Projection Geometry Part I

February 13, 2014 in AV Design Tips, The Basics by Sam Davisson

geometry_1237605Time to get back to the "Basics". I took some time away for the holidays and have been suffering a bit of blogger envy. There really are some very interesting changes happening in the industry to blog about but I think it’s important to get back to the basics now and then. No matter how much the industry changes, the basics are always going to be important to understanding how things work.

The projection screen is one of the key elements in a successful presentation space. If the audience is squinting in the back or the folks in he outer row seats are forced to contort their bodies or twist their necks to see the presentation, the space simply isn’t working. If the presentation and the available screen space isn’t properly matched it can be equally as horrendous. Distracting black lines or blank space around the carefully prepared presentation or by the projected image spilling off of the screen and onto the surrounding walls simply ruin the presentation. Therefore specifying the right projection screen is extremely importance to creating a functional presentation space.

Projection Screen Size Matters

The first step to creating a space that is an effective presentation room, where each member of the audience can easily view the presentation, is to select a screen that is the right size. An appropriate screen size is determined by taking both the seating arrangement and dimensions of the room into consideration.

A couple of decades ago, the standard practice was to establish the image width of the projection screen first. The image width of the screen was determined by measuring the distance from the screen to the most distant viewer and then dividing by six. Previously, projection screens just needed to be big enough for everyone to see, now they must be big enough for everyone to read and evaluate data, especially that guy who likes sitting in the very back row on the outter edge of the room.

The rule of thumb today dictates that designers begin by determining the image height dimension of the screen. The rule for conference rooms and classrooms, or any room where viewing text, is that the screens image height should be equal to one-sixth of the distance between the screen and the most distant viewer. Once you establish the image height it must be checked against the dimensions of the room.

Another basic rule for most presentation area’s is that the bottom of the image should be a minimum of 4′ (48") above the audience floor, allowing audience members seated in the back of the room to see the entire screen. It is also recommended that there be 12" of top black masking between the ceiling and the top of the image or 18" if black masking is not to be used. When specifying a screen for a project, verify that the height of the screen plus top black masking plus the empty space below the bottom of the image area do not exceed the room’s ceiling height.

Basically what that means is that in a typical room with a 10′ ceiling the maximum image height will be 60"

Once the image height is established, the width of the screen will be determined by the format of the information that will be projected and its associated aspect ratio. The aspect ratio is a ratio of width to height in which the material was recorded and will be projected.

The Aspect Ratio Determines Width

Aspect ratio is defined as the relationship to the width respective of the image height. In todays commercial environment, video’s high definition resolution (1920×1080) seems to have gained the upper hand. Doing a little math that comes out to a 1.78:1 ratio aka 16:9. The other popular aspect ratio 16:10 (1.6:1) which seems to be used mostly in schools. In the computer world this represents WXGA and all of it’s derivitives.

If your dealing with someone who hasn’t jumped into HD, the standard definition aspect ratio was / is 1.33:1 and for those designing a presentation space for movies you may want to consider cinemascope which is 2.35:1

If your designing a home theater you’ll often want to go with a cinemascope screen. It should be noted that cinemascope is wider and narrower than an HD image. So you’l probably want to decide whether to go with a constant width or constant height system for your projection screen and add masking as appropriate. If you decide on a constant height then you’ll want side masking for displaying HD content. Conversely, if you go with constant width, then top and bottom masking would be the wise choice for those wide screen videos.

So, in the conference room discussed above where the image height was 60" and you are looking for a 16:9 (1.78:1) image size you would multiply the image height (60") by 1.78. The image width would be roughly 106.8"

Viewing Angles

To determine the viewing angle, imagine that a member of the audience is sitting in line with the center of the projection screen. That viewer is considered on-axis. Now imagine that the projection screen represents one axis and a second axis is drawn from the center of the projection screen through the on-axis viewer, creating a right angle. If the screen specified for the project has a viewing angle of 60 degrees, then a 60 degree angle can be drawn to both the right and left of the center viewer. These two angles define what is commonly referred to as the viewing cone. No seats should be planned outside of the viewing cone, because the image that the people in those seats will have of the screen will be distorted.

Projection screens today are manufactured to offer a wide range of viewing angles. A 60-degree viewing angle is considered especially wide. A 45-degree viewing angle is considered moderate. A narrow viewing angle will measure about 30-degrees.

Screen Surface

Because the screen surface goes hand in hand with the selection of the projector and I won’t be discussing projector selection until part 2 of Projection Geometry, I’ll hold off the discussion until then but I wanted it clear that screen surface is a very important area of discussion when selecting your screen and wanted to mention it.

Physical Characteristics

The final step in specifying a projection screen is to decide upon the different physical characteristics of the screen. These characteristics need to match the particular needs or uses of the projection screen and the presentation space.

Front or Rear Projection
There are two projection methods: front and rear projection. Front projection occurs when the projector is placed out in front of the projection screen. The projector shoots its visual image toward the projection screen, which then reflects the image back into the eyes of the audience.

With rear projection, the projector is positioned behind the projection screen. Instead of using the projection screen to reflect the image back into the audience, a rear projector transmits its visual message through the rear projection screen and into the audience.

Each of these projection methods has it’s strengths and limitations. A brief compare and contrast of these two projection methods reveals that the real estate requirement is perhaps the largest difference between front and rear projection. In front projection, the projector sits in the same room as the audience, either perched on a conference table, on a cart in the middle of the room, or suspended overhead. Rear projection requires that a dark room exist behind the projection screen that provides enough room for the projector to project an image of the necessary size. The larger the image, the deeper the dark room must be to support it.

However, rear projection allows you to remove the noise of a projector outside of the room. Rear projection also makes it impossible for the presenter to walk in front of the projectors light beam, nearly blinding themselves and blocking out the visual image in the process. Rear projection also manages ambient light better. A front projection screen cannot differentiate between the light from the projector and the light from overhead, it reflects all of it back into the eyes of the audience. With rear projection, the dark room behind the screen absorbs ambient light like a black box. The ambient light passes through the screen into the dark space, while the projected image passes through the projection screen into the auditorium, classroom, or conference room.

Electric or Manual Projection Screens
Today, there are a variety of ways to control both the small and large projection screens electrically. It can be as simple as touching a button on the wall to lower and raise a projection screen; as convenient as a handheld infrared or radio frequency remote control; or as intricate as a fully integrated control system with touch panel. With most integrated solutions, it is possible to touch only one button to make the room presentation ready. The projector can drop out of the ceiling turn itself on, the projection screen descends from its recessed hiding place and the lights dim to a pre-selected level.

Because of the convienance of integrated control systems, manual screens are rarely used any more but you will run into them on some legacy projects.

Wall-Mounted, Ceiling-Mounted, or Recessed
Mount projection screens on a wall or a ceiling for easy access and then tuck them away in their attached protective case when not in use. If the space demands something a little more discrete, projection screens can be easily recessed into the ceiling, completely removing it from the visual field when not being used.

There are a few applications ceiling or recessed mounting would not be a fitt. For example, recessing a projection screen in an application with an especially high ceiling, such as the sixty foot ceilings in some churches, is not an especially viable option. When the desired screen size is only a 9′ x12′, it does not make any sense to attach enough fabric to it to drop the projection screen to its desired position 30 feet below.

Portable or Fixed Frame
Is the projection screen being shared between multiple rooms or should it be an aesthetic fixture in the space? Both are possible with today’s technology.

Portable screens are available with black backing. This black backing reduces the amount of light interference incurred from the other side of the screen, improving the quality of the image projected on these mobile presentation screens. The portable case is made small and light so it is easier to move projection screens from one presentation to the next.

Fixed frames are good fits for rooms that host presentation after presentation and where the clients do not want to put the screen away or rooms with extremely high ceilings. Mounted to a wall like a picture, this type of frame does not need to be hard-wired into the space like its electric counterparts. Fixed frames accommodate both front and rear projection methods.

Screen Borders
Human eyes focus better onto a particular image when it is bordered. Bordered images are also perceived to have more contrast, brighter colors, and be, overall, a sharper image. That fundamental feature of our visual system is responsible for frames around art and margins on paper. Borders around a projection screen also enhance the professionalism of the presentation by allowing the projected image to, in print terms, "bleed-off" of the screen.

To provide the best viewing environment for an audience, specify that black borders be included on the projection screens in the space. Without the clearly defined boundary of the screen, the images can get lost as audience members try to disseminate the edge of the fabric from the supporting wall behind the screen.

Drop
The term drop in the projection screen industry refers to extra fabric that is added to the top or bottom of the screen to position the screen surface within normal viewing heights. An example of when this may be used is in a building with high ceilings. If there is a 15 foot ceiling and a 6′ x 8′ projection screen is going to be mounted so that the bottom of the screen is 4 feet or 48 inches off of the ground, the project demands 5 feet of drop to be added to the top of the screen and mounted to the ceiling.

Which comes first the projector or the projection screen? Thats like asking which came first the chicken or the egg. For the most part, I make those determinations simultaneously as most the time you considering the same information for each. Although, I usually start with the screen sizing and work from there. In Part 2 we will discuss the decision making process at greater length and work through more of the projection geometry.

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