector from the surface is the ONLY factor determining size.  Most Short Throw Lenses are less than 1:1, which would be 1’ of throw distance would yield 1’ of projected width.  A 0.8:1 lens would mean that you would need approximate 9-1/2” of throw distance for each 1’ of desired width.  0.73:1 would be each 8-3/4” throw distance for each 1’ of width.  The smaller the first number, the less distance required.  These calculated distances are from lens to surface, and typically have a margin of error, so don’t count on any absolutes until you have it up and working.

Imagine taking a 50º ellipsoidal and mounting it on a line set close to the plaster line and shining it directly towards the cyclorama.  Use the shutters to form a wide rectangle that wastes as little light as possible and fills the area on which you desire to project scenery.  Now raise that line set so the fixture is at trim and aim the fixture downward, so the top of the rectangle is illuminating the cyc at the desired height of your projected scenery.

The rectangle beam of light will have formed a trapezoid, being much larger at the bottom, and probably spilling onto the stage.  This is exactly what the projector will do.  You have the ability to adjust the shutters to regain your rectangle shape, and you can accomplish the same function utilizing Keystone Correction with the video projector.  Just like with the shutters on the ellipsoidal, reshaping the bottom of the light beam, Keystone Correction eliminates some of your output.  The difference is that projector Keystone Correction also electronically modifies the output so the adjusted image will remain true to image you send the projector.

Keystone Correction

All projectors have vertical Keystone Correction, which adjusts the top or bottom of the image, many have horizontal correction, however, the projectors that are capable of the output lumens you will need for this application typically have Corner Keystone Correction, which allows you to adjust each corner individually.

Placement of the projector is critical to the width of the projected image, Keystone Correction can only reduce the image corners, not increase them.  It is also important to place the projector lens in the center of the desired image area.  Some projectors have the lens positioned off center, and although you probably can corner correct, all Keystone Correction results in a reduction of output, so you want to keep it limited to only what is necessary.

Along with being properly centered, it is important that the projector, and hence the projected image be level.  Projectors mounts have pitch and roll adjustments for this, and most rental projectors, of this size, will come in what is known as a stacking cage.  This device has some very useful adjusting mechanisms, and it makes the projector easy to mount to a couple of adjacent line sets.

Optimal Projector Placement

Optimal projector placement is known as ‘On Axis’, which would be placing the projector with the lens exactly aligned with the center of the screen, both top to bottom, and left to right.  This would make each corner of the projected image the exact same distance from the lens, and there would be no need for Keystone Correction.  This is not practical for our front projection applicable. 

It is worth noting that on axis projection requires the absolute least distance from lens to surface.  As you raise the height of the projector, even though you are aiming it downward, the distance of lens to the top of the projection surface decreases as the distance to bottom increases.  Keystone Correction can fix the bottom, however, you lost about a foot of throw distance at the top, and the projected image will be smaller.

Lens Shift

Most projectors also have a function called Lens Shift, which allows you to remotely adjust the vertical output of the projector.  Some projectors also offer Horizontal Lens Shift.  Lens Shift has certain limitations, and typically the projector manufacturer recommends the projector be placed somewhere perpendicular within the top or bottom of the projected area.  In other words, don’t mount the projector above the intended projection surface.  There is a little wiggle room for this, however, go too far above and focus from top to bottom becomes challenging.

The great thing about Lens Shift is that you can adjust the image up or down without the requirement of as much additional Keystone Correction as required when physically aiming the projector downward.  One ‘gotcha’ with Lens Shift, especially with short throw lenses, is that you can shift the image too much, so the inside of the lens starts to cut off the image output.  For this reason, a combination of downward pitch of the projector, Lens Shift, and Keystone Correction will be required to achieve the desired projected image.

Some projectors do a better job of lens shifting up, and if that is the case with your projector, you will need to mount the projector upside down, so the upward Lens Shift actually lowers the projected image.  The mounting attachment points of the projector are on the bottom, so this is straightforward.  There is a function in the projector Image Set-Up for Ceiling mounting, and that will flip the image you are sending the projector.  Some projectors have the Ceiling invert function in a special Installation section.  Do keep in mind that although your image is inverted, some of the functions on the remote MAY still work with the normal orientation of the projector, but you are Theatre People, so you get it that Left is Right, and Up is Down.

What if Front Projection won't work?

Something is occurring on stage that makes front projection impossible.  You are flying a prop, or a person within the projection cone?  This brings us to Rear Projection.

For projecting scenery, I prefer RP for a couple of reasons:

Unlike a cyclorama or other front projection screen, which is typically white, a Rear Projection Surface is gray, and that tends to reject stage lighting more efficiently, so you can achieve better contrast.

Placement of the projector is typically On Axis, and as such, the necessity for dramatic Keystone Correction is negated, if not totally eliminated.

Of course, the downside to rear projection is the upstage space you have to sacrifice for the projector and projection cone.  In the case of front projection, the projector and cone can occupy the same vertical stage space as the performers, with only a relatively small upstage area off limits.  Rear projection requires depth for the projector and the cone.  This will likely also add an additional expense, as most performance spaces are not equipped with a large RP Screen, yet.

Rear Projection

Thinking of our desired 20’ width, even with the 0.73:1 lens, that still requires about 14’-6” from lens to surface, and the projector itself is almost 3’, hence you are giving up at least 17’ of upstage area behind the projection screen.  It is highly unlikely that the Administration will approve your request to knock a hole in your upstage wall, but you can certainly ask.  You can still use this area for limited storage, as well as a crossover, provided performers and technicians avoid walking through the projection cone.

There are rear projection mirror systems, which are as complicated to set up as they are expensive to purchase.  These systems would allow for the projector to face towards the upstage wall, and the reflection off the mirror is what shines on the screen.  At a very minimum, they eliminate the depth of the projector, so our 17’ is now down to around 14’.  These utilize very specialized First Surface Mirrors.  Unlike regular mirrors which would produce one reflection off the glass, and a second off the mirror, giving you unacceptable double image. 

Enter the latest in lens technology, the Ultra Short Throw or UST lens.  These lenses are typically in the 0.38:1 to 0.4:1 fixed range, effectively cutting the lens to screen distance in half.  They come in two form factors, both of which afford what is essentially periscopic functionality.  One very unique lens I have worked with utilizes a right angle, so along with the short throw distance required, the projector is positioned sideways, further reducing required distance from back wall to projection surface.  To project the 20’ wide by 12-6” image requires 9’-9” from the upstage wall.  This lens works best when positioned On Axis, with the center of the lens directly behind the projection screen.  We mounted the projector on a short truss between two rolling stands, which allowed for easy height adjustment.  We then incrementally moved the screen until the entire surface area was filled at the bottom.  The center of the lens was slightly below the center of the screen, so only minimal keystone correction at the top corners was necessary.  I did make a few physical left-right adjustments with the projector, to get the lens in the most optimal location.  After taking final measurements of the projector position, we determined that a small rolling scaffold would have achieved almost the same projector placement.  This amazing piece of technology does come at a cost, this lens alone sells for about $20,000.

The other UST lens functions in a manner similar to that of a whiteboard projector.  The projector is mounted above the intended surface, with the lens as close to the upstage wall as possible.  This lens projects the image downward at approximately a 55º angle, so precise projector height is absolutely critical to where the projected output intersects the surface.  I had considered this lens for the above referenced application; however, the projector would have needed to be mounted at approximately 18’, and it would have saved no more than a couple feet upstage.  This lens is a viable option for front projection, however, two things need to be considered, because of the steep angle of projection, the surface must be very straight, taught and unmoving, because at that angle, even slight ripples or movement are very noticeable to the audience.

Another consideration for rear projection, with a short throw lenses, especially when the projector is on axis, is that the distance from the lens to the center of the screen is significantly less than the throw distance to the outer edges, which tend to make the image appear brighter in the center of the screen, known as a Hot Spot.  You are also projecting the image through a semi-transparent material, so some of the light is pointed directly at audience members.  It does not matter from where the image is viewed, a direct line from lens to eyes will always appear to be slightly brighter.  From onstage left or right, the opposite side of the screen will appear quite a bit dimmer, because the light from the lens is being projected away from you.  Some rear projection screens are made from a material that is better for use with a short throw lens, however, access to such a screen on a rental basis may be challenging.  Fortunately, this is less obvious to audience members.

Content & Delivery

Media content and how to deliver your scenery and background images to the projector.

The preferred method of professionals would be to load the images on a Media Server, which allows you to call up a specific image with a command from a DMX lighting console.  Image recall can be cued into the lighting console just like any automated lighting fixture, albeit a rather complicated one.  Media Servers can have multiple layers of video working at the same time, and typically require hundreds of DMX parameters.  Media servers allow for the manipulation of the images or video files to an amazing degree, or you can simply use it like PowerPoint to make clean transitions from one image to another.

Presuming you have already spent a sizable chunk of your budget procuring the projector, lens and possible screen, let’s explore some more cost-effective options to the Media Server, which is a little like using a high-performance sports car to run to the store for milk.

Either Microsoft’s PowerPoint or Apple’s Keynote are two great tools to serve up images with smooth transitions, however, there are no off the shelf options to trigger a presentation via DMX.  Rosco used to make something called Keystroke for that function, however, they may still be out there for rental.

Q-Lab by Figure 53 is another way to load and recall audio, video and lighting cues from a Mac computer.  The License is available to purchase or rent for your show.  You can download the app and build your show, and only need to rent the license for the actual performance and tech time the projector is in operation.  If you have used Q-Lab for audio cues, you will be familiar with the workflow.  If you have an older version, you will need to upgrade to add the video functionality.  Please be mindful that you are asking this computer to do a lot of processing, so if you are using an older Mac, it could get bogged down.

From a connection standpoint, keeping the device serving up the images close to the projector is preferred. With a Media Server, this is a no-brainer, place the server near the projector and run DMX to the server from the console.  If you are using a computer with an operator, best to put them backstage with a headset to hear the Stage Manager call cues.  There will be a strong desire to put the computer in the tech booth, and run a long cable to the projector, however, I caution against this.  Long cable runs often produce a degraded signal, which results in diminished image brightness or quality.

If you are producing you own content, be mindful of the audience’s perspective.  They are viewing these images behind performers, so size and angle are critical.