Angles Of Reflection
Ambient Light, Transmit or Reflect?
If you have been in the audiovisual industry for any length of time, it is likely that you have been amazed at the advances in video projectors over the last five to ten years. I know I sure have. Today’s video projectors are lightweight, very powerful, extremely bright, have better uniformity and are much easier to install than their cumbersome CRT relatives that we used in the past. These advances have been welcomed with open arms by the audiovisual industry and the technicians who were once required to install and calibrate those dinosaur projectors.
During those CRT days, the one thing we commonly faced was situations where we needed a fairly large, say 120" diagonal, projection screen in rooms that were very bright and contained large amounts of ambient light. So, it became necessary at the time for us to consider rear projection as a way to help combat the low output of the projector and the high amounts of ambient light in the rooms. This worked very well and there have been many systems installed over the years that are similar to this exact scenario.
With the advances made in video projectors, it appears that there is an assumption being made today that, because projectors have become so bright and powerful, rear projection is not necessary anymore. Many think that you can just overpower whatever ambient light is present. I contend that these statements are false and in fact, rear projection is still an option which one should consider every time a system is designed. Some of you may be thinking I am a bit off my rocker for making such a claim. However, I challenge anyone to show me a rear and front projection system side by side, using the same exact projectors, sources and gain of screen and tell me that the front screen image is better than the rear. I guarantee you will agree that the rear projection system looks better, especially with ambient light present. Why is this the case and how can I be so confident? To answer that question, let us take a moment and contrast the two different methods of projection.
First, let us discuss front projection screens in terms of their advantages and disadvantages. A front projection screen’s main objective is to reflect light that is incident to its surface in a pattern consistent with the amount of reflective, refractive or diffusive elements placed on the screen. From past articles, we have learned that with a diffusive screen, the light is bounced off equally in all directions and with a reflective/refractive screen, the light is directed off the screen in a narrower pattern to concentrate the light towards the audience. Therefore, the lower the gain, the wider the dispersion pattern and the higher the gain, the narrower, or more directional, the dispersion pattern becomes. Given this knowledge, we can now discuss what happens when not only the light coming from our video projector strikes the screen but what happens when ambient light strikes the screen. The ambient light is handled by a front projection screen in the exact same way as the projected light. In other words, the screen cannot discriminate between the projected light and the ambient light. Therefore, it scatters, reflects or refracts all light incident to its surface in the same manner.
The way in which a front projection screen works can be both good and bad, depending on many different factors. If we are placing the screen in a room that is very dark and the walls, ceilings and floors are covered with materials which are dark in nature and absorb light, then nearly any type of front projection screen will perform exceptionally well. One of the reasons why most screens will work well in this environment is because we have an exceptional “system black level”. The “system black level” is how black the screen will be with a given amount of ambient light and the projector muted such that it is attempting to create a black screen. Remember, one of the main ingredients to making a good video system is to have one with enough contrast. Contrast is most affected by the black level so, therefore, a good “system black level”, very low in the gray scale, is the goal of any good visual display system.
Now let us change things a bit. Say that our room with little or no ambient light and dark walls, ceiling and floor now has a fairly substantial amount of ambient light and some highly reflective surfaces around the room. Those reflective surfaces can include glass from picture frames, a white ceiling, beige carpet and even off-white walls. What do you think has happened to our “system black level”? As you would expect, it is higher up the gray scale into an area ever approaching white. While both of these scenarios are at the opposite ends of the spectrum, they are presented to point out the fact that the only way to get an acceptable “system black level” with a front projection screen is to have control over the ambient lighting and secondary reflections off materials in the room.
Other disadvantages of a front projection system are environmental in nature. First of all, if the room is being used as a training or presentation room, then the person providing the presentation may at some point walk into the projection path. This not only blocks the light from reaching the screen but it can also be very blinding to the presenter. Many presenters move around the room when discussing their topic and, if this is the case, a front projection screen may not be the best choice. In addition, the projector’s fan tends to make noise. Try as they might, projector manufacturers still have to move air through their units in order to keep them cool. This can be a source of noise in the room which can raise the NC (noise criteria) level of a given room and require higher levels of speech reinforcement in order for the message to be conveyed. One other thing to consider in front projection is the fact that in order to combat high levels of ambient light, a very powerful and typically expensive projector is necessary. Here, instead of lowering the black level in our contrast ratio, we are attempting to raise the white level to help achieve a better ratio. The results of this method are much more risky and not typically as effective.
While we have been discussing the disadvantages of front projection, there are still a number of advantages as well. They include such things as less space required for a large image. In front projection, the presenter and audience share the room with the projection equipment and as a result less real estate is needed. Another advantage to front projection is the fact that many installers are more familiar and comfortable with front projection systems. Therefore, the time it takes for the installation could be shorter than a comparable rear projection system.
Now that we have established some of the pros and cons of front projection let us discuss rear projection in the same manner. Like we did with front projection, it is only fair that we discuss how a rear projection screen presents an image. What a rear projection screen does with light is transmit it through thousands of tiny particles suspended on the screen which then diffuse the light in a given pattern. Similar to front projection, the concentration of these particles has much do to with the dispersion pattern of the light being transmitted. However, in this case, the higher the concentration of the particles, the lower the gain of the screen and the wider the dispersion pattern. Conversely, with a lower concentration of particles, the higher the gain and the narrower the dispersion pattern.
While the gain and dispersion pattern of a rear projection screen is important and has everything to do with choosing the correct gain of the screen, it still does not explain why rear projection would be superior to front projection. Look back again at the last paragraph. One key word was used in the way a rear projection screen works. That word was transmit. When light hits any rear projection screen most of the light is transmitted through the screen towards the other side. If we have large amounts of ambient light in the room, this is no small factor. This transmittance is what causes the ambient light from the room to go through the screen into the projection booth behind the screen and allows light from the projector to travel through the screen and into the audience’s eyes. This optical characteristic is very significant to the success of a rear projection screen.
As opposed to front projection, where the competing light sources are both traveling towards the screen and being reflected off in the same fashion, in a rear screen the competing light is traveling in a direction that is opposite to the projection beam and is absorbed within the dark room behind the screen. This provides us a number of benefits. The first is that the projected light is being aimed directly at the audience’s eyes and we are not relying on the screen to bounce it back towards them. The second benefit is the fact that the ambient light is not significantly reducing our “system black level”. This will provide us with a much better contrast level on the screen and make the image appear brighter.
In addition, there are secondary benefits to utilizing a rear projection system. The installation looks cleaner. You do not see the projector as a source of glare or distraction. The image is easier to interact with from a presenter’s point of view. The presenter is not going to be blinded by the light of the projector. And lastly, the noise from the projector is now tucked away in a room that is dedicated to the equipment and may even have a separate cooling system.
Wow, this is great news, right? Well, yes, as long as you consider the two issues facing rear projection. The first and possibly one of the biggest objections to using a rear screen system is space. There is a space need associated with choosing this projection method. However, that hurdle is easily overcome with the introduction of a mirror system. Many screen manufacturers have a rear projection module in their product line and Da-Lite is no exception to that comment.
Also, one of the things you have to work through, as a designer, and ironically the second issue facing rear projection, is the possibility of a more costly overall system. When working through a design, one must weigh the cost associated with the front versus rear projection. If you use a more expensive, higher output projector to overcome the ambient light, how does that compare to using a rear projection system with a more reasonably priced projector which has fewer lumens? Or better yet, will you need to use two front projection screens to cover the audience because of the higher gain and directionality needed to overcome the ambient light? These are all questions to consider when making your choice.
Incidentally, there is a rule of thumb that can be used for determining if you have enough space behind the screen for a rear projection mirror module. This rule will provide you with a very rough estimate of the space required. First, take the throw distance required for the size of screen chosen. Then add to that the overall depth of the projector specified. Divide that total number in half and this is approximately the space a one mirror system will take. One can also draw a scaled version of the projector and its needed throw distance on a piece of paper. Cut out the drawing and then place it within a drawing of the proposed room (same scale). By folding the light path you will see how a rear projection mirror system performs its magic. However, if you need something a bit more exact in nature, it is best to complete a Mirror Drawing Request form and allow the optical engineers to design your system in a CAD-based program. This will ensure that every aspect of the system is worked through in great detail.
From all of this, you can see that rear projection is still a valid choice even with today’s bright and powerful projectors. Though we try to overcome them, the laws of physics still exist and it is better to transmit ambient light than reflect it.
-- Blake Brubaker