I see that a lot of people make them-self the projection screen. I have the intention to do the same. But as a lot of people I want to use a material that maybe is not initialy made for projections, (something as a blackout cloth or someone mention a new material named "thermo-suede")
My question is how to I calculate the screen gain of a material as those mention above.
Thank you.
My question is how to I calculate the screen gain of a material as those mention above.
Thank you.
from
http://www.display-optics.com/pdf/screen_gain.PDF
So there you go, SG is how "bright" the screen looks. If you want to measure your SG, all you need to do is get yourself a perfect Lambertian screen. =/
#1 on Google. Fer cryin out loud, ppl, at least _pretend_ to look before asking.
http://www.display-optics.com/pdf/screen_gain.PDF
So there you go, SG is how "bright" the screen looks. If you want to measure your SG, all you need to do is get yourself a perfect Lambertian screen. =/
#1 on Google. Fer cryin out loud, ppl, at least _pretend_ to look before asking.
Well... everybady knows that SG is how bright the screen is and that 1.0 is when transmit all the light that it reveives. But first, why should I buy a perfect Lambertian screen? this is unpractical... I buy that screen then what? I make my-self another screen and then I compare them? This is unpractical and unprecise. Second, I know that it is a way to mesure it, in your quote sais: "regardless of where it is mesured" so it is mesured in some way.
And for your information I did my research but found nothing relevant;
Screen Gain = PI x Luminance of the screen / Illuminance incident on screen
But I don't know how to mesure the luminance of the screen and its illuminance incident.
Anyway thanks! And pls don't stop at first search on google you gave me just a definition! (and in this forum I suppose this is common knowledge)
And for your information I did my research but found nothing relevant;
Screen Gain = PI x Luminance of the screen / Illuminance incident on screen
But I don't know how to mesure the luminance of the screen and its illuminance incident.
Anyway thanks! And pls don't stop at first search on google you gave me just a definition! (and in this forum I suppose this is common knowledge)
Ok, sorry bout that. I was having a bad night. The comment about buying a perfect Lambertian screen was sarcasm: I doubt there is such a thing.
What I was trying to say was that, while SG is used as a measure of "brightness" of a diffuser (=screen), I don't think that you'll find an accurate way to measure it without a lot of expensive equipment.
Here's why: the measure not only has to do with the amount of light being reflected/diffused, but also uses the angle relative to the screen, both vertical and horizontal. If you look at the second page on that link, you'll see charts describing this.
It seems to me that just about any home-spun method we could come up with wouldn't be much better than a semi-guess.
So, what's the fun in leaving it there, eh? If you can come up with a measure of the <a href="http://www.natmus.dk/cons/tp/lightcd/light_cd.htm">candela</a> output of the screen, then you can solve:
What I was trying to say was that, while SG is used as a measure of "brightness" of a diffuser (=screen), I don't think that you'll find an accurate way to measure it without a lot of expensive equipment.
Here's why: the measure not only has to do with the amount of light being reflected/diffused, but also uses the angle relative to the screen, both vertical and horizontal. If you look at the second page on that link, you'll see charts describing this.
It seems to me that just about any home-spun method we could come up with wouldn't be much better than a semi-guess.
So, what's the fun in leaving it there, eh? If you can come up with a measure of the <a href="http://www.natmus.dk/cons/tp/lightcd/light_cd.htm">candela</a> output of the screen, then you can solve:
Whups, sorry, hit submit too soon:
nits = candela/m^2 (area over which candela are measured)
SG = screen gain
lux = (projector lumen output)/(screen area in m^2)
(lux * SG)/pi = nits
SG = (pi * nits)/lux
SO there you go. If you know how many lumens your projector puts out, and you can measure candela, then you can figure out the SG for one point (one specific angle). Do that multiple times for many different angles, and you can figure out the actual SG for your screen.
Me personally, I'd stop at the "looks pretty good to me" stage.
nits = candela/m^2 (area over which candela are measured)
SG = screen gain
lux = (projector lumen output)/(screen area in m^2)
(lux * SG)/pi = nits
SG = (pi * nits)/lux
SO there you go. If you know how many lumens your projector puts out, and you can measure candela, then you can figure out the SG for one point (one specific angle). Do that multiple times for many different angles, and you can figure out the actual SG for your screen.
Me personally, I'd stop at the "looks pretty good to me" stage.
Come to think of it, that was probably an unsatisfactory reply. I think what you're asking, on some level, is "how can I tell what would make a good screen material".
People think of a screen as "reflector", reflecting the image from the projector back at them. In actuality, the screen is a "diffuser", that takes that projected image and reflects it back in a 180deg arc (vertical and horizontal). An "ideal" diffuser would have a uniform screen gain of 1.0.
Think of it this way: a mirror is an almost-perfect reflector, but a terrible diffuser. A beam of light directed at it's surface is reflected back at a complementary angle almost perfectly, but the beam is barely visible from anywhere outside that complementary angle.
So part of the question should also be what _doesn't_ make a good screen material.
You want a material that reflects a uniform spectrum of light without being "shiny". Your basic "good' reflector is a straight wall painted with a flat white paint. White reflects almost the entire spectrum, nature of the flat paint means it diffuses the image in all directions.
However, this is not really what you want. When you are talking projections, any light that is not reflected/diffused towards the observers is wasted. The more light that is wasted, the dimmer your image will appear. Ideally, you would want all the light reflected/diffused in the direction of the observers (aka "on-axis").
This is where you start seeing screen gains of greater than 1.0. The value is not referring to the value of the _entire_ screen gain, but the screen gain within this "on-axis" area. Off-axis areas would have to have screen gains of less than 1.0 in order to make this possible.
Again with the mirror analogy: the reflected beam that follows the complementary angle would have maximum screen gain, whereas gains outside that angle would quickly approach zero.
In my opinion, finding material that was not intended as projection material that just happens to have a screen gain of >1.0 is improbable. Some of the material that people claim to have SG > 1.0 are blackout cloth and the paint-on screen material (<a href="http://www.goosystems.com/">Screen-Goo</a>). Also there is a plastic wallboard called "Plas-Tex" some people have liked. All these materials may or may not have screen gains exceeding 1.0. Many people use these products, but I don't think anyone has ever managed to measure them for sure, attesting to the difficulty in ascertaining true screen gain.
One note: notice that people don't stick with white screens. Grey screen material seems to improve contrast, especially for LCD projectors. This has more to do with the spectrum of light reflected from these surfaces than the screen gain.
Complex enough for you ?
People think of a screen as "reflector", reflecting the image from the projector back at them. In actuality, the screen is a "diffuser", that takes that projected image and reflects it back in a 180deg arc (vertical and horizontal). An "ideal" diffuser would have a uniform screen gain of 1.0.
Think of it this way: a mirror is an almost-perfect reflector, but a terrible diffuser. A beam of light directed at it's surface is reflected back at a complementary angle almost perfectly, but the beam is barely visible from anywhere outside that complementary angle.
So part of the question should also be what _doesn't_ make a good screen material.
You want a material that reflects a uniform spectrum of light without being "shiny". Your basic "good' reflector is a straight wall painted with a flat white paint. White reflects almost the entire spectrum, nature of the flat paint means it diffuses the image in all directions.
However, this is not really what you want. When you are talking projections, any light that is not reflected/diffused towards the observers is wasted. The more light that is wasted, the dimmer your image will appear. Ideally, you would want all the light reflected/diffused in the direction of the observers (aka "on-axis").
This is where you start seeing screen gains of greater than 1.0. The value is not referring to the value of the _entire_ screen gain, but the screen gain within this "on-axis" area. Off-axis areas would have to have screen gains of less than 1.0 in order to make this possible.
Again with the mirror analogy: the reflected beam that follows the complementary angle would have maximum screen gain, whereas gains outside that angle would quickly approach zero.
In my opinion, finding material that was not intended as projection material that just happens to have a screen gain of >1.0 is improbable. Some of the material that people claim to have SG > 1.0 are blackout cloth and the paint-on screen material (<a href="http://www.goosystems.com/">Screen-Goo</a>). Also there is a plastic wallboard called "Plas-Tex" some people have liked. All these materials may or may not have screen gains exceeding 1.0. Many people use these products, but I don't think anyone has ever managed to measure them for sure, attesting to the difficulty in ascertaining true screen gain.
One note: notice that people don't stick with white screens. Grey screen material seems to improve contrast, especially for LCD projectors. This has more to do with the spectrum of light reflected from these surfaces than the screen gain.
Complex enough for you ?
Before I built my first screen, I searched the gain question too. I never did find anyone who actually measured the gain of the common diy screen materials. I finally discovered that for me anyway, fabric selection ultimately boiled down to how the fabric looked in use and if the image ended up bright enough to support the screen size I wanted.
I have constructed a few screens with a couple of different materials and always return to blackout fabric, due almost entirely to cost. I can tell you that blackout material is not quite as white as a matte white fabric from Stewart, Dalite or Draper (I did a visual fabric comparison with my projector), and it is not quite as bright as the 1.0 to 1.5 gain fabrics I looked at, but it is remarkably close.
I think that some people who use LCD and DLP do use some variant of a gray screen, but I have found that the type and color of screen used generally is pretty variable and is more directly related to taste. For example, there is a large camp of LCD owners who swear by Dalite High Power fabric, which at 2.8 gain flies in the face of conventional wisdom for a bright projector with not really good blacks. But the look of the High Power (a retro reflective fabric) is tough to beat if you can deal with it’s unique negative characteristics. Unfortunately, one of them for me is the $117 per square yard price tag.
I have used Thermal Suede, manufactured by Rolcon, with great success. If you don’t need over 54” in one dimension, it is a great solution. I paid under $30 for the fabric on my first screen and another $45 for an 88” x 52” stretcher frame from an art supply house. It only took an hour to construct the frame and staple the fabric (which incidentally goes on very easily). More recently, I rebuilt my screen using a larger frame in a 2.35:1 format and used the the same type of screen fabric.
Granted, it’s not a high end solution, and probably not as good as a real engineered fabric, but you will be pleasantly surprised how good it does look for under a hundred bucks.😉
I have constructed a few screens with a couple of different materials and always return to blackout fabric, due almost entirely to cost. I can tell you that blackout material is not quite as white as a matte white fabric from Stewart, Dalite or Draper (I did a visual fabric comparison with my projector), and it is not quite as bright as the 1.0 to 1.5 gain fabrics I looked at, but it is remarkably close.
I think that some people who use LCD and DLP do use some variant of a gray screen, but I have found that the type and color of screen used generally is pretty variable and is more directly related to taste. For example, there is a large camp of LCD owners who swear by Dalite High Power fabric, which at 2.8 gain flies in the face of conventional wisdom for a bright projector with not really good blacks. But the look of the High Power (a retro reflective fabric) is tough to beat if you can deal with it’s unique negative characteristics. Unfortunately, one of them for me is the $117 per square yard price tag.
I have used Thermal Suede, manufactured by Rolcon, with great success. If you don’t need over 54” in one dimension, it is a great solution. I paid under $30 for the fabric on my first screen and another $45 for an 88” x 52” stretcher frame from an art supply house. It only took an hour to construct the frame and staple the fabric (which incidentally goes on very easily). More recently, I rebuilt my screen using a larger frame in a 2.35:1 format and used the the same type of screen fabric.
Granted, it’s not a high end solution, and probably not as good as a real engineered fabric, but you will be pleasantly surprised how good it does look for under a hundred bucks.😉
Thank you to both of you. Yes it helps a lot!! I will try the thermo-suede material (seam that gives good results) and I will maybe to a comparation with a professional screen to see how bad or good is. And I will try to use the formulas maybe I can come up with something...
Thank you.
Thank you.
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