eBay Screen Paint Roundup - Page 3 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #21 of 23 Old 10-28-11, 08:58 PM
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Re: eBay Screen Paint Roundup

Harpmaker wrote:
When people say that a mix must have a peak gain of at least 1.0 to not have negative gain they simply don't know what they are talking about.

Cali wrote: View Post
Pls explain further.
You want more explanation on gain or on those other people not knowing what they are talking about? OK, I'll play nice.

All gain values (and this almost always means the peak or maximum gain a screen is capable of) are relative. The standard reference is called a Unity target which can be made of several things, but most are made of barium sulfate or magnesium carbonate. These substances (usually in the form of a powder compressed into a disk or square) are as close to a perfectly diffusive white surface as can be made. This is a target that will reflect all the light that hits it and reflect it in all possible directions with equal intensity. They are called Unity targets because the reflected light from them during gain testing is considered to be 1.0 (Unity). To get a good idea of what a Unity target looks like just open a bag or box of corn starch and look at it under a bright light.

To test a screen for gain, after a reading is taken from a Unity target that target is replaced by the screen sample under test. The screen or the light meter is adjusted for angle until the highest (brightest) reading is obtained on the light meter. This is the peak reflectance value for that screen. To get the gain value of the screen the screens reflectance value is divided by the Unity targets reflectance value taken under the exact same lighting conditions (which should be a dark room and a directional light source).

The problem with using only peak gain based on a Unity target is that it can lead to false assumptions about screen performance. Peak gain alone cannot tell you what shade of gray a screen is, it only tells you the maximum brightness of the screen compared to a Unity target (white screens with a gain of 1.0 are also used by some, but the results are usually not as accurate since such screens are rarely as white as a Unity target and have a bit more gloss to compensate). It also cannot tell you the true reflective properties of the screen since only a on-axis reading was taken. To get a more accurate sense of screen performance we need to take reflectance readings off-axis as well. These can then be charted and we can see the screens "viewing cone" and determine it's "half angle" (the angle where the screen reflects one half of the brightness of the on-axis measurement) if it has one.

To say that a screen with a peak gain greater than 1.0 is brighter than a Unity screen is true, but unless we know the viewing cone of the screen we don't know for how long it is true as viewing angle changes. Many DIY screens that claim to have gains over 1.0 may in fact do so, but the ones we have tested have had a viewing cone where that brightness diminished quite rapidly and in the case of a Silver Fire screen the brightness of the screen past 15° was less than a Black Widow™ screen. The formulae for making SF has totally changed since the time of this gain testing. Has that screen mix improved? No one knows since the SF developers don't do any sort of color or gain testing except "by eye" which is demonstrably inaccurate.

If we have two screens that both have a gain of 1.0 and one is white and the other is a dark gray do they both have the same gain? The answer can be yes if the white screen had very little gloss and the gray screen has a lot of gloss (or other reflective elements), but the two screen will behave quite differently. Think of a matte white screen compared to a Black Diamond or Supernova.

As I demonstrated in my last post most gray paints will have a peak gain at least slightly higher than their TDG (totally diffusive gain) because they almost always have a bit of gloss to them and the more gloss they have the higher their peak gains will be, but the more gloss they have the greater the likelihood that they will hot spot. Both C&S™ Ultra and BW™ increase gain by using a base paint that has a bit of gloss, but we also add another paint that has reflective particles in it, thus their peak gains are considerably over TDG, but the trick is to do so without hot spotting or introducing other screen artifacts. Even though BW™ has a peak gain of less than 1.0 compared to a Unity target it has a large amount of positive gain compared to it's TDG. We aren't "fudging the numbers" or "spinning" the figures, to get an idea of how a screen will perform one needs to take all these things (peak gain, geometric gain, viewing cone, viewing angle) into consideration.

Oh yes, geometric gain is TDG divided by peak gain. The GG (geometric gain) of C&S™ Ultra is ~1.32 and the GG of BW™ is 1.82.
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post #22 of 23 Old 10-29-11, 09:34 AM Thread Starter
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Re: eBay Screen Paint Roundup

I knew you'd come through Harp! Thanks for taking the time to write that up!

As you can see Cali, Harp is a lot better at explaining things than I. I know them and understand them, I'm just not as gifted when it comes to typing it all out.
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post #23 of 23 Old 10-29-11, 12:09 PM
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Re: eBay Screen Paint Roundup

All clear now Thanks Harp.
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ebay , paint , roundup , screen

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