sheetrock question - Page 2 - Home Theater Forum and Systems - HomeTheaterShack.com

Old 06-02-10, 01:20 PM
Moderator Emeritus
Don

Join Date: Oct 2007
Location: Central PA
Posts: 3,772
Re: sheetrock question

Quote:
newscreen wrote: View Post
By loss of focus, I think I meant a less sharp picture--will this also happen?
With a 141" diag. screen each pixel will be around 0.064" square (assuming true HD content). I'm not sure, but I don't think the human eye can resolve that small a size from 17' away.

Quote:
If I were to double screen size to 184" x 104", I come up with 5L, which I gather is far too low.
Yeah, 5 fL. is getting pretty low for all but a white screen in a perfect "bat cave" HT.

Quote:
If I wanted a minimum of 12 lumens with a new lamp (thus ending at 6 lumens before failure), and using Scorpion N8, I find a screen size of 122.7 x 69.3 (141" diagonal with 11.7L). Am I good to go?
Lumens is the amount of light the projector is putting out, fL. (foot-lamberts) is the brightness of the screen. Lumens is a constant, but the fL. will change with the size of the screen.

My understanding is that while most PJ lamps lose around 50% of their brightness over their life span, the lose isn't linear and most of it occurs within the first half of bulb life.

To put this in photographic terms, a PJ lamp will lose about 1 f-stop of brightness over it's life.

When we start talking about minimum brightness levels for an acceptable screen things start getting VERY subjective. I would be happy with your above scenario, but you may not be. If you really intend on having a screen that is only being hit with 6 fL. of light for an extended period of time I would recommend Cream&Sugar™ (N9).

Quote:
I have been told that a screen wider than twice the seating distance from the screen can result in a loss of detail, hence my original 106" diagonal screen for a seating distance of 17'. Is this a good rule to follow, or am I OK with a 141" diagonal screen at 17'? If 141" diagonal is good, would you stay with Scorpion N8 or go to N8.5 or C&S N9? Thanks in advance.
According the the THX Guide, the "best" seating distance would be at 168 inches or 14 feet (after rounding slightly). This is just a guide and proper seating distance is another subjective thing. A spreadsheet that will let you play with HT setup can be found here.

If you are planning on using the screen with an old lamp for an extended time then use C&S™, but you will be giving up some of the attributes of a darker gray screen.
Harpmaker is offline

Old 06-02-10, 02:37 PM Thread Starter
Shackster

Join Date: May 2010
Posts: 28
Re: sheetrock question

MatrixDweller and Harpmaker--Thank you.

Is there any kind of table listing recommended ranges for foot-lamberts at screen for each of your paint formulae, perhaps listing them as "acceptable," "good" and "preferred"?
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Old 06-02-10, 03:36 PM Thread Starter
Shackster

Join Date: May 2010
Posts: 28
Re: sheetrock question

Harpmaker,

The spreadsheet you linked gives a recommendation of 114" diagonal (99.4 x 55.9) for 16.1 foot-lamberts and an ideal seating of 14.8'. THX link gives ideal seating 0f 11.5'. Any reason to choose one over the other?
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Old 06-02-10, 04:48 PM
Moderator Emeritus
Don

Join Date: Oct 2007
Location: Central PA
Posts: 3,772
Re: sheetrock question

Quote:
newscreen wrote: View Post
Is there any kind of table listing recommended ranges for foot-lamberts at screen for each of your paint formulae, perhaps listing them as "acceptable," "good" and "preferred"?
Not that I know of. About the closest "official specification" is one from the film industry (SMPTE) that states, IIRC, that in a movie theater the screen should be producing 16 fL. from an "open gate" (similar to a PJ showing a 100IRE White image) when viewed from the center seat of the middle row of the theater.

To quote a study: "The THX Theater Alignment Program surveyed over 300 theaters in the U.S. and found the average fL. measurement to be 11 foot-Lamberts with some as low as 7-8 foot-Lamberts, far from the recommended 16 foot-Lamberts". So while the "perfect" fL. level to simulate a movie theater should be 16 fL., a lot of theaters fudge that number and the average was only 11 fL.

I just found a very interesting PDF file from Runco (who makes your PJ) here that details how they determine the specifications of their projectors. I am quite impressed by this. It spills the beans on how PJ manufacturers can fudge the ANSI measurements to produce unrealistic numbers for brightness and contrast. Runco doesn't do that and their PJ specs are what you will actually get in a properly set up HT. I tip my hat to Runco.

BTW, to compare two screens to determine fL. multiply the number obtained by dividing lumens by screen size (which finds the fL. rating of a 1.0 gain screen) by the gain of the screen under consideration. For example, if your PJ is producing 12 fL. with a 1.0 gain screen (also called a Unity or Unity Gain screen) that screen would actually be reflecting about 10.7 fL. if it was painted with Black Widow™ (which has a gain of about 0.89). We have not done extensive gain testing of our other screen mixes yet, but being lighter grays they would have higher values.

Also, don't get hung up on gain values (especially thinking that higher is better, as some other DIY screen forums preach). ANY screen that has a gain greater than 1.0 WILL have a narrower viewing cone than a Unity screen; and most screens I have seen that have gains over 1.5 have such a narrow viewing cone that they actually hot spot.

Quote:
Harpmaker, the spreadsheet you linked gives a recommendation of 114" diagonal (99.4 x 55.9) for 16.1 foot-lamberts and an ideal seating of 14.8'. THX link gives ideal seating 0f 11.5'. Any reason to choose one over the other?
Not really. Those are simply recommendations from people that have studied the problem, and while I'm not saying to ignore them it is a fact that some people have other preferences. Myself, I prefer a smaller screen than is normally recommended because I don't like to have to turn my head, or even move my eyes rapidly a great distance, to see what is happening from one side of the screen to the other. Others prefer even larger screens than recommended by the "experts".
Harpmaker is offline
Old 06-02-10, 07:41 PM Thread Starter
Shackster

Join Date: May 2010
Posts: 28
Re: sheetrock question

Harpmaker, thanks again. By the way, for the spreadsheet you linked to, that I used for the above calculations, I used .9 as a guess for the gain. I believe I read elsewhere, Black Widow is similar to GrayHawk which Stewart lists as .9 also. So with Scorpion, I should have a safety margin to start with and as the bulb ages. Regarding screen gain as a topic, thanks to your site I had already learned enough not to overdo it.
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Old 06-02-10, 08:28 PM
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Don

Join Date: Oct 2007
Location: Central PA
Posts: 3,772
Re: sheetrock question

Quote:
newscreen wrote: View Post
Harpmaker, thanks again. By the way, for the spreadsheet you linked to, that I used for the above calculations, I used .9 as a guess for the gain. I believe I read elsewhere, Black Widow is similar to GrayHawk which Stewart lists as .9 also. So with Scorpion, I should have a safety margin to start with and as the bulb ages.
Correct on all counts!

Quote:
Regarding screen gain as a topic, thanks to your site I had already learned enough not to overdo it.
Cool!

It's quite frustrating when I read others saying things like "Any screen that has a gain below 1.0 will not give optimum results". Such people truly don't understand how front projection screens work and are living in the past when underpowered PJ's were the norm. Todays PJ's don't need high gain screens except in very special circumstances.
Harpmaker is offline
Old 06-02-10, 08:52 PM Thread Starter
Shackster

Join Date: May 2010
Posts: 28
Re: sheetrock question

Harpmaker,

Thank you again. I think that I was equating screen gain with N as a source of my confusion.
newscreen is offline
Old 06-02-10, 10:07 PM
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Don

Join Date: Oct 2007
Location: Central PA
Posts: 3,772
Re: sheetrock question

Quote:
newscreen wrote: View Post
Harpmaker,

Thank you again. I think that I was equating screen gain with N as a source of my confusion.
No problem.

While a screens N level will affect it's gain (the lower the N level the less the gain), this can be compensated for, up to a point, by adding reflective elements to the paint. When this compensation is taken too far we start to get negative effects such as a granular appearance in the image, shimmering in bright areas of the image, and at the worst, hot spotting. We do quite extensive testing of our mixes to make sure none of these effects are present before introducing them in the DIY Screens forum here at HTS. The Elektra™ mix was posted a bit early in this process since it answered an urgent need by a member, but it is on the Screen Developers forum here to reflect that it was still in testing (it hadn't even been named yet!). A number of people were kind enough to risk using it to make their screens and it has proven to be a good mix. All that has to be done before moving it to the DIY Screens forum is to get some comparative photos taken (which I will do when time permits).
Harpmaker is offline
Old 06-05-10, 08:31 AM
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1canuck2

Join Date: Jul 2009
Posts: 117
My System
Re: sheetrock question

Quote:
newscreen wrote: View Post
Another question. I have been told that a screen wider than twice the seating distance from the screen can result in a loss of detail, hence my original 106" diagonal screen for a seating distance of 17'. Is this a good rule to follow, or am I OK with a 141" diagonal screen at 17'? If 141" diagonal is good, would you stay with Scorpion N8 or go to N8.5 or C&S N9? Thanks in advance.
IMHO, 17' back (front row I assume) is too far back for a 106" screen. You'll end up wishing you'd gone bigger...

I am projecting on a 125" diagonal screen and my front row's eyes are only 11' back. I started at 11.5' back and actually moved my front row forward 6 inches...

FYI, I am projecting onto drywall. My drywaller used two long sheets with the seam horizontal. You can see a good shot of the drywall here:
http://www.avsforum.com/avs-vb/showt...9#post16825509

The drywaller did a good job during the mudding stage and I honestly cannot tell where the seam is now I did two coats of Kilz2 primer and three coats of Elektra screen paint.

Screen shots from my HT (featuring Elektra DIY screen paint).
1canuck2 is offline
Old 06-05-10, 08:41 AM Thread Starter
Shackster

Join Date: May 2010
Posts: 28
Re: sheetrock question

1canuck2--Thank you. I am thinking of a 120" diagonal in Scorpion or Elektra N8 or 8.5 (most likely Scorpion N8) and framing to 114" or 106" if desired. Seating can easily be adjusted (one row only) from 11' to 17'.
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