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garyfritz
Joined: 08 Apr 2006
Posts: 12088
Location: Fort Collins, CO
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| Posted: Wed Jun 30, 2010 2:19 pm Post subject: |
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Yes, that's a pretty small screen, 65" wide. I think most people here are running 84-100"+ wide. My screen is 84" wide which means I have 69% more screen area than you, so you'll have 69% more brightness than me before considering gain.
3.5 is a very aggressive screen gain. Unless you build a good torus, you'll get terrible hotspotting from that. That gain is jumping your ftL from just over 10 to 37. But even your 10 ftL is surprisingly high considering you started with ANSI lumens. Your small screen is helping there.
SC said he gets 12 ftL on a 100 IRE window. That's about what I measure too, and that's basically measuring peak lumens. You'd get a much higher ftL reading if you measured a 100 IRE window. But if you use a full-screen white field, the brightness drops dramatically because of current limitations in the tubes. That happens no matter what screen you use -- it's just an inherent property of CRTs. If you had a 3.5 gain 65x36 screen you would probably see 37 ftL or thereabouts on a full-screen white field, and you'd burn a hole in your screen if you displayed a 100 IRE window. 
I think 37 ftL is brighter than most direct-view CRT or flat-panel displays, which can be watched in a bright room. That's waaaayy more light than you want in a cave. I think a basic Wilsonart DW screen would give you more than enough light for that size of screen. You'd still be getting about 13 ftL *ANSI*, or (since you have 1200 peak lumens) about 1200/240 * 13 = 65 ftL peak!? Jehosephat.
By comparison, I have an 8500 with 1200 peak lumens / 225 ANSI. My screen is 16:9 so I use 1.33/1.77 = 75% of the tube for 900 peak / 169 ANSI. I have an 85"x48" = 28 sq ft blackout cloth screen with a 1.0 gain on a good day. 0.9 is probably more accurate. So I get 0.9 * 900 / 28 = 29 ftL peak!? 0.9 * 169 / 28 = 5.4 ftL ANSI? Waaaitaminnit here...
Something's wrong and I'm not sure what. I know I don't have that much brightness. I've measured it with 2 light meters and I get around 10-12 ftL (peak) on a 100IRE window. I'm sure the ftL = gain*lumens/area calculation is right -- see e.g. http://htrgroup.com/main.php?section=brightness . I'm sure I've calculated this in the past and got a more reasonable number. If somebody sees my error, set me straight willya?
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Kiev Savoie
Joined: 25 Oct 2007
Posts: 432
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| Posted: Thu Jul 01, 2010 2:20 am Post subject: |
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I'm sorry. I was in a rush last night so i did not explain myself properly. That was just a practice run. The screen i was calculating is my Draper Diamond screen for rear projection, the fresnel/lenticular type. I knew it was bright but i always figured it was in the 20's not the high 30's. My plan is to mount an electric roll down in font of it for a movies only FP setup that will be professionally calibrated. I'll use my RP setup for TV and videogames, but when it's time to get serious and watch some quality films that's when the the screen rolls down and I rev up the FP projector. I drew up a diagram and made the calculations. I'll post it soon. I just have to get the dang thing scanned.
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Spanky Ham
Joined: 22 Mar 2006
Posts: 5643
Location: Comedy Central
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| Posted: Mon Jul 05, 2010 4:43 pm Post subject: |
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| donaldk wrote: | | Spanky needs to go find an old Vutec 120". Gain is only 13 at dead center. |
If you know of one that is somewhat close by, then I would jump on it. If I ever get my G70 installed, then I will probably make one for cheap. If I could have coordinated logistically, then I would have tried to grab the one in Tampa last year.
Kiev,
I am going to be in BR on Wed and leaving Fri. I could probably drive out on Thu.
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Person99
Joined: 09 Mar 2006
Posts: 4899
Location: Flower Mound, TX
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| Posted: Wed Jul 07, 2010 9:36 pm Post subject: |
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| garyfritz wrote: | | Yeah, I'd say 10 ftL is a bare minimum. Like I said, I wish I had more. I know some people run lower than that but I wouldn't. 14 would be good, 16 would be better, but I doubt many CRT owners are getting 16 or more without stacking, blending, or high-gain screens. Unless they have a really small screen. |
Are you really sure you are getting 10?
I was getting under 8 with my Zenith 1200 on a 1.3 (claimed, actually 1.2 ) gain screen at 52" x 92". Not many CRTs are capable of 10 in 16:9 on a decent sized screen (100" diag or better). I don't think there are many with 8" machines getting more than 8 (with most around 5-7) and many 9" machines getting much more than 10.
Less than 8 was tolerable to me before, but I now have over 14 on a 16:9 image (which I like--but would not mind 18) and just a touch over 10 on a 2.35 image. The 10 is a little dim for me for many movies, so I sometimes kick up to high power to get 12. The difference between 8 and 14 is HUGE!!! That difference alone has made almost everyone that has seen the last two set ups prefer the new one because just like louder music just sounds better to humans, brighter pics just look better.
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Dave
A train station is where the train stops. A bus station is where the bus stops. On my desk, I have a work station....
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garyfritz
Joined: 08 Apr 2006
Posts: 12088
Location: Fort Collins, CO
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| Posted: Wed Jul 07, 2010 11:00 pm Post subject: |
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Two different sensors (SpyderII and Minolta 2150) agreed on it. That was a window reading, of course, not full field. I'm sure I get nowhere near 10 ftL on white scenes like Ice Age.
Your screen was 17% bigger than mine, so you lost most of the benefit of your 1.2 gain compared to my setup. You should have still had more light than me.
But yes, more light would be Good. I keep thinking I should make myself a DW screen so I can at least get a ~1.3x bump over my cheezy BO cloth...
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ecrabb
Forum Moderator
Joined: 13 Mar 2006
Posts: 15909
Location: Utah
TV/Projector: JVC RS40, Epson 5010
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| Posted: Wed Jul 07, 2010 11:04 pm Post subject: |
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| garyfritz wrote: | | I keep thinking I should make myself a DW screen so I can at least get a ~1.3x bump over my cheezy BO cloth... |
Yes, you should. There's no think about it... You know you should! Major difference, Gary. Just do it. 
SC
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Kiev Savoie
Joined: 25 Oct 2007
Posts: 432
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| Posted: Thu Jul 08, 2010 3:11 am Post subject: |
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What's a DW screen?
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AnalogRocks
Forum Moderator
Joined: 08 Mar 2006
Posts: 26706
Location: Toronto, Ontario, Canada
TV/Projector: Sony 1252Q, AMPRO 4000G
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garyfritz
Joined: 08 Apr 2006
Posts: 12088
Location: Fort Collins, CO
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| Posted: Thu Jul 08, 2010 3:17 am Post subject: |
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...which has been spectroradiometer-measured to be near-perfect white, 1.24 gain, near-indestructible, cleanable, and pretty cheap. You can get a sheet for a normal-sized screen for around $100-150. Mount it to some rigid backing or hang it from a frame and you've got a darn good and darn cheap screen.
Now if it would just magically appear in my HT I'd be a happy guy. 
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Person99
Joined: 09 Mar 2006
Posts: 4899
Location: Flower Mound, TX
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| Posted: Thu Jul 08, 2010 1:40 pm Post subject: |
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| garyfritz wrote: | | Two different sensors (SpyderII and Minolta 2150) agreed on it. That was a window reading, of course, not full field. I'm sure I get nowhere near 10 ftL on white scenes like Ice Age. |
Was that a 10% window? I would usually do a 20% window also as I think it is a more accurate representation of the APL a "bright" scene. 100% white is not accurate as there are few movies even close.
EDIT: I should note that I put "bright" in scare quotes because an APL of 20% is a very typical scene in a movie--an indoor daylight scene, etc. Measuring a 20% windonw on a CRT give you a FAR better representation of what you will experience in the average movie than a 10% window does.
Certainly, there are much brighter scenes (Ice Age, Happy Feet, some fade to whites, etc), but not many movies go much over an APL of 30% or so. So, I think measuring a 30-35% window is good to do to know what you are getting at the typical extreme, but 20% is a good representation of real world ftL you get when watching a movie.
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A train station is where the train stops. A bus station is where the bus stops. On my desk, I have a work station....
Last edited by Person99 on Thu Jul 08, 2010 3:38 pm; edited 1 time in total
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Person99
Joined: 09 Mar 2006
Posts: 4899
Location: Flower Mound, TX
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| Posted: Thu Jul 08, 2010 1:43 pm Post subject: |
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| garyfritz wrote: | ...which has been spectroradiometer-measured to be near-perfect white, 1.24 gain, near-indestructible, cleanable, and pretty cheap. You can get a sheet for a normal-sized screen for around $100-150. Mount it to some rigid backing or hang it from a frame and you've got a darn good and darn cheap screen.
Now if it would just magically appear in my HT I'd be a happy guy. |
It has slight blue shift but you can adjust it out.
I love my DW screen. I went from a commercial screen to the DW and I love the way it looks. Very sharp and nice picture. I build a 48" x 113" curved screen out of it. Total cost of about $250 bucks which includes a wide black velvet border, etc. I think it is superior to my much more costly DaLite screen in almost every way.
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Dave
A train station is where the train stops. A bus station is where the bus stops. On my desk, I have a work station....
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Person99
Joined: 09 Mar 2006
Posts: 4899
Location: Flower Mound, TX
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| Posted: Thu Jul 08, 2010 3:53 pm Post subject: |
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Although I edited my post above I wanted to underscore the point that using a 10% window for calculate your real world ftL on a CRT is not a good idea. I've attached a screen grab from Gladiator. This is a scene most people would call a realatively "dim scene". Its APL is exactly 15.02%. So you can see how you can hit 30-35 in movies with about 20 being a "decent" average.
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Dave
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garyfritz
Joined: 08 Apr 2006
Posts: 12088
Location: Fort Collins, CO
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| Posted: Thu Jul 08, 2010 4:06 pm Post subject: |
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| Person99 wrote: | | Was that a 10% window? I would usually do a 20% window also as I think it is a more accurate representation of the APL a "bright" scene. 100% white is not accurate as there are few movies even close. |
Not sure -- it might have been a 10% window, which may be why I got a higher level than you did. I probably used either GetGray or AVSHD709 -- don't think I used Avia or DVE. (I have too many calibration disks.  ) I think GetGray uses 20% windows.
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Kiev Savoie
Joined: 25 Oct 2007
Posts: 432
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| Posted: Mon Jul 19, 2010 10:40 pm Post subject: |
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Well here is what I drew up to work out what my screen size should be. After looking at these numbers I think I would be pretty unsatisfied with the brightness of even a 1.3 roll-down. What you see is a rough representation of my RP screen and two FP screens representing the smallest size I would accept and the largest size I can hang on my electric assembly. The masked off area is the only thing I calculated for as these would be the A.R.'s most used. That's 1.78 for the RP and 2.35 for the FP, of course all you have to do is add 25% to get the Ftl. for the FP's unmasked at 1.78. I'm looking at MAYBE 10Ftl. for the smaller screen which still matches the height of the RP screen.
So, now i'm thinking about a torus. A torus would allow me similar or maybe even better gain than the RP and I could make it significantly larger, which would decrease the Ftl. some but would allow for a larger sweet spot(from what i've read so far). The only problem is I have no idea how to calculate the gain for a torus screen, given the different curvatures, sizes and materials etc.
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dvh99
Joined: 25 Dec 2009
Posts: 2158
Location: nederland
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| Posted: Mon Jul 19, 2010 11:07 pm Post subject: |
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i am guessing that calculating gain for torus screens only requires a correction factor when given the screen size of the torus and the flat screen. it will be an increasing correction factor with bigger screens
maybe projection screen manufacturers have graphs with those data.
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VideoGrabber
Joined: 09 Apr 2006
Posts: 933
Location: Michigan
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| Posted: Tue Jul 20, 2010 5:09 pm Post subject: |
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Kiev commented:
> The only problem is I have no idea how to calculate the gain for a torus screen, given the different curvatures, sizes and materials etc. <
Bending a screen surface into a torus does not change the gain of the surface material. It just makes it more uniform... close to the same at the sides as in the central hot-spot. So you need to start with a higher-gain material, that wouldn't be appropriate for a flat screen, to get the benefits of a torus. Most look for a base gain of 3 or more.
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garyfritz
Joined: 08 Apr 2006
Posts: 12088
Location: Fort Collins, CO
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| Posted: Tue Jul 20, 2010 5:51 pm Post subject: |
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Yup, exactly. A torus just (ideally) spreads the hot-spot across the whole screen. A 3-gain screen material is almost unusable as a flat screen because you have such a bright hotspot in the center. Now curve the screen to reflect that hotspot at you from the whole screen surface, and the whole screen should be as bright as that hotspot was.
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WanMan
Joined: 19 Mar 2006
Posts: 10270
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| Posted: Tue Jul 20, 2010 6:09 pm Post subject: |
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| Person99 wrote: | | Although I edited my post above I wanted to underscore the point that using a 10% window for calculate your real world ftL on a CRT is not a good idea. I've attached a screen grab from Gladiator. This is a scene most people would call a realatively "dim scene". Its APL is exactly 15.02%. So you can see how you can hit 30-35 in movies with about 20 being a "decent" average. |
Some things the brain refuses to remember, like spelling restaurant (Jesus, I just spelled that right for the first time in my 45 year life). I cannot remember what APL stands for. So, does the amount of light coming off of this image at 15% like measure a white rectangle that represents a 15% window?
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Kiev Savoie
Joined: 25 Oct 2007
Posts: 432
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| Posted: Tue Jul 20, 2010 11:46 pm Post subject: |
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OK, so...
lumens = 240 * 0.57 (1.33 *2.35) = 136
Gain = 3.1 (Vutec Pearlescent White)
Screen Area =
A) 96 x 41 = 8' x 3.42' = 27.33 SqFt.
B) 108 x 46 = 9' x 3.83' = 34.47 SqFt.
C) 120 x 51 = 10' x 4.26 = 42.55 SqFt.
A= 15.42 Ftl.
B= 12.23 Ftl.
C= 9.9 Ftl.
Hmm... seems a bit low, but the 8' foot wide screen ought to be pretty sweet. By the way, do my numbers in the drawing look right?
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WanMan
Joined: 19 Mar 2006
Posts: 10270
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| Posted: Wed Jul 21, 2010 12:06 am Post subject: |
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I just ordered a 5x12 sheet and look forward to seeing how this works. Cost was $153.60+Tax. About i/10 the cost of the screen I had seriously considered but continued to not order.
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