Angles Of Reflection
If you attended the recent Infocomm trade show in Anaheim, CA there is no doubt that you saw several projector manufacturers touting that fact that they have the brightest projector on the market. There was one such manufacturer claiming to be outputting 30,000 lumens from their large venue projector. Obviously, these manufacturers are looking for new and more creative ways to get as much brightness out of their units as possible. With that in mind, is there anything we need to be concerned about from a screen selection point of view?
Uniformity - Revisited
Interestingly enough, I was recently posed a question about whether or not it was appropriate to use an extremely bright projector in a certain visual display system. The person who asked was concerned because they were using what they considered to be a moderately sized screen. The main focus of the question had to do with the potential of a “ hotspot” with this extremely bright projector. My quick response to that question was to ask further questions about the project in order to help determine if we would, indeed, have a problem. After a bit of investigation, we determined that there were no significant issues or concerns which would point to the phenomena known as “hotspotting”. I think the one item that struck me most about this conversation was the fact that there seems to be a misunderstanding about why a hotspot exists and the fact that more light output from the projector triggers one to think that it could be present. Let us examine those concerns a bit more closely to see if there is merit in them.
In order to evaluate these concerns, we must first understand how a projection screen works and, furthermore, how it is that a hotspot can exist. One of the most basic principles we need to understand about a hotspot is that the screen itself is not the factor at fault for the hotspot’s existence. Therefore, it must be the projector. Correct? Well, not exactly. The cause of a hotspot has more to do with the fact that we are using a small lens to project an image onto a large screen and can further be exaggerated by a screen which has a high gain. Let us break this down further to examine both the screen and projector/lens portions of this display to see why this occurs.
There are three basic functions which a front projection screen can do with light rays that are incident to its surface. They are: Scatter, Refract or Reflect. A screen which scatters light has, as one of its main elements, Magnesium Carbonate (MgCO3). It just so happens that this is also the material used as a reference point for determining gain of a projection screen. The reason MgCO3 is used for both of these applications is its ability to scatter light incident to its surface in a fashion that is equal in all directions. The best way to think of this is to imagine one tiny little light ray hitting the surface and then being broken into countless other smaller light rays that bounce off the screen equally in all different directions such that one cannot detect from where the incident light ray originated. In other words, since this type of screen performs in what we call an Isotropic fashion, light incident to the surface will be bounced off equally in all directions regardless of the angle of incidence. With that in mind, one might already come to the conclusion that a screen which scatters light could not exaggerate a hotspot. While indeed this is true, let us look further into the other two types of surfaces. It is worth noting at this time, that screens which scatter light are typically those exhibiting a gain of 1.0 also commonly known as Matte White.
A screen which is said to have Refractive characteristics is one which has some type of glass structure to it. Do not, however, confuse this type of screen with a rigid rear projection glass screen. That is not the type of glass we are referring to for this example. Instead, we are talking about front projection screens which have glass beads either adhered to their front surface or imbedded into the structure of the screen itself. These are better known as glass-beaded or High Power screen surfaces. In Figure 1 below, we see what occurs with these surfaces as light incident to their surface passes through the thousands of tiny little beads and is bent at angles dependent upon where they strike the bead. After passing through the bead, the light ray strikes a surface similar to Matte White where it is scattered and then passed back through the beads before exiting the screen surface. This type of screen is also referred to as a Retro-Reflective screen because it bounces the light back towards the direction in which it originated. So, could a hotspot occur here? Before we answer that question, let us look at the third and final type of front projection screen.
Figure 1
If a screen is said to have Reflective characteristics, then it usually has a gain higher than 1.0. The make up of a Reflective surface is such that it has a specular material added to its surface which acts as an enormous number of tiny little mirrors. As the light strikes these mirrors, it is reflected off of the surface and back in a fashion which coincides with the degree by which the surface is coated with these mirror-like particles. In other words, the higher the concentration of the particles, the more reflective the surface will become. However, as we have learned through our infancy in the audiovisual marketplace, a projection screen cannot create more light and, therefore, the added brightness one receives from a Reflective screen has an associated fee attached. That fee is the directionality of such a surface and a narrower viewing angle. A Reflective screen does not reflect all of the light incident to its surface equally in all directions. It is, indeed, much more directional and deliberate about the way in which it reflects the light rays incident to its surface. To be more precise, that “way” is such that the angle of reflectance is equal but opposite the angle of incidence. See Figure 2 below. Imagine, if you will, the cue ball on a billiard table being put into motion such that it strikes the side bumper of the table. When it does so, it will bounce off at an angle that is both equal but opposite to the way in which it struck the bumper. This too is how light rays incident to a Reflective screen behave. Some of the more commonly known reflective screens are: Cinema Vision, Pearlescent, Video Spectra 1.5, Silver Vision and Silver Matte.
.jpg)
Figure 2
Very well then, what does the way in which a screen scatters, refracts or reflects the light incident to its surface have to do with the potential for a hotspot? It actually has everything to do with it. You see what we need to consider when discussing a hotspot is the fact that light coming from the projector is striking the screen surface at many different angles. Take for instance a ceiling mounted LCD projector. The center most light ray is striking the screen at an angle that is downward in the vertical dimension but for the most part perpendicular in the horizontal dimension. Contrast that with the light which is emitted from the corner of the lens, specifically the upper right side of the lens as we look at the screen. These light rays are striking the upper right hand corner of the screen nearly perpendicular in the vertical dimension. However, the way in which it is striking the screen in the horizontal dimension is very different than the light rays emitted from the center of the lens. Also, take into consideration that a shorter focal length lens will compound these angles. If you bring the projector closer to the screen, the angles off to the side become increasingly harsh.
Alright, we now know the three characteristics of a front projection screen and the basics of how light from a lens strikes a screen surface. Let us put the two together and see if a brighter projector automatically creates the potential for a hotspot. Before we do, there is at least one item we must presume about the projector. With only a few exceptions, LCD, DLP and other “digital” projectors manufactured today provide uniformity across their fields that may vary from center to edge by as little as 10%. So, for the purposes of discussion, we will say that the projector we have chosen meets these criteria. This may seem a bit trivial, but it is very important when determining the uniformity of your overall visual display. If the projector itself is not uniform, then we could have severe problems with our display. Remember the old CRT projectors?
If we were to place the light coming from a very bright projector onto a scattering or Matte White screen, do we have the potential for a hotspot? The answer to this is definitely not. Remember even if we have harsh bend angles from the light striking the screen, a scattering screen disperses the light back in a fashion that is equal in all directions. So the potential here is non-existent.
Let us examine further a screen which is Refractive. Here we have a different scenario. Given that a Refractive screen will send the most concentrated portion of the light back towards the source, we do have the potential but not perhaps as great as one might think. Since this type of screen is very directional in its dispersion pattern, our audience will likely be seated within the parameters of the screen and this minimizes the potential.
As we learned above, a Reflective screen behaves such that the angle of incidence is equal and opposite of the angle of reflection. This then, provides a scenario where we could be off axis of the “sweet spot” of the reflection for differing portions of the screen. Given that, we can assume that our potential for a hotspot increases slightly as we use a Reflective screen. However, does more light from a projector infer that we will have a uniformity issue? The quick answer to that is also no. Remember a hotspot is not created by the brightness output of a projector. It is created by the relationship between the projector’s lens function and the screen’s directionality. Providing that we use a long enough focal length lens (by my experience at least 1.6), we should not have any issues with uniformity regardless of the screen chosen. If then, the display requires a short focal length lens, we now know that we must use a low gain “Scattering” screen to help minimize the potential for hotspotting. So remember, brighter does not always mean that we will have uniformity issues. We must make our decisions of screen selection and lens selection based on this knowledge.
-- Blake Brubaker
Download in PDF Format (133.8kb)