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Discussion Starter · #1 ·
I've been reading a lot here about about DaLite's HiPower fabric. I currently have a VideoSpectra 1.5 surface on my Model B. The thing that bothers me most about the screen currently is the waves that have developed in the fabric. This makes for annoying visible variations in picture quality with camera pans or solid light colored backgrounds. I can imagine that the HiPower material probably isn't any less likely to develop waves, but will the waves be less visible in a projected image due to the reflective nature of the fabric???


Also, where can I find information about the viewing cone of the fabric? From my seated position, the bottom of my screen is typically about eye level. I just wonder how much variation in brightness there will be.
 

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Hi Jwtseng,


On a reflective material (most screens at 1.5gain or less), you will notice waves as they change the angle of reflection of the light. Basically, the light bounces off at a different angle (at the wavy area) than all of the surrounding light. You're eye percieves even slight waves on a standard reflective surface, because the change in the angle of reflection yields significant changes in the gain across the wave. Some light will focus right back at you, increasing the gain at that spot, and other light will scatter much more than expected decreasing gain right next to it.


Now the highpower is a retroreflective surface, not reflective. The highpower surface is essentially an ultra-fine coat of glass beads, on the order of 90 times denser than the beads on a typical glass bead screen. This means that for all practical purposes, you can treat it like a regular screen, but it treats light like a glass beaded screen. Each bead on a glass beaded screen gathers input light, and focuses it onto a small point at the back of the bead, where it then reflects, and travels back out of the bead in the same direction at which it entered. This phenomenon is very apparent in a small sample of a highpower screen, or even in common street signs. Even if you shine a light on the screen from a sharp angle, it refelects back straight at you, instead of carreening off some other direction. This means that waves don't cause a drastic shift in gain, it is only your distance from the projected light source that will cause shifts in gain (which is therefore the source of the viewing cone) So even if a highpower does "wave," which in my experience is less than other screen materials due to the added strength of the glass bead coating, the waves will be almost unnoticeable.


When considering highpower as a screen of choice, you'll need to mount your projector as close to your eyes as possible for best effect, the retroflective nature of the screen will not produce variations in brightness, but the viewing cone drops sharply over the first 20 degrees, at which point it becomes very close to the gain perceived on a matte white surface.


Hope that helps,

Andy
 

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Andy,

By far that was the most concise and most "plain english" explanation I have heard so far explaining the Hi Powered screens. No offense to the other members of the forum but sometimes alot of the explainations are just alittle over my head. Anyone agree?


Ken
 

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Discussion Starter · #5 ·
So if my projector is mounted on the ceiling, I may be already be outside the "viewing cone" of the hipower screen? Will the waves be more visible outside this viewing cone?
 

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Ken-

Thanks! I appreciate it.


jwtseng-


Regarding the ceiling mount, yes, you will most likely be outside of the viewing cone. How much outside depends on your distance from the screen and how high above you the projector sits. If you have a teenage kid nearby, give them the numbers as a trigonometry assignment :)


Regarding the noticability of waves, my education tells me that they should be no more prevalent no matter the axis of viewing, but my experience is lacking. I'll try an experiment tonight to test it. In theory, when viewed on axis, each bead is sending ~100% brightness back at your eyes, and when viewed off axis it would send, let's say 40% back to your eyes. However, that 40% should be very uniform across the whole screen, even in wavy areas. A standard reflective screen, however, would lose some gain outside of the viewing cone, but the scattering effect of the wave (I was talking about in my previous post) will yield a similar pattern of brightness/darkness that it causes on axis.
 

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Discussion Starter · #7 ·
Andyspix:


Thanks so much for your excellent explanations! Can you report back on your experiment when you get a chance? The biggest gripe I have with my screen is the visibility of waves. I don't think I have a problem with picture brightness. I mean, I think my projector would produce fine images even on a 1.0 gain matte white screen. So even if I *was* outside the viewing cone on a high power screen, I think the image would still be plenty bright...as long as brightness was uniform from any given viewing angle.


So if everything I'm hearing is true, I think that the HiPower fabric may be a good upgrade for me (and my budget).
 

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Hi all,


I did a simple experiment in which I taped a sample swatch of highpower to my full highpower screen, with a small bulge in the sample to simulate a wave. On axis, the swatch was almost invisible, but off axis it became more and more noticeable, looking for all intents and purposes like a wave. At about 30 degrees off and beyond, I had a hard time telling the difference between it and a matte white model B I had before. However, when the ambient light was minimized to almost complete darkness, the wave faded to almost nothing


I've thought on this some, and believe I know the reason. Basically, there is no such thing as perfect retroreflectivity. Beyond a certain angle, ambient light approaching the screen from the sides starts to reflect off of the surface of the bead instead of entering the bead and retroreflecting. It's similiar to looking through a window straight on, vs looking from an angle. The sharper the angle, the more the glass acts like a mirror, showing you reflections, and less it appears transparent. I tried taking a few pictures, but my ancient camera just doesn't show the differences well enough, you can't tell anything about the waves at either angle.


In short, it appears that off axis, a highpower cancels waves slightly better than my matte white screen did due to the effect of the ambient light, but not much. On axis the waves are not noticable at all, and off axis waves in complete darkness are very minimalized. In the end, if you truly loathe waves, get a fixed wall screen (da-snap etc) , not a pull down, of highpower and I doubt you'll ever see another wave


Cheers,

Andy
 

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In my experiences with both a Dalite pulldown glass beaded screen and a Dalite pulldown HighPower screen, the Highpower is dramatically better (i.e. less noticable) at waves in the screen.


If you ceiling mount your projector with a Highpower, the results will still be excellent... (as are mine)
 

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Discussion Starter · #10 ·
Andyspix:


Thanks for your continued observations. I'm still contemplating the High Power fabric. Despite your off-axis findings, I feel fairly certain that waves would still appear less pronounced than on my current Video Spectra screen. I have my own swatch of High Power fabric coming from DaLite.


Getting back to the whole "off-axis" discussion, I noticed that DaLite describes the viewing angle of the High Power fabric as 25 degrees. I wonder what exactly this means. I assume that the term "viewing angle" describes the angle formed by the projector to the screen center and back out to the viewer's eye. But I have read that the gain of this High Power fabric actually drops *below* 1.0 somewhere around 20 degrees. Here is a thread I found that contains a graph of gain measurements for High Power fabric as well as a viewing angle calculator (excel spreadsheet).

http://www.avsforum.com/avs-vb/showt...hreadid=201122


Anyone have any thoughts? By my estimations, my viewing angle is about 20 degrees (ceiling mount). I think that this calculation of viewing angle must be different from what DaLite is calling "viewing angle".
 

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jwtseng-


I believe the viewing cone of 25 degrees means the half gain point.


This means that at 12.5 degrees to the left or right of the projector, the highpower should yield a gain of about 1.4. beyond that the drop is more gradual, much like a gaussian (bell curve). My eyes can't tell the difference beyond about 12 degrees either way. I register either "really bright" or "about matte white" The dropoff beyond the viewing cone is too gradual for me to distinguish, and even viewing the screen from 90 degrees (essentially standing next to the wall) the image is plenty bright in my setup.


Cheers,

Andy
 

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Discussion Starter · #12 ·
So the 25 degrees number refers to left-right orientation only and not top to bottom? I'm assuming that the high power fabric is oriented in a specific direction if your statement is true. I think I read something about the fabric having grooves that must be oriented vertically.
 

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Discussion Starter · #14 ·
That's what I figured. 12.5 on either side of center. I guess I'll just have to bite the bullet. It can't be any worse than the waves I'm dealing with right now.
 

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Andy,


Thanks for your highly informative posts. Are you sure that the 25-degree "viewing angle" of the High Power fabric means a cone of +/- 12.5 degrees? Ever since I read the Da-Lite specs page, I have been wondering whether it meant +/- 25 degrees or +/- 12.5 degrees...
 

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Well, here is the optical characteristic of the Kikuchi Super Grain Beads 260G fabric (gain 2.6), which is somewhat similar to the Da-Lite High Power. It says the half-gain angle is +/- 15 degrees. The gain also seems to fall under unity outside the cone. Just FIY...
 
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