Angles Of Reflection
Properly specifying a projection screen is a bit like finishing a crossword puzzle. Both are done through the systematic use of interrelated clues to arrive at the best solution, but with “Projector Brightness” taking the place of hints like “Erstwhile Acorns”. Although some careful forethought is always recommended regardless of the task, rigid rear projection screens require special care because, as when using a pen to fill in the squares, errors can be especially difficult to correct. In the interest of avoiding costly mistakes, let’s take a quick look at what goes into…
Rear Projection Planning
To begin, assume a twentieth floor conference room located in a building surrounded by a bustling city. An integrator (we’ll call him Dale Eitscrene) has been asked to install an in-wall rear projection screen. The owner wants the screen to be as big as the room will allow, envisioning an image that will span from floor to ceiling. Dale wisely notes the fact that they are standing on floor twenty in an office building with doorways, elevators and stairwells that would struggle to accommodate an 8-foot high piece of nearly inflexible acrylic. Add to that the fact that most screens are better off standing roughly 48” from the floor and it is clear that the owner will need to modify his expectations.
To get a better sense of an appropriate size for the screen, Dale measures the room height and the distance to the farthest seat, following the recommendations made previously in this very same series of articles. He also measures the small projection room behind the conference room and finds that the projector intended for the display needs more space than is available to fill the necessary image area. One option is to reduce the size of the screen to match the projector’s capabilities but that may result in a display too small to be seen comfortably by everyone in the room.
A better option is to use a mirror system to effectively fold the projected light’s path, allowing a short space to accommodate a longer throw distance. To ensure that he specifies the right mirror, Dale’s next step should be to fill out a mirror request form, which collects the sorts of measurements he has already taken so that we can design the most efficient mirror system and projector cradle (collectively referred to as a Rear Projection Module) to suit his needs. This will ask for the model of the projector, the thickness of the wall holding the screen, the distance from the bottom of the screen to the floor, etc. The more information he can provide, the better the results will be.
This information will also allow us to recommend an appropriate surface type for the screen. The process for doing this is largely similar to the one used to find the right front projection screen with the gain and half angles being the most important factors. However, it is important to note that these two measurements do not necessarily relate to each other the same way in both front and rear projection.
Case in point: DA-100, a coating that has the same 1.0 gain as a Matte White screen but does not share the same half angles. This is because, while the composition of this particular coating does allow it to transmit an equivalent volume of light compared to what Matte White reflects, it does not diffuse it in exactly the same pattern. In other words, this and most other rear projection screens allow off-axis light to fall off more quickly compared to front projection screens with a similar gain. In response to this phenomenon, we have developed a line of wide angle coatings that more closely match their front projection counterparts. Their wider half angles allow for relatively large screens with good uniformity, even when there is not much room to accommodate a long throw distance.
Since Dale is working with a limited throw for his screen, he would definitely benefit from a wide angle coating. Also, since bright, affordable projectors are now widely available, it may be advisable for him to look for a coating with a gain below 1.0. Generally speaking, the lower the gain, the more uniform the image. While it is possible for a screen to be too dim or too bright, it can never be too uniform. The same formula for converting lumens to foot lamberts applies in rear projection. As long as the projector is bright enough to give good results, a good low gain coating will almost always look best.
Besides gain and half angles, the other relationships worth understanding are those between a rear projection screen’s viewing area, its panel size, and its overall dimensions. There is no single answer to how these factors will relate, since these will depend greatly on how the screen is going to be installed. If using one of Da-Lite’s frames, approximately one half inch on each of the four sides of the screen is needed for the frame to grip the acrylic panel. This adds an inch to both the width and height of the actual panel beyond the viewing area.
The frame itself will determine the real overall dimensions and the size of the opening needed to hold the screen. Different styles will give different results, so it is generally advisable to confirm the size before making a decision. In cases where a frame is not needed or wanted, it is also a good idea to detail how the screen will be supported. After all, not every rear projection screen is going to be placed in a wall; some are the walls, as is the case with flight simulators, or are the top of an interactive work surface, just to name a couple of alternative options. In these instances, we can offer our advice on how to make the best use of a projection screen to suit.
That is our goal with any projection screen, actually. Nobody likes to make mistakes and even when they can be fixed, they can be cost time, money, and confidence. They are better avoided in the first place, which is what makes planning so important. Adequate planning helps to ensure that Dale will have installed a great rear projection screen just as surely as erstwhile acorns have become oaks.
-- Adam Teevan
ateevan@da-lite.com
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