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| Projector Screens | DIY Screens Gain and other confusing topics.Discuss Gain and other confusing topics. in the Home Theater | Audio and Video forum; Gain and other confusing topics. There is always an interest in gain when it comes to screens. I have recently seen many discussions and questions ...  |
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 08-16-07, 05:30 PM
| #1 (Link) | |||||
| Gain and other confusing topics. There is always an interest in gain when it comes to screens. I have recently seen many discussions and questions about gain. Some statements are valid, some are conjecture, but there does seem to be a lot of confusion about screen gain and the various ways it is achieved. More importantly there seems to be some questions about the pros and cons of gain and how it is achieved. First a brief bit of history about the ‘Silver Screen’. At the turn of the century, 1909 to be specific, motion pictures were becoming the rage. One problem that plagued the fledgling theater industry was that projectors and screen materials at that time were extremely limited and crude by today’s standards. This resulted in images being very dim and quite hard to see, but the public was still fascinated with moving pictures. Adele DeBerri owned a theater in Chicago during this era. She was a unique individual, remember this was an era when women typically did not own and operate a business. Not only was she a pioneer in that respect, but she was an innovator as well. Adele had the idea to paint the screen image area with a silver paint that was highly reflective and therefore would reflect more light back at the viewing audience. That’s how the ‘Silver Screen’ was born. What many may not be aware of is Adele went on to developed a silver painted canvas projection screen that quickly became the standard for the industry. Da-Lite Screen Company, Inc. is the successor to the business founded in Chicago in 1909 by Adele DeBerri. The reason for high gain screens date back to the earlier example of turn of the century projectors that needed something to focus the light and make the image brighter. Today projectors are much more sophisticated and ten fold better than the old ‘moving picture’ projectors back at the turn of the century. The projectors currently available are so bright that if a person was to look directly into the lens it could cause permanent eye damage. So if our newer projectors are significantly brighter than even projectors made ten years ago let alone turn of the century technology, and gain is to produce a brighter image- why do people still seek a high gain screen? Situation and setting is often the main reason. Excessively large screens also come to mind. Why does “higher gain” make a screen appear better? Or does it? And how is “gain” achieved? First, gain is the ratio of brightness of your projection screen material to a white standard such as barium sulfate or magnesium carbonate. These materials are used by the industry to set a flat white “Lambertian” light distribution where every point in the audience would see the same image brightness. A gain of 1 represents a screen as bright as magnesium carbonate. A gain of 1.1 means it is 10% brighter and a gain of 2 means a screen is twice as bright. 
Like the saying, “Nothing in life is free” and a screen cannot create light. Gain is not the creation of light, it is the focusing and redirecting the diffused light back at the viewer. The price of this increased brightness is field of view, or viewing cone. The center of the screen and on axis viewing in the ‘sweet spot’ looks great, but anyone sitting off to the side will see a darker, sometimes even unwatchable image. So to increase the gain, or image brightness on axis, the light has to come from somewhere. Since a screen cannot create light, the off axis light is refocused back at the viewer. Too much gain has other negative effects too. Those viewing the image directly in front of the projector screen may have to endure a movie that is uncomfortably bright. And then there is the effect called “hot spotting” where the image literally shows a bright circle in the center of the image. Gain in front projector screen materials is made by adding “mirror-like” materials that will reflect light back at the projector instead of diffusing the reflected light in a Lambertian distribution. In the beginning it was silver, then came glass beaded products. In 1954, the first pearlescent materials were introduced. Pearlescents were innovative because they were clear and sparkly instead of silver. (That’s right, pearlescents aren’t something new or revolutionary) Pearlescents work by the process of interference. The first interference pigments were obtained and produced by Nature. Mother of pearl is an excellent example. Some seashells also have the interference effect. Interference is the separation of white light into its component colors much like what a prism does when it refracts light. By making a series of thin layers that are clear and refract light, an interference pattern is set up. Sparkle and shimmer are two ways to describe pearlescence. Man made pearlescents are made from the mineral mica. The mica is processed into small particles and then coated with a very thin layer of titanium dioxide. The layer is so thin that it actually allows light to pass through instead of acting like a normal pigment when it is used in paint. Here is the problem: each layer reflects a small percentage at the front face of the particle, most of the light is transmitted (passed through the mica flake) and refracted. When light hits the back surface of the particle, a small percentage is again reflected and most continues on through. If the particles get stacked one upon another, the number of light reflections gets to be quite large. The resulting sparkle can look bright and somewhat impressive. If the concentration of flakes isn’t dense enough, then the screen can look ‘sparkly’ when hit with high powered projector light. The down side is color shift and light separation. There is a newer solution and way around this prism effect and to increase gain without introducing color shifting other than 1954 technology and methods, and that is by the use of non-interference pigments. Instead of being based on mica, the key particle is aluminum oxide. It is thick enough so that it is opaque and will not allow light to pass through it as mica flakes (Pearlescent) do. It is then coated with the same thin layer of titanium dioxide that the interference pigment was but the optical results are much different. The reflections are reduced and the degree of color separation is minimized. Also the prism effect is eliminated, which in turn eliminates or greatly reduces any color shifting.  Specular gain is gain that is not created from the base color. In other words, a bright white screen will have a higher natural gain than a darker gray screen. If a shiny surface coating is applied over the same two screens, the gain on the white screen would be higher than the gray screen since the base itself is adding to the overall gain. The base gray also adds to the overall gain, but at a lower ratio, therefore it will have a higher specular gain. This can be confusing, and people may say “But how can a screen with lower gain hot-spot when a higher gain screen with the same surface doesn’t?” As explained this is due to the surface sheen of the coating on the gray contributing more to the gain than the base. Here are some graphs from when this was discussed about the laminate screens that hopefully will make more sense: The pie charts as a whole represent the total gain. With the same surface coating for each, what changes is the ratio's of the specularity to the surface color and the basic gain of the color. If the coating remains the same, and the color gain decreases, then the ratio of specularity goes up even though the coating hasn't changed or added extra sheen.  So even though the surface coating is the same for both materials, it makes up a high percentage of the overall specular gain for Fashion Grey than it does with Designer White. With more of the gain being from the coating, the specularity has increased, and that can increase the possibility of hot spotting. Hopefully this shed some 'light' on a few things and this thread is intended to discuss some of the confusing aspects about screens, color, and light. "Make everything as simple as possible, but no simpler." - Albert Einstein "If all else fails, spin the cat."- Grzboken | |||||
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| 08-16-07, 10:42 PM
| #5 (Link) | |||
| Re: Gain and other confusing topics. Thanks guys  I'll make a link from the data thread to here and maybe recopy it, but a link should suffice. I figure people can use this thread as a common discussion thread for various topics that they may find confusing. I'll work up something on viewing cone since a lot of people ask questions about that too, like questions like "Why is gray scale accuracy more important than Contrast Ratio? Things like that. "Make everything as simple as possible, but no simpler." - Albert Einstein "If all else fails, spin the cat."- Grzboken | |||
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| 08-17-07, 11:11 AM
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| What do the experts say? There seems to be constantly changing buzz words in the Screen world such as 'ambient rejection', viewing cone, texture- but one that always stays at the top of the list is gain. Since we started this thread off about gain and some of the ways that is achieved, I thought it would be interesting to show what the experts have to say. Quote:
Okay times up! This is from the makers of the High Power (HP) screen that has a gain of 2.8. But why would they say this if they make a high gain screen? High gain does have its uses but is more of a specialty screen. The typical theater brightness is between 12 to 22 foot Lamberts of light at the screen. To put that in perspective, Televisions produce 35 fL of light and some are even higher.
 Most Home Theater setups only require a screen with gain ranging from .85 to 1.8. The typical range is 1.0 to 1.6. Above 1.8 things start to get tricky and sticky. Most screens over 2.0 in gain switch over to Retro-Reflection screens. Okay I know some are saying 'Hold on, you're throwing more terms around again'. A retro-reflection screen is specially made so the light hitting the screen is reflected back along the projection axis. An angular reflective screen the projected light reflects at an angle equal to the projected light's incident angle. Think of angular reflection like a pool table and you're making a bank shot into the side pocket, and retro-reflective is like a traffic sign- when your car's headlights shine on the sign the light is reflected back at your eye and the sign appears bright. Here is what these two setups look like, this graphic is one I am sure everyone has seen many times: The example on the left is a retro-reflective screen and on the right is an angular reflective screen.   So a quick recap:
So what do we need? As stated, the typical screen gain used for Home Theaters is between 1.0 and 1.6. That isn't to say there aren't special situations that may require higher gain, but that is more the exception than the rule. 12fL of light at the screen is the minimum recommended amount of brightness for a fully light controlled room. When initially designing your Home Theater layout, use a 1.0 gain as your reference and starting point. Once you determine what the fL of brightness at the screen will be for your proposed setup you can start looking at gain. If the fL are below 12 then a screen with enough gain to bring it to that level of brightness is strongly recommended. If your setting has ambient light issues, then the screen will need to have even more fL of brightness at the screen as well as some other specifications that we'll get into later. What I have found is that 12fL is fine for controlled lighting, 13-14 is even better. Much more than that and the image starts to become too bright and can cause eye fatigue. If 12fL of light is the minimum for a light controlled room, I would say 14 fL is the minimum for ambient light, 15-16 is ideal. Ambient light has always been a problem though with projectors. In this case gain isn't necessarily the answer. Gain is only a tool to get the required fL needed. If your projector is bright enough to produce 14-16fL of light at the screen then gain isn't as big of a factor. Gray screens are typically lower in gain and many are less than 1.0, so take that into consideration with your calculations. I would drop to .85 for the gain in the equation. Again, if you can hit 14-16fL you are fine. If not, then a brighter projector, smaller screen, or higher gain is needed. Here is an example of 14 fL on a gray screen with an enormous amount of not just ambient lighting, but ambient sunlight.  ...and a few more of different images and angles...        These were from a 1700 maximum Lumen rated projector (video optimized is around 470 fL) on a properly balanced neutral gray screen with a matte finish. The reason I posted these was to demonstrate that ambient viewing is very attainable and easy too as long as certain guidelines are met. I personally don't watch things on the screen during the day, but many people want to so I test for that environment too. The best thing anyone can do when it comes to direct sunlight is to try and kill it at the source. Blackout Cloth is very popular for screens, but it also works extremely well for its intended use, which is to block incoming light during the day. Some people though do not want their living room to look like a cave during the day. This obviously is a compromise they have accepted, but there are things that can be done to help. Window film can knock down a lot of incoming light and I highly recommend it. Here are some examples of how much light various films allow through. VLT - Visible Light Transmittance, tells how much light can pass through the film. The lower the number, the darker the film.    From left to right the VLT ratings are 20% VLT, 35% VLT, and 50% VLT. So with the right setup where the projector and screen meet the 14-16 fL criteria and with a little effort to knock down the incoming ambient light, a projector is usable even during the day. Keep in mind it will never look as good and stunning as it does when the lights are out, but a very watchable image can easily be achieved. We covered a lot of ground quickly with the first post and this one. I will get something up for viewing cone and a few other topics, but feel free to jump in at any time with any questions. This thread is not totally an informational thread like the stickies tend to be and is an open format thread. For specific questions about a particular commercial screen or DIY application, please direct those to the appropriate person or thread. This one is more for those annoying general questions and confusing issues that a lot of people have. "Make everything as simple as possible, but no simpler." - Albert Einstein "If all else fails, spin the cat."- Grzboken | ||||||||||||||||
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| 08-18-07, 12:34 AM
| #7 (Link) | |||
| Viewing angle and viewing cone Well, since you asked...please find below my questions/thoughts about viewing conce and viewing angle. I hear these terms thrown out very often on this and other forums, and want to make sure that I understand the true meaning of them. So, let's discuss the viewing angle and viewing cone for the following scenario.  If I want at least 50% brightness at every seat, do I need a screen with a viewing cone 53.2 degrees (2 x 26.6)? The thought is that I can calculate the needed viewing cone based on the audience position, and then directly compare that to the viewing cone of candidate screens when I am selecting the screen for my application. For viewing angle, since 90 degrees is "ideal", is his/her viewing angle 62.4 degrees (90 - 26.6), even though his angle to the left part of the screen is 45 degrees and to the right side of the screen is 90 degrees? Viewing angle could also, I suppose, be defined as ideal at 0 degrees (meaning no deviation from perpendicular), which would shift these values accordingly (in this case, to 26.6 degrees). If the viewing angle is defined how I think it is (relative to the axis through the center of the screen), it seems like a misleading statistic to me. For the room layout above, I can be 26.6 degrees off of the center axis, but my angle to the full extents of the screen can vary significantly. In the sketch below, I've added more viewing positions to the same room.  Position #1 is the same as in the sketch above...26.6 degrees off center axis, and a 45 degree angle to the left side of the screen. Position #2 is still 26.6 degrees off-axis, but the angle now is much less than 45 degrees to the left edge of the screen. The opposite is true for position #3. Still 26.6 degrees off center axis, but now an angle of much greater than 45 degrees. So, all 3 seats can have the same viewing angle, yet the picture from Position #2 may look wonderful at the same time view from Position #3 may be unwatchable (due to the left portion of the screen being very dark at an angle of significantly more than 45 degrees). For the sake of argument, let's ignore the fact that Position #3 is too close to the screen to be a realistic seating position. The point remains, that the viewing experience from different seats that are the same angle off-axis can be very different. The further away from the screen that you are, the better off you will be (as far as a uniform image across the screen). For this reason, I think viewing angle should me defined as the angle between the viewer and the vector that is perpendicular to the far edge of the screen (edge on the opposite side of the viewer). That definition would give the worst angle from a given seat location to any portion of the screen, and in the illustrated case would be 45 degrees from seat #1, < 45 from seat #2, and > 45 from seat #3. The bottom line is that I recognize that viewing cone is typically listed relative to a percentage drop off in viewing brightness...e.g. a 50% viewing cone of 60 degrees, meaning that the gain is half on the on-axis gain when someone is 30 degrees off-axis. But, since in the process of selecting of a screen, the customer would typically uses the viewing cone spec in conjunction (comparing to) the viewing angle of the audience, that comparison can be flawed (for the reasons that I outline above). A number that only represents the number of degrees off of screen-axis (which is how I believe viewing angle is defined) simply does not capture the effect of angle to the full width of the screen on the viewing experience. Hopefully, you can follow my logic, Bill...my intent was to make this clear with the illustrations, but I'm not sure that I did so. | |||
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| 08-18-07, 01:14 AM
| #8 (Link) | ||||
| Re: Gain and other confusing topics. Viewing cone is the half gain of a screen. It is the point where the image is half as bright as it is when viewed directly on axis, or at zero degrees.  This is an extreme graphic example to try and illustrate this concept. The red lines represent the angle of half gain. As you have heard many times anyone sitting outside the the angle where the gain drops by 50% will definitely be in the 'cheap seats'. If I am understanding you though, your question is the half gain and viewing cone the the same, or is the viewing cone the total number of degrees or just to one side? Is that what you are asking? I can answer that if it is.  If we assume a half gain of 30 degrees, then the viewing cone total is 60 degrees. This is a very reasonable viewing cone and would accomodate most Home Theater setups. This was just assuming a arbitrary half gain but I think it illustrates the concept well. Like everything in life there are trade offs and compromises to be made but fortunately I think there are enough options to accomodate your setup. Quote:
"Make everything as simple as possible, but no simpler." - Albert Einstein "If all else fails, spin the cat."- Grzboken | ||||
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| 08-18-07, 01:24 AM
| #9 (Link) | |||
| Re: Gain and other confusing topics. Hi Bill, As you may recall, many people have tried to tell me that the addition of a matte poly top coat simply adds surface sheen and therefore boosts the gain. I will agree that it may add some sheen but that is not what I think is happening. At least it is not the significant thing that's happening. If you consider your sketch above, it shows all the light being reflected. I believe that some of the light is absorbed by a painted flat finish surface. By top coating the flat paint, we reduce the amount of light absorbed and therefore the screen appears brighter. So basically what I think happens is the wetting of the flat finish by the polyurethane increases the reflective efficiency. Does that make any sense to you? | |||
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| 08-18-07, 01:47 AM
| #10 (Link) | |||
| Re: Gain and other confusing topics. The graphic was just part of the explanation of Lambertian reflectance, not necessarily any specific screen surface. I agree that light is being absorbed by the base color. That agrees with total gain which includes the base color and any surface sheen that increases reflectivity. The poly coating is certainly adding a reflective coating to the screen surface, but it is only part of the total make up of the equation, the base color also comes into play. Look at the tests done on UPW. It has a higher gain to begin with without any poly being added as a top coating. When the poly is added there is a slight boost but the UPW is still carrying most of the weight. My question and curiosity is this- Since most applications are dealing with flat paints, the matte poly is bringing it up to a similar finish as a matte paint. What would be interesting to see, and sounds like something to test down the road would be to get a neutral in a matte finish and also in a flat finish. Then put a poly coating on the flat finish and see if it performs the same as the matte alone or better. My best guess is it will be similar in performance, but that is something that would have to be tested. A little side note, maybe a slight teaser  ...I have screen samples of DIYTheatre and actual paint samples from Digital Image (both the base and top coating) The base doesn't appear to be anything extraordinary, just a nice bright white paint or polymer. The top coating though looks interesting. It isn't a clear coating that's for sure. I spoke to the owner of Digital Image and he was telling me he spent the better part of two years developing and testing this. I hope to be able to make my test panels this weekend since my daughter has gone back to PA and the house is quiet (dead) now. "Make everything as simple as possible, but no simpler." - Albert Einstein "If all else fails, spin the cat."- Grzboken | |||
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| 08-18-07, 07:56 AM
| #11 (Link) | |||
| Re: Gain and other confusing topics. I'm sure most people can recall finding a really nice colored stone at the beach or in a stream. You brought it home and when it dried t no longer had much color. What do you do to get the stone to look nice like it did when in the water. You varnished it right. No doubt the varnish may have added sheen but the more significant thing was the wetting of the stone's surface. I think the matte poly does a similar thing with the flat paints. I agree with Bill that if you top coat flat UPW and eggshell UPW that are both tinted to the same color, you can't tell them apart. I top coated some gray panels with semi-gloss poly and gloss poly. Then I top coated both with the matte poly. You cannot tell them apart. There is something that takes place at the paint poly interface that I think changes the reflective efficiency of the surface. I just don't have the expertise to explain this. Last edited by Tiddler; 08-18-07 at 06:07 PM. | |||
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| 08-18-07, 04:40 PM
| #13 (Link) | ||||
| Re: Gain and other confusing topics. Quote:
Would you agree that, in my original sketch, the screen brightness of the left portion of the screen looks very different depending on if you are in Position 1, position 2, or position 3? Yet, all 3 positions are at a "viewing angle" of 26.6 degrees. Therefore, my contention is that "viewing angle" alone is not enough to compare against a screen's "viewing angle" when selecting a screen. One must also take the overall seating and screen geometry into effect...because viewing angle is defined relative to the center of the screen, and has nothing at all to do with how (in my sketch, for instance) the left side of the screen will look. Is my comment any clearer? | ||||
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| 08-18-07, 06:20 PM
| #14 (Link) | ||||
| Re: Gain and other confusing topics. I'm not sure if this help to clarify this idea of viewing angle and screen viewing cone but this was an attempt to show how the brightness is varying with position across a screen when the material has a narrower viewing cone. Quote:
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