Gain and other confusing topics.

[wbassett]

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.

If a surface exhibits Lambertian reflectance, light projected on it is scattered such that the apparent brightness of the surface to an observer is the same regardless of the observer's angle of view. More technically, the surface luminance is the same regardless of angle of view.

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.

Surface sheen is another way to quickly increase the gain, but it also raises the specular gain which almost always results in hot spotting. The higher the projector’s Lumen output, the more noticeable the hot spotting becomes.

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.

[rufinatti]

Excellent write up. Thanks for the history lesson re: the "silver screen".

[mechman]

Bill,

Excellent write up! This should be a sticky, either on it's own or within the data thread.

mech

[cynical2]

mechman wrote:

Bill,

Excellent write up! This should be a sticky, either on it's own or within the data thread.

mech

I AGREE

[wbassett]

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.

[wbassett]

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.

Da-Lite wrote:

Gain, we must always remember, does not imply amplification. No screen can add power to the display. All available brightness is created by the projector and only the projector. So when a gain screen exhibits increased brightness at its center, it is certain that it has robbed that extra energy from somewhere else. With all diffusion screens, therefore, the higher the gain, the lower the uniformity. This is the principal reason to recommend low gain screens whenever possible.

I'll let that sink in for a minute.

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.

		What is a foot Lambert or fL? That is the amount of light at the screen. To calculate this use the formula Keep in mind that this is Video Optimized Lumens, not the maximum Lumen rating of the projector- those two numbers are very different. Also the above formula is assuming a screen gain of 1.0. To calculate for a different screen gain, multiply the result by the screen gain.

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:

• Gain is the ratio of brightness of your projection screen material to a white standard such as barium sulfate or magnesium carbonate. The higher the gain, the lower the viewing cone becomes.
• Retro-Reflection screens are a high gain screen that reflects light back towards the source, like a stop sign.
• Angular Reflection screens reflect the projected light away from the light source, like a bank shot in pool.

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.

[cynical2]

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.

[wbassett]

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.

cynical2 wrote:

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)?

So the answer is yes, for your scenario you'd want a 53.2 degree viewing cone, or a half gain of 26.6 degrees.

[Tiddler]

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?

[wbassett]

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.

[Tiddler]

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.

[mechman]

Another interesting thing to note is that I applied matte poly to get rid of the sheen and the hot spots.

mech

[cynical2]

wbassett wrote:

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.





So the answer is yes, for your scenario you'd want a 53.2 degree viewing cone, or a half gain of 26.6 degrees.

Thanks, Bill. I am having trouble conveying my point...let me try this another way.

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?

[Tiddler]

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.

tiddler wrote:

I took the sample panel with the two coats of 2xPearl-Folkart and applied one coat of just matte poly. The idea was to eliminate any possibility that the Folkart Pearlizing Medium had added sheen to the surface. As I have said before anything other than a very low luster sheen is the enemy of rolled DIY Screen Solutions.

30 Degrees Off Axis:
|. . Before Poly . .| |. . After Poly . .|






It appears to have muted the warm spot between the first and second position.

When considering the viewing of a movie from a less than ideal angle one must realize that it may not just be a less bright image it may also vary in brightness considerably across the screen as in this example:

tiddler wrote:

Here ia an attempt to demonstrate the variation in rightness with viewing angle.

I started by leting the camera decide on settings for the retractable screen with white light projected. Then I set the camera in manual to lock the settings. I the took picture of the retractable alone and with the Folkart sample panel in the four hanger locations. This should give some indication of brightness uniformity and viewing angle characteristics. I Think?

On Axis:






30 Degrees Off Axis:

[wbassett]

I think I know what you are getting at but might not be sure...

The only thing I can say is screen manufacturers measure from the center as on axis and the 0 degree mark, they don't measure from off angles like that for gain measurements for their reference point, only for half gain. Not something I'm making up, it's straight from the horses mouth. They also go on a few assumptions as known parameters, such as there is a recommended viewing distance formula and based on that the viewing cone should remain the same. That is 1 1/2 times the width of the screen as the optimal seating position. Now if people sit closer and at off angles, say on the floor, then they are no longer at the normal and recommended distances and yes I can see where that could be a problem.

Another reason is the projector light is the strongest in the center. For a unity gain screen you could move all over and still get the same readings from a light meter, but once gain is introduced things change. Now when you move around you get different readings because the light is being directed on a specific viewing angle, which is based on that center axis reading and not an off center reading. Does that make sense? This way they have a set standard that screens can be measured and you can go from one manufacturer to another and be able to relate their specs and know what to expect.

cynical2 wrote:

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.

You bring up a good point, but unfortunately you'd have to convince every screen manufacturer to change their way of doing things and thus create a new standard and that's obviously not going to happen. I'm not saying you're thinking this, but what is presented isn't made up, it's what the commercial companies define as their standards and they all are pretty good at adhering to them so us as consumers can have usable specifications to compare things. Otherwise it would be a mess and each screen company could just say whatever they wanted and even change their stance on something if they suddenly feel it would bring more sales.

Maybe this will help some, this is from Da-Lite-

Da-Lite wrote:

When most of us see a movie at the local cineplex, we generally like to sit somewhere between half way back and in the middle of the screen. This, we feel, is the best seat in the house. No doubt this is in most cases just that. However, show up a little late to the screening of that “must see” new movie and you will find that the only seats left are those in the front row and perhaps are the seats that are to the far left or right of the screen. These are by most standards considered the worst seats in the house. Why is that the case? Well, as you would assume, the angles at which you are required to watch the movie, both horizontally and vertically, are sometimes uncomfortable. The human eye’s visual field of view is 135° High by 160° Wide. Although this is a very impressive range of vision, it is possible, as we know from the movies, to be too close for comfort. So, exactly when is this the case in a commercial boardroom or a residential home theater?

For most commercial applications that the closest we should sit to the screen is 1½ times the width of the screen and furthermore we learned that this row could be 2 screen widths across. So, does this still apply in the 1080p revolution? In order to answer this question, we need to see how this rule came into being. The calculations behind this recommendation were based on the off-axis angle at which a text character becomes more elliptical and less recognizable. The maximum angle at which we can acceptably view this character is 45º. Therefore, by drawing sight lines from a respective screen out from the left side of the screen to the right side of the audience and vice versa, we end up with an ever increasing cone which has an intersection point that is .5 widths out from the screen. At this point, only one person would be within the acceptable viewing position. Taking this out, further reveals our rule of 2 screen widths at 1½ times the width back from the screen. So as you can see, this rule has nothing to do with the resolution of the screen. It has only to do with the angle of which we are viewing the screen. Therefore, in the commercial world, we can make the assumption that our guidelines are still applicable.

As for the residential side of the equation, things become a bit stickier. If the room in which the screen is located is one that is multi-purpose and has seating that may be off-axis at harsh angles, the rules we use for the commercial world should be applied. However, if instead, we have a dedicated theater room with seating arranged much like the local cineplex, the rules change just a bit. If we apply the same logic that was used for the commercial boardroom application above, then we would say that a row that is 1 width back from the screen is able to be 1 width across. After all, the math works correctly.

However, let us think about this from a real world perspective. As an example let us look at the same 45" x 80" screen that was used above. Our normal rules for sizing a screen say that we should be no further back than 3x the height or 11.25 feet. In order to then determine the closest seating distance, we would say that it is equal to the width or 80". Is this too close? According to our maximum off-axis viewing angle, no it is not, but what about the pixels? Will we see them seated this close? In order to answer this question, we need to look at the human visual system. If we are lucky enough to have 20/20 vision, that basically means that we can clearly distinguish one arc minute of contrasting information, from 20 feet away. Converting that to inches, tells us that in order for us to distinguish the contrast of that item, in this case the gap between two pixels, the pixel will need to be larger than 0.069 inches in height.

Looking at our examples from above, we learned that our pixel is 0.0625 inches in height for the lesser of the two resolutions and 0.0417 inches in height for the greater of the two. As you can see from 20 feet back, neither one of them becomes an issue. However, once we move forward on the 1024 x 720 resolution we begin to have potential issues where as the 1080 x 1920 image does not cause problems until we get to somewhere around 12 feet from the screen. So as you can see the scenario where we would be seated one screen height or 80 inches from the screen is way too close and we would likely be able to see the pixels. Since we have determined that at 12 feet is where we will potentially begin to see the pixels, let us use that information to determine the optimum seating area for the screen and ultimately provide us with a formula for determining the proper screen height. By taking the 12 feet and dividing our screen height of 45 inches, we have determined that the optimum viewing distance for a 1080p projected image is equal to 3 screen heights. So, our 1/3 rule that we have been using in the residential world is indeed still applicable and is not too close for comfort.

I know they weren't talking about gain and viewing cone there, but they really do put some thought into optimal viewing distances and angles of viewers. Based on their formulas, they calculate a normal distance from the screen and anyone seated within the viewing cone (for your example say 53.2 degrees) will be fine. If a person moves outside of that range, then they are changing the conditions, just like getting stuck in the front row off to the side at the theater as in their example. (Or being forced to park WAAAAAY off to the side at our local drive in and barely be able to see a good image just because we have a pickup truck! )

Hopefully I understood and answered your question this time!

[mechman]

cynical2 wrote:

Thanks, Bill. I am having trouble conveying my point...let me try this another way.

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?

Let me try to figure this out. Your basic question, as it seems to me, is this: Is the screen brighter up close than far away? Yes. How's that? However, if you want to be within the 'cone of utilization' (I just made that up! Sounds good though!) for your particular screen, than yes, you have to be within it's viewing cone. And yes if you're sitting 5 feet from the screen it's gonna be a bit brighter than 30 feet on the edge of the viewing cone.

One thing I'd like to add to Bill's post is that it should say unity gain screens or less you could move all over and notice no drop off in performance. This shot is way off axis and there's no drop in performance at all.



Now if I was to add gain by putting a pearl topcoat on...

mech

[cynical2]

I think Bill understands what I was getting at...not that brightness increases as distance from the screen changes, rather that the angle to the edges of the screen (at the same "viewing angle") is very different depending on distance from the screen.

I've clarified my point in the sketch below.



Note that all 3 seating positions are at the same viewing angle of 26.6 degrees. However, note that their off-axis angle to the left edge of the screen is VERY DIFFERENT, depending upon seat location (ranging from 38 degrees to 60 degrees).

So, for a screen with gain, the person that is closer to the screen may have an extremely dark image on the left side of the screen compared to the person sitting further away...even though all 3 viewers are at the same exact viewing angle and all viewing the same screen.

So, my point is that viewing cone and viewing angle aren't enough to fully define the expected viewing experience...seat position, screen size, etc also drive the viewer's results.

In other words, one could buy a screen and make sure that their seats are all within the screens viewing cone, and the view moving towards the edges of the screen could still be very dark/unwatchable.

[wbassett]

mechman wrote:

One thing I'd like to add to Bill's post is that it should say unity gain screens or less you could move all over and notice no drop off in performance. This shot is way off axis and there's no drop in performance at all.

Long post and easy to over look I know...

For a unity gain screen you could move all over and still get the same readings from a light meter, but once gain is introduced things change.

[mechman]

Jim,

I understood completely! You want to

cynical2 wrote:

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).

So that it justifies this:

cynical2 wrote:

So, for a screen with gain, the person that is closer to the screen may have an extremely dark image on the left side of the screen compared to the person sitting further away...even though all 3 viewers are at the same exact viewing angle and all viewing the same screen.

and this

cynical2 wrote:

So, my point is that viewing cone and viewing angle aren't enough to fully define the expected viewing experience...seat position, screen size, etc also drive the viewer's results.

because we're doing this

cynical2 wrote:

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 only way I see it is that you want Da-Lite, Draper, Stewart, Carada, HoloDisplays, etc. to come up with a new standard for viewing angle based upon the assumption that we're going to sit too close to the screen.

Seriously though! Before I head to the north country this aft I'm gonna hang the High Power and test your theory out. Luckily my FG has 180 degree viewing cone!

mech

[cynical2]

wbassett wrote:

Long post and easy to over look I know...

Wow...you two are scary.

Were you separated at birth???

Bill, I'm looking forward to hearing back from you on changing the industry's explanation of viewing cone and it's limitations when compared to viewing angle...



Seriously, thanks for your explanation...I do think you understand what I was trying (unsuccessfully) to communicate. Hopefully, my last sketch will clear up what I meant for anyone else that's interested.

[wbassett]

In all honesty cynical, the same problem exists with a lot of rear projection TVs. This is something that isn't going to change until technology changes (maybe with the new laser projectors). The problem with projectors is everything get's amplified when things are blown up to monsterous sizes. Any flaws are more noticable.

That even goes for movies and the High Def format. I've seen some movies where on standard definition it looked great, but on high def or upconverted to 1080p you can actually see the makeup or the lines where they tried to blend in a wig or bald skin head cap. The bigger and clearer things are they more we notice things.

[wbassett]

cynical2 wrote:

...not that brightness increases as distance from the screen changes, rather that the angle to the edges of the screen (at the same "viewing angle") is very different depending on distance from the screen.

...

So, for a screen with gain, the person that is closer to the screen may have an extremely dark image on the left side of the screen compared to the person sitting further away...even though all 3 viewers are at the same exact viewing angle and all viewing the same screen.

Oh, here is a mind blower too if you factor this in... a screen can also look different if you are sitting on the floor or standing up depending on the composition and specifications of that particular screen.

So, my point is that viewing cone and viewing angle aren't enough to fully define the expected viewing experience...seat position, screen size, etc also drive the viewer's results.

In other words, one could buy a screen and make sure that their seats are all within the screens viewing cone, and the view moving towards the edges of the screen could still be very dark/unwatchable.

I've reread your posts and what you say is true, but if you follow the recommendations for viewing distances, there is a minimum and maximum distance. As long as the viewer is within that range and also within the viewing cone, the screen image should look uniform to them. Outside of those recommended distances and angle, everything changes. So even if you are technically within the viewing cone, but closer or further than the recommended distances, you won't see an optimal uniform image.

Keep in mind that in addition to screens, projector's also have a uniformity rating. So everything is a balancing act that's for sure. It is totally possible for two people to have rooms laid out identical, and have the same screen but different projector's and have a very different look. One person may have to opt for a screen with a different viewing cone or properties than the other, so it's everything taken into consideration together as a whole rather than individual components.

I feel people misunderstand gain and some see it as a 'brightness adjustment' when it's not. It's a tool and used to achieve the recommended screen brightness in fL and from there you control the brightness, CR, and color levels with the projector.

cynical2 wrote:

In other words, one could buy a screen and make sure that their seats are all within the screens viewing cone, and the view moving towards the edges of the screen could still be very dark/unwatchable.

That really shouldn't be the case if the screen's viewing cone value is correct. Again anyone within that specified set of parameters should see a good image. Yes off to extreme sides and on the fringe there could be a slight drop off, but that actually is to be expected to some extent and even is an issue with some HDTV's as was previously mentioned. Gain doesn't stay at say 1.6 up until the half gain point and then suddenly drop off to 50%, it gradually gets lower (therefore dimmer) as the viewing angle increases.

Now, it is possible that some companies may have fudged their viewing cone angles to make a screen look more appealing, but none of the reputable and established screen companies would do that. There is too much at stake in the form of bad reviews and customer satisfaction, as well as their reputation once word gets out. DIY? As good as some of the methods are it doesn't always adhere to industry standards so one method could be wildly different than another method with the same color and reported gain. Things have gotten a lot better with DIY though and for some a set of standards is starting to be followed and I personally feel that is nothing but good news for everyone!

Did I even come remotely close to answering your question? I think we've just about exhausted this one, at least I know I am exhausted!

[cynical2]

wbassett wrote:

<snip>

Bill, Yes...sorry I should have been more explicit. You answered my question with your previous post. Given my (extremely) limited knowledge of screen performance, I was just thinking about things from a theoretical standpoint...namely that seats at the same "viewing angle" (per it's definition) can have very different views of the edges of the screen, depending on where in the room the seats lie.

I also understand that it's probably impossible to implement a standard that would take that difference into account, given that it is room specific. Based on your input (which I trust much more than my own, quite frankly), it sounds like within reasonable seating distances you should be OK using viewing cone relative to viewing angle when choosing a screen. Sounds reasonable to me.

So, yes, as far as I'm concerned we can sunset this discussion. I am (personally) satisfied with the explanation. My intent wasn't to stir the pot regarding the validity of industry-standard parameters, only to educate myself (and hopefully others who may have similar questions) on how those standards can be considered complete when other factors are at play. I think the bottom-line answer of (paraphrased) "they are legitimate within the range of recommended seating/screen configurations" about sums it up.

Again, my apologies for dragging it out, but I wasn't sure I was conveying my question clearly. And, thanks to you, mech, and tiddler for your patience in getting me straight.

With that, I'll shut up now.

[mechman]

wbassett wrote:

I've reread your posts and what you say is true, but if you follow the recommendations for viewing distances, there is a minimum and maximum distance. As long as the viewer is within that range and also within the viewing cone, the screen image should look uniform to them. Outside of those recommended distances and angle, everything changes. So even if you are technically within the viewing cone, but closer or further than the recommended distances, you won't see an optimal uniform image.

I'm going to assume I don't have to set up the High Power now and take pics from within it's narrow cone...

It can wait until I return from the three day getaway.

mech

[wbassett]

cynical2 wrote:

Again, my apologies for dragging it out, but I wasn't sure I was conveying my question clearly. And, thanks to you, mech, and tiddler for your patience in getting me straight.

It was a valid question and one that seemed to be getting kicked around a lot at a few places.

I think you summed it up well.