Projection Screen Gain Factor Question!

[john81]

Hello All,


I am in the process of buying one of two possible motorized projection screens for my basement home theatre. The home theatre has 3 halogen potlights (about 8 feet in front of screen) and 3 scances (w/ dimmer switch). There is not much natural daylight entering the room at all. However, I do want to use the projection screen without having to turn off many lights at all. I also would like to maximize viewing angle, however not having to turn off the lights is more important than the max viewing angle. My choices are between a 1.8 gain factor screen and a 1.1 gain factor screen from the Elunevision brand sold by the east-porters. Which gain factor would suit my scenario?


Also, since I plan on using a 92" screen with a viewing distance of 14 feet, my screen diagonal to viewing distance ratio is 1.83. Is that acceptable from the experience of others?

I haven't seen much of Elunevision on the forums lately, so I'm wondering if this is the best value for a screen. Have to say they seem pretty affordable.
Thanks for any input. Cheers.

 

[JimMcC]

The screen dealer should be able to advise you on which screen gain to get. For sure, you will need a gray screen if you want lights on. Assuming your projector is either 720p or 1080p, a 92" diagonal screen is a little small for a 14' seating distance. I'd go with about 106" diagonal.

 

[John Kotches]

Let's explain what's meant by screen gain first. The gain is relative to the reference material, which is a block of magnesium carbonate. So the light output of a 1.3 gain screen is 1.3x brighter than a 1.0 gain screen. There's no free lunch with this though, what happens is that the light gets focused into a narrower plane as you increase gain. This works against wider viewing angles, i.e. off-axis viewers have a significantly dimmer image to look at.

When one decides which screen gain to used it should be based on a few factors. One of them is brightness "at the seats". SMTPE guidelines for screen brightness (at the seats) is 13-16 foot lamberts. Commercial theaters are relatively dark environments ;-) If you are going to be watching this with a lot of ambient light, you will probably be better suited with a flat panel.

So, what's the right screen? I don't know as we don't know what projector you intend to use. From the projector, we will know the light output (lumens) and we can calculate ft lamberts based on screen size, screen gain and lumens.

ft lamberts = (lumens * screen gain) / surface area (in square feet)

You said a 92" screen -- is that width or diagonal? A 92" diagonal is an 80x45 screen which is 3600 square inches. Divide by 144 to get square feet == 25 square feet. A 92" width screen is 92 x 51 (approximate) is about 33 square feet.


If we want a target of 16 ft lamberts we can work through this to find what lumens we need with each screen to hit our target brightness. Let's work with the 25 square foot screen.... Target the high end (16 foot lamberts) and with the 1.13 gain material.

This is good old algebra in action. To solve for lumens, the equation gets shifted around to be

lumens = (ft lamberts * surface area) / screen gain

lumens = 16 * 25 / 1.13 or about 350 lumens.

For the 1.8 gain screen it looks like this:

lumens = 16 * 25 / 1.8 or about 220 lumens.

I leave the calculations for the larger screen up to you. One thing I should tell you is that if you are using this with ambient lighting you will probably want 20 or so ft lamberts off the screen.

Cheers,

 

[John Kotches]

The screen dealer should be able to advise you on which screen gain to get. For sure, you will need a gray screen if you want lights on. Assuming your projector is either 720p or 1080p, a 92" diagonal screen is a little small for a 14' seating distance. I'd go with about 106" diagonal.

A 92" wide 16:9 screen is a 106" diagonal. I don't know if the OP was talking about the width or the diagonal.

Most quality FP screens are sold by width rather than by the diagonal.

 

[mike123abc]

I have a 92" diagonaly screen. It is grey with a 1.0 gain. It works very well with moderate lighting (not pointed at the screen). But, it is far better with the lights off. Perhaps a dimmer on the lights would be a good compromise. With a 1.0 screen I have 0 problem with off angle viewing, it looks just as bright at any angle. You have to also consider a lot of light is thrown back into the room from the screen, your lights can be a lot dimmer than you might think at first.

 

[john81]

Thanks very much for the replies.

My screen size will be a 92" diagonal. The two screens screens I am debating on are found on the "eastporters dot com" site. They are the 1.1 and 1.8 motorized screens.

After seeing the online manual for choosing a screen, I found out the 1.8 screen is actually their premier screen. Like said previously, it does cut down on the viewing angle to 100 degrees (rather than 160 for the 1.1), however they claim the 1.8 has more vivid life-like colors. And it happens to be $30 cheaper with their current sale. I havent seen my actual projector yet, but it is used, so I planned on getting this 1.8 screen in case the luminosity is weak on it also.

So i then called the sales rep, and the guy said i should stay away from the 1.8 screen cuz the glass beads would create noticable pixellation if I plan on viewing it from 14 feet away. I asked him how far back I should be, he said almost 20 feet! I couldn't believe him, cuz why would anyone want a 92" screen at a viewing distance of 20 feet. Sounded like a sales pitch for me to buy the more expensive 1.1 screen.

Can anyone certify that he is telling the truth? I'm sure there are other people with glass bead screens watching it at a distance closer than 20 feet!

I do have a dimmer, so that can be controlled, however the more ambient light I can get away with the better! Would the screens be distinguishable regarding this considering they are both Grey anyways?