[David600]

here it is: my Dalite is a 10ft 4:3 screen and has a gain of 1.1 or something. It's not perforated.


What will be the gain in brightness ( FLamberts?) for a 1000lumens CRT ( peaks ) if I use a 3meter 1.8 gain perforated screen ( 0.5mm diameter holes, density of 28,000 holes per square meter ). will that 1.8 gain help compensate for the microperforations ?


any idea how to calculate that ? thanks a lot.

 

[Matthew Todd]

The formula for screen brightness in footlamberts is to take the projector brightness (in ANSI lumens, not peak) and divide by the screen area in square feet. Take that figure and multiply by the screen gain. To account for a perforated screen figure about 10% loss, so multiply that figure by 0.90 to get the final screen brightness.



You mentioned a 10 foot screen. Is that the diagonal measurement or the screen width? For the examples below, I'm assuming it is width.


For example:


On a 10 foot WIDE 4:3 screen, screen area = 10*((3/4)*10)= 75 sq ft.


Screen brightness with a 300 ANSI lumen CRT and screen gain of 1.1 and no perforations would be (300/75)*1.1 = 4.4 ft lamberts.


On a 3 meter (about 10 feet) WIDE 4:3 screen, screen area is the same as above (75 sq ft)


Screen brightness with a 300 ANSI lumen CRT and screen gain of 1.8 and perforations would be (300/75)*1.8*0.90 = 6.48 ft lamberts.


If you use a 16:9 screen instead of a 4:3 screen the area for a screen of the same width is reduced, and so brightness is increased. For example, a 10 ft WIDE 16:9 screen will have an area of (10*((9/16)*10) = 56.25 sq ft, so the brightness will increase by the difference in ratio of area ((75-56.25)/75) = 25% (so brightness increases by 25% when you use a 16:9 screen vs. a 4:3 screen of the same width.)


Matt


[This message has been edited by Matthew Todd (edited 07-26-2001).]

 

[David600]

thanks a lot Matt!


The screen I seek is an Harkness Hall cinema scree but with 0.5mm microperforation and density of perforations is 2.2%, which much less than the microperforated from Stewart for instance (which has 3 times more perforations per square meter). The PM180 from HHall is destined to work with cinema speakers, which means horns. I have those so this suits me more and help reduce the loss in brightness I would have with a classic home theater perforated screen.


Can I assume by this density of 2.2% that the loss in brightness from the 1.8 gain it adds, would be 2.2% also ?


As for the screen format, I intend to go with a 2:35:1 format, constant height thus. I fear though the CRT I aim at( a Sony 1272 which as only 140 ANSI lumens) , even with this format and the gain, would have pain at giving me a bright image, without pushing hard the tubes....


thanks for your detailed initial answer, it was much appreciated.


David

Belgium

 

[KFung]

David,


I don't think cinema perforations are a good idea at your seating distances, Stewart's microperf has 0.125 mm perforations (compared to the 0.5 mm cinema perforations) and you can see the perforations up until about 10' away from the screen. Cinema perforations will be visible much father than that (I think at least out to 14-15'), I would say that you should probably evaluate their visibility in your application with a sample before going for cinema perf...


Regards,


Kam Fung

 

[David600]

hi


I"m not talking about cinema perforations (1.2mm diamater) but microperforations, 0.5mmm ( though bigger than Stewart new 0.127mm diameter perforations ).


I've seen this 0.5mm type screen from ORAY at 3meters viewing distance: no problem.

My viewing distance ranges from 4meters to 7meters for a 3meter screen.

 

[KFung]

Stewart's microperf has always been 0.005"/0.125 mm so it's not exactly new. http://www.avsforum.com/ubb/smile.gif If you find that you don't notice the perforations at your viewing distance, then I guess it's not a problem!


Regards,


Kam Fung

 

[Matthew Todd]

Hi David. Where in Belgium are you? I spent 2 years in that area (Southern Belgium and Northeastern France) from '93 to '95.


I don't know exactly what the light loss would be for the perforations you're talking about, but I think it will be more than the 2.2%.


The 10% number I used was for Stewart's microperf screens. I have info that Stewart's cineperf (which uses 1.2mm diameter holes) has a loss of about 8%. All this is off Stewart's website.


Doing a little math something interesting becomes apparent. For the microperf screen (hole D = .5mm and density = 30,000/sq ft) the percentage area of the screen that is holes is 4.2%.


For a Stewart cineperf screen (hole D = 1.2mm and density = 5500/sq ft) the percentage area of the screen that is holes is 6.7%.


Funny thing is the screen with more empty space has less light loss!


It seems to me that the relationships between hole density and diameter and light loss are not simple. (unless I made a mistake somewhere? -- which is possible)


Since your perforation density is lower (28000/ sq m = ~2650/ sq ft) but your hold diameter is about the same as the Stewart microperf I can't really make a good guess as to where light loss will be, but I would think you will have a lower light loss than that 10%, but I can't really say how much. It will probably be between 5-10%.


KFung, are the Stewart microperfs really .125mm diameter? My info says they are .02in, which is closer to .50mm.


Anyway, the bottom line is I think you will still have 5-10% light loss with the screen described by David.


Matt

 

[KFung]

Matt,


No you're right, I misremember. It is 0.020"/0.5 mm, I must have been thinking 0.005 metres, not inches! http://www.avsforum.com/ubb/smile.gif My calculations yield ~6.5% light loss for microperf though... If you back calculate 4.2% you get a hole diameter of ~0.016"/0.4mm.


The 10.2% is probably what they get in real work testing, I imagine there are other factors that affect the light loss. I don't think that the perforation process is perfect, maybe there is some pucker left over from the pins and it is reflecting light away at an angle. 10.2% light loss is consistent with a hole diameter of ~0.025", which is not too far off their spec; so local deformation around the perforating pins might explain their higher real-world light loss along with tool tolerances.


Anyways, back on topic. I just checked the Harkness Hall site and the acoustical performance of that screen is not so good for critical listening. 15 dB at 16 kHz is going to require a *lot* of equalization even with more efficient horn speakers. The fall off is pretty bad, almost linear after ~2 kHz. I would say, throw a swatch of it over the speakers you are going to use and make sure it is acceptable to you. That's the problem with such a small perforated area, you lose a lot of performance in the high end.


Regards,


Kam Fung

 

[David600]

hi guys


Matt: I live in Brussels. But love the USA ( have relatives in NY ).


the PM180 fro HHall is like this: density of holes is 2.2% ( which made me conclude, given the diameter of 0.5mmm and the area formula of Pi x radius times 2 = 28,000 holes per square meter, compared to the Stewart which claims 30,000 holes per square foot; the Stewart has holes of 1/20th of an inch, which is 2.54cm divided by 20= 0.127mm if I am correct ).

So the PM180 has indeed less holes per square foot or meter and holes are more than twice as big in diameter.


The graph showing the dB loss is for an unequalized measure while the one for that Stewart is AFTER equalization a friend told me ..... http://www.avsforum.com/ubb/biggrin.gif so ??


It seems clear however that the smaller the holes, the more you need of them as compared to less holes but bigger ones for cinema screens ( which are 1.2mm in diameter for standard viewings ( which is about 7 or 8 meters for the first row, at which distance you don't notice the holes ).


So the Harkness Hall might have almost not visibility of the 0.5mm holes but has not enough of them, compared to the Stewart, though the brightness loss is less I would guess, consequently, on this PM180 compared to the Stewart... is this clear ???? http://www.avsforum.com/ubb/smile.gif


it's a question of compromise.....;


Now, one trick is to have the left and right speakers right on the sides of the screen and just the center speaker behind it and certainly equalized but certainly too of the horn, high sensitivity type, while the Stewart can accepts standard speakers ( no horns ).


ARGH!!!!!!!!!!!!

 

[KFung]

David,


The microperf holes are spec'ed at 0.020', so around 0.5 mm, same as the Harkness Hall (if their number is correct). The relationship between sounds attenuation and perforations has a lot to do with the percentage perforated area, not just size and number of perforations (although I guess you could say they are related). You will get better performance if you have a larger amount of perforated space (which makes sense). The small perforated area of the Harkness Hall screen is most likely the reason why there is a great deal of fall-off.


I couldn't say for sure whether the attenuation graph for the Stewart Microperf is corrected or not, I'd have to look into it more. However, I do know that it's performance is much better than what Harkness Hall claims. I don't think the microperf'ed screens have any problem with horn speakers... The loss in brightness is not as much as you think, and you gain a lot more performance in audio. I don't believe the Harkness Hall perforated screen is really designed for home use. You really should get samples of both and take a look.


Regards,


Kam Fung