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# All Screen Gain Calculator

I just completed a new screen gain calculator that can handle angular reflective screens as well as retro reflective screens. I call it the All Screen Gain Calculator. It replaces and adds functionality to my High Power Screen Gain Calculator, which many of you have downloaded and used.

What can you do with this calculator? Well, for one thing, you can tell which type of screen (angular or retro) best suits your needs. For another, having picked a screen type, you can determine the best position for your projector and seating. You can also see what brightness uniformity (or lack thereof) you can expect from side to side and top to bottom of your screen.

To use the new calculator, you need only answer five technical questions about the screen you wish to evaluate, and seven questions about the geometry of your HT setup. I hope you will find it useful. For those of you who are curious (or skeptical) about how the new calculator works, I offer a write-up a few posts down.

I am providing calculator versions in both Excel (preferred) and MS Works Spreadsheet (for those who don't have access to Excel). Either version will also run in the spreadsheet portion of Open Office, a free download.

Note that setup values for the Da-Lite High Power are in the next post below.

I see that about 125 of you have downloaded my calculator in the first 6 days after I posted this thread. As you may have noticed, many screen manufacturers do not publish all the parameter values required by the calculator. In particular, the screen gain at the maximum viewing angle, and the minimum off-axis gain may be difficult to find. You often must either request these parameter values from the manufacturer, or determine them yourself from a published screen gain graph (if you can find one).

Making matters worse, different manufacturers define “maximum viewing angle” differently. My calculator defines maximum viewing angle as the angle to ONE SIDE (Definition One) of a line perpendicular to the screen at which the gain decreases to a “predetermined level,” such as one-half the on-axis gain. Some manufacturers define it that way, while others define it to be TWICE that angle (Definition Two). Some appear to define it both ways, depending on the particular screen model. In addition, not all manufacturers use the same “predetermined level.” This is why you need to understand which definition the manufacturer is using and what the gain is at that angle. (If the manufacturer is using Definition Two, divide the angle by two to obtain the value to use with the calculator.)

It would be very much appreciated if those of you who have determined the correct parameter values for a particular screen would be kind enough to help the rest of us out by posting them here. When we have a number of screens characterized, I will try to consolidate the information in one place to make it easier to find. I will begin by posting the parameters for my own screen below. Thanks in advance for your help.

SCREEN NAME: DA-LITE HIGH POWER (OLD)

SCREEN TYPE: RETRO REFLECTIVE

ON-AXIS GAIN: 2.8

MAX VIEWING ANGLE (ONE SIDE): 15 DEGREES (AT ONE-HALF THE ON-AXIS GAIN)*

GAIN AT MAX VIEWING ANGLE: 1.4

MIN OFF AXIS GAIN: 0.75

SOURCE OF INFORMATION: DA-LITE HP SCREEN GAIN GRAPH (Received Oct. 2007)

* Da-Lite’s website states the maximum viewing angle is 30 degrees, but the Da-Lite High Power Screen Gain Graph shows the half-gain point to be at 15 degrees to one side of the screen perpendicular. Their website appears to be using Definition Two for viewing angle--at least for this screen.

--------------------------------------------------------------------------

Here are the parameters for the new 2.4 gain HP screen.

SCREEN NAME: DA-LITE HIGH POWER (NEW)

SCREEN TYPE: RETRO REFLECTIVE

ON-AXIS GAIN: 2.4

MAX VIEWING ANGLE (ONE SIDE): 30 DEGREES (AT ONE-HALF THE ON-AXIS GAIN)*

GAIN AT MAX VIEWING ANGLE: 1.2

MIN OFF AXIS GAIN: 0.9

SOURCE OF INFORMATION: DA-LITE graph supplied here.
Since no one has responded yet with parameters for any particular angular reflective screen, I decided to post the following parameters for a "generic" angular reflective screen. These parameters are useful for getting a general feel for the performance of the angular reflective screen. They are also useful for determining the PJ height which will produce the best top-to-bottom screen brightness uniformity for ANY angular reflective screen.

SCREEN NAME: GENERIC SCREEN

SCREEN TYPE: ANGULAR REFLECTIVE

ON-AXIS GAIN: 1.5

MAX VIEWING ANGLE (ONE SIDE): 40 DEGREES

GAIN AT MAX VIEWING ANGLE: 0.75

MIN OFF AXIS GAIN: 0.5

If anyone knows the values of the parameters for an actual make and model of angular reflective screen, please post them here.
Nice tool, when I get closer to purchasing, I will test it out, and post the data in the format you requested.

Thanks!
That would be great. Thanks for your help!
How the All Screen Gain Calculator Works (Moved from Post #1)

I. Basic Description

The All Screen Gain Calculator can calculate the screen gain for both angular reflective and retro reflective front projection screens. The calculator will not work correctly for screens that diffuse light asymmetrically, such as ribbed or profiled screens. I wouldn't use it on the SilverStar, either. If in doubt about your screen type, please consult the manufacturer.

This explanation begins similar to the explanation of my earlier High Power Screen Gain Calculator. Those of you already familiar with that write-up may want to skip ahead a few paragraphs to Section II, labeled Differences Between Retro Reflective and Angular Reflective Screens.

First, I had to pick a 3-D coordinate system (x, y, z). While most any coordinate system would do, I wanted to pick one that was easy to visualize and use. I decided my x coordinate would represent horizontal distance parallel to the screen, and my y coordinate would represent vertical distance parallel to the screen (no big surprises here). I further decided to place the origin (zero value) of the x axis at the horizontal center of the screen, and the origin of the y axis at the floor of the room. My z coordinate represents distance perpendicular to the screen surface, and the origin of the z axis is at the screen surface.

Using the above coordinate system, a point on the screen 50 inches above the floor and at the horizontal center of the screen, for example, would be represented by the x, y, z coordinates (0, 50, 0). As a second example, a point defining a projector lens aligned horizontally with the screen center and placed 72 inches above the floor and 144 inches from the screen would be represented by (0, 72, 144). As a third example, a point defining a viewer's eyes positioned horizontally 24 inches to the right of the screen center at a height of 37 inches above the floor and 120 inches back from the screen would have the coordinates (24, 37, 120). Note that had I said to the left of the screen center, the coordinates would have been (-24, 37, 120). Well, you get the picture, dont you?

After accepting the data entered by a user, my calculator first defines the x, y, z coordinates of a Screen Point (a point on the screen at which the gain is to be evaluated), a PJ Lens Point (a point defining the front and center of the projector lens), and a Viewer Point (a point at which the viewer's eyes are located). The calculator then calculates a PJ Vector by subtracting the coordinates of the PJ Lens Point from those of the Screen Point. The PJ Vector has a direction parallel to a line from the PJ Lens Point to the Screen Point. The length of the PJ Vector is the distance along that line from the PJ Lens Point to the Screen Point. In a similar fashion, the calculator calculates a Viewer Vector by subtracting the coordinates of the Viewer Point from those of the Screen Point.

The angle (herein called Error Angle) between the PJ Vector (or a slightly modified version thereof called the Best View Vector) and the Viewer Vector is what determines the screen gain. Before the calculator can compute the Error Angle, it must create unit vectors (vectors of length = one) parallel to the PJ Vector and the Viewer Vector. This is begun by calculating the respective lengths of the PJ and Viewer Vectors (length = the square root of the sum of the squares of the elements of each vector). The calculator then divides the elements of each vector by the vector's own length, thereby deriving the PJ Unit Vector and the Viewer Unit Vector, which are unit vectors parallel to the PJ Vector and the Viewer Vector, respectively.

II. Differences Between Retro Reflective and Angular Reflective Screens

Here is where I describe the Best View Vector. This vector defines the viewing direction that will realize the highest gain from the screen at a particular Screen Point. The Best View Vector for a retro reflective screen is different from that for an angular reflective screen. For the retro reflective screen, the Best View Vector is identical to the PJ Vector, because the retro reflective screen is designed to send most of its reflected light right back in the direction of the projector.

So how do we determine the Best View Vector for the angular reflective screen? First, we must understand how the angular reflective screen reflects light. The angular reflective screen can be simplistically described as a big mirror immediately behind a translucent light-scattering layer. The mirror reflects light according to the familiar angle of reflection equals the angle of incidence rule, while the light-scattering layer scatters the reflected light so that it can be seen (with decreased brightness) at angles other than the precise angle of reflection. To realize the highest gain at a Screen Point, the Best View Vector for the Angular Reflective screen must point directly into the Screen Point and parallel to the angle of reflection. To calculate the Error Angle between the Best View Vector and the actual Viewer Vector, we will need to use their respective Unit Vectors.

Once we have determined the PJ Unit Vector for a Screen Point on the angular reflective screen, calculating the Best View Unit Vector is easy. Let the PJ Unit Vector at a Screen Point be represented in our x, y, and z dimensions as (X, Y, Z). The Angle of Reflectance Unit Vector is then (X, Y, -Z), because the mirror reflects the (perpendicular) z-axis component of the light without changing the x and y components. The Best View Unit Vector then is the reverse (negative) of the Angle of Reflectance Unit Vector, or (-X, -Y, Z). Thus to compute the Best View Unit Vector for the angular reflective screen, we simply multiply the x and y components of the PJ Unit Vector by -1, while leaving the z component intact!

The calculator next calculates the dot product of the (appropriately determined for screen type) Best View Unit Vector and the Viewer Unit Vector. (The dot product = the sum of the products of the corresponding elements of each of the unit vectors). The Unit Vector Dot Product is a scalar (a real number), which conveniently happens to be equal to the cosine of the Error Angle between the two unit vectors. (See the tutorial linked below.) The calculator then calculates the Error Angle as the arc cosine of the Unit Vector Dot Product. The above described calculations are performed in parallel three times to obtain values for Screen Points at screen left, screen center, and screen right.

The calculator uses a mix of linear and exponential interpolation to estimate the screen gain corresponding to the Error Angle at each Screen Point. Between on-axis (zero Error Angle) and the manufacturer-recommended maximum viewing angle, the gain is assumed to vary linearly. At angles greater than the maximum viewing angle, the gain is assumed to decay exponentially towards a value equal to the minimum gain for large off-axis angles. While this approach is somewhat less accurate than using a table look-up of screen gain versus error angle data for specific make and model screens, it is a more practical approach to designing a generic calculator that is usable for a wide range of front projection screens for which basic gain parameters are available. The accuracy is deemed more than sufficient to make comparisons of projector and viewer positions, as well as screen type, for home theater performance optimization. Those who want to check the calculator-estimated gains should feel free to use the Calculated Error Angle values (which are highly accurate) to look up the corresponding gain values on a screen gain chart for their particular screen(s) of interest.

1. Be sure to specify the correct type of screen. Glass bead and High Power screens are retro reflective; most other gain screens are angular reflective. Should you specify the wrong screen type, the calculated results will be erroneous. If uncertain, consult the screen manufacturer.

2. The Screen Left and Screen Right gains are calculated for the far edges of the screen. If it makes you feel better, you can have the calculator compute these gains at points removed somewhat from the screen edges. To do this, just specify a smaller-than-actual horizontal width of viewing area. For example, if your screen's actual viewing area is 87 inches wide, and you want to calculate the Screen Left and Screen Right gains at 6 inches inside the screen edges, just specify a viewing area width of 87-12=75 inches. (The Screen Center gain will not be affected.)

3. Projector position is extremely important for the retro reflective screen. Ideally, the projector lens height should be near viewer eye level to get maximum screen gain. If other factors force a higher projector position, try to place it as low as possible to avoid losing much of the potential screen gain. It is perhaps less well known that the distance from the projector lens to the retro reflective screen is also important for gain uniformity across the screen in regard to off-center viewing positions. All on-center positions will see excellent gain uniformity with the retro reflective screen. You can add two equally-off-center positions to that happy list by placing the projector just a few inches behind the row of off-center seats. The off-center seats will, of course, experience lower gain than the center seats, but the gain advantageously will not change much from one side of the screen to the other. Use this calculator to experiment with various projector positions to see what I mean.

4. Projector height is important for the angular reflective screen, as well--if you are interested in making centerscreen your brightest area, top-to-bottom.. For example, when the projector is at too high an angle, the top half of the angular reflective screen will be brighter than the bottom half. If, for an on-center viewer, the calculator computes Screen Center gain at less than your specified on-axis centerscreen gain value, then the projector is not at the optimum vertical angle for that viewer, and the projector should be adjusted upward or downward. A greater projector distance (i.e., throw) from the angular reflective screen will generally produce better gain uniformity in all directions. This has to be traded off with reduced overall illumination (due to the slower zoom lens setting required for a greater throw). You can use the calculator to explore gain uniformity with different projector positions.

5. If you should happen to run the same input parameters in this calculator and in my older High Power Screen Gain Calculator, you will get somewhat different screen gain results (but should get the same Computed Error Angles). The reasons are twofold: (a) The older calculator is based on earlier gain specs that are a little different from today's published specs for the High Power screen; and (b) the older calculator uses a table-look-up method, which can produce slightly more accurate screen gain values than the method used in the new calculator, as discussed above.

6. If any users see anything that they think may be questionable about the calculations and assumptions used in this calculator, please let me know either in the open forum or by PM. I do not want to spread misinformation. Also, if any of you are fortunate enough to own a one-degree spot photometer, I would be delighted to have you measure a real-world set-up and see how closely the measurements compare with what the calculator predicts.

7. Readers seeking more in-depth information on the calculation techniques described above are referred to Chapters 0 through 10 of an excellent Vector Math tutorial from Central Connecticut State University, which is linked below.

http://chortle.ccsu.edu/VectorLessons/vectorIndex.html
This is a great tool, thanks - I hope I'm using it correctly.

Most have told me not to get the DaLite High Power Screen unless I can position it in the middle of the screen, because of the retro-reflective nature.

But located at the upper 1.3 of the screen, I'm still coming up with a center gain of 2.12 - am I doing something wrong, because that doesn't seem bad at all, and I wanted to use the retro-reflective screen to minimize reflections from my lighter colored ceilings.

My ceiling is nine feet high, I will drop the projector to six feet from floor, and center of screen will be at 60 inches from floor

I've posted an image of the calculation - please tell me if I'm screwing up, or if all the warnings that the gain is gone on the DaLite High Power with less than perfect placement are perhaps a bit overstated?

Thanks
Quote:
Originally Posted by thrang

This is a great tool, thanks - I hope I'm using it correctly.

Most have told me not to get the DaLite High Power Screen unless I can position it in the middle of the screen, because of the retro-reflective nature.

But located at the upper 1.3 of the screen, I'm still coming up with a center gain of 2.12 - am I doing something wrong, because that doesn't seem bad at all, and I wanted to use the retro-reflective screen to minimize reflections from my lighter colored ceilings.

My ceiling is nine feet high, I will drop the projector to six feet from floor, and center of screen will be at 60 inches from floor

I've posted an image of the calculation - please tell me if I'm screwing up, or if all the warnings that the gain is gone on the DaLite High Power with less than perfect placement are perhaps a bit overstated?
Thanks

Thanks. Yes you appear to be using the calculator correctly, except that your lens height input of 60 inches does not match your stated height of 6 feet (=72 inches). With a lens height of 72 inches the gain drops to around 1.68. That's still a respectable gain, but just not as much as you could have with a lower lens height.

It is not quite correct to say that the best PJ position is center screen. It is more accurate to say that the best PJ position is near viewer eye level. The retro-reflective screen reflects the most light back towards the source of the light (the PJ). That statement is true no matter how high or low the screen is (within reasonable limits). The issue with placing the PJ too high on a white ceiling is that in addition to getting lower screen gain, more light will be reflected back up to the white ceiling and thence onto the darker areas of the image, degrading black level and contrast.
Quote:
Originally Posted by FLBoy

Thanks. Yes you appear to be using the calculator correctly, except that your lens height input of 60 inches does not match your stated height of 6 feet (=72 inches). With a lens height of 72 inches the gain drops to around 1.68. That's still a respectable gain, but just not as much as you could have with a lower lens height.

It is not quite correct to say that the best PJ position is center screen. It is more accurate to say that the best PJ position is near viewer eye level. The retro-reflective screen reflects the most light back towards the source of the light (the PJ). That statement is true no matter how high or low the screen is (within reasonable limits). The issue with placing the PJ too high on a white ceiling is that in addition to getting lower screen gain, more light will be reflected back up to the white ceiling and thence onto the darker areas of the image, degrading black level and contrast.

Thanks - I think it was just revealed that I cannot multiply reliably

I'm hoping three feet down from the ceiling is substantive enough to reduce light spillage/bounce from the projector onto the ceiling, and that the retro-reflective screen is the better choice for minimizing off-axis light scatter off the screen. Would you say I'm generally correct with this assumption?
I always muck up the easy math, too.

Your assumption about a retro-reflective screen's ability to minimize light scatter from the screen to the ceiling is a good one IF you mount the PJ low enough to ensure that the ceiling is indeed far enough off-axis.

Actually, you can answer your question for yourself with my calculator. Just tell it temporarily that your viewer eye level is 108 inches. That will show you the screen gain at the ceiling height and at the viewer's distance from the screen. I just tried that with your numbers and with the PJ at 72 inches, and the screen gain at the ceiling above your viewers' heads is higher than the screen gain at the viewers' eye level. That can't be a good thing.

This issue can be corrected in one of two ways. If you feel a screen gain of about 1.7 is sufficient, you can darken the ceiling. If you want a higher gain and don't want to darken the ceiling, then you can lower the PJ another foot or so (two feet would be even better). At least one forum member has bought a telescoping extension pole for his ceiling mount, so he can lower the PJ to an optimum height for the HP only when he needs to use the PJ. For best results in either case you will need to darken the rear wall (behind the viewers) with drapes, paint, etc., as the HP screen is most sensitive to light from that direction.

I love my HP screen and wouldn't trade it for any other screen I know of. It does require a bit of creative effort to set it up correctly. I ended up using a pedestal mount for my PJ. My lens is 39 inches above the floor, and the PJ shoots between my two main seats. This works very well in my family room, which has a white ceiling and white walls. My rear wall is mostly covered by dark drapes.
Quote:
Originally Posted by FLBoy

I always muck up the easy math, too.

Your assumption about a retro-reflective screen's ability to minimize light scatter from the screen to the ceiling is a good one IF you mount the PJ low enough to ensure that the ceiling is indeed far enough off-axis.

Actually, you can answer your question for yourself with my calculator. Just tell it temporarily that your viewer eye level is 108 inches. That will show you the screen gain at the ceiling height and at the viewer's distance from the screen. I just tried that with your numbers and with the PJ at 72 inches, and the screen gain at the ceiling above your viewers' heads is higher than the screen gain at the viewers' eye level. That can't be a good thing.

This issue can be corrected in one of two ways. If you feel a screen gain of about 1.7 is sufficient, you can darken the ceiling. If you want a higher gain and don't want to darken the ceiling, then you can lower the PJ another foot or so (two feet would be even better). At least one forum member has bought a telescoping extension pole for his ceiling mount, so he can lower the PJ to an optimum height for the HP only when he needs to use the PJ. For best results in either case you will need to darken the rear wall (behind the viewers) with drapes, paint, etc., as the HP screen is most sensitive to light from that direction.

I love my HP screen and wouldn't trade it for any other screen I know of. It does require a bit of creative effort to set it up correctly. I ended up using a pedestal mount for my PJ. My lens is 39 inches above the floor, and the PJ shoots between my two main seats. This works very well in my family room, which has a white ceiling and white walls. My rear wall is mostly covered by dark drapes.

Thanks for staying with me on this one...

However, I'm not following your hypothetical completely, in terms of what I'm "testing" for. I see what you're saying about the theoretical higher screen gain at 108, but the reality is the top of the screen will be about 22 inches from the ceiling (108 inch ceiling, 22 inches between top of screen and ceiling.

Was your hypothetical trying to say I would be bouncing more light against the ceiling assuming the screen was near the 108 mark? Since is will be close to two feet down, does that change the concern? Sorry for my confusion on this....
Any idea who makes the telscoping projector mount?

I could, as you do, pedestal mount, back about 225 inches from the screen, assume a 96 inch wide screen, at 50 inches form the floor, and get a 2.4 gain. This might put the Epson out of consideration, because my understanding is that it's a tad noisier than the Sony VW60 or JVC RS2 I'm considering, and this pedestal position puts the projector within a few feet of my head.

Using a projector lumens formula I was provided that takes into account viewing diagonal, target foot lambert, and screen gain, I arrive at a 320 lumen requirement from the projector at that distance.[/indent]
Ignoring the topic of whether I'm not using the sweet spot of the lens by zooming in so much, the Sony and JVC should give me this, Epson maybe not even the noise turns out not to be an issue.

Now to verify the minimum and maximum throws of each...
thrang- Light scatters from a screen in all directions. The difference with the HP is that more light scatters towards the PJ. A viewer has a line of sight to a point on the screen. The PJ has a different line of sight to the same point. The angle between the viewer's line of sight and the PJ's line of sight is called the error angle. The smaller the error angle, the higher the screen gain, and the more light gets to the viewer.

What I was testing was the screen gain if a hypothetical viewer--let's call him Mr. Fly--were sitting on the ceiling at 108 inches above the floor and 180 inches from the screen. The screen gain at that point would be representative of the amount of light getting to that point. We would like the screen gain to be low at that point, preferably close to 1.0 or less.

The test shows that with the PJ at 72 inches above the floor, Mr. Fly at 108 has a screen gain that is greater than your human viewer at 37 inches. Bring the PJ down to pedestal height, say 39 inches, and Mr. Fly's screen gain drops while your human viewer's screen gain increases. That's what we want. We want to send the screen's light to the human viewer, not to Mr. Fly on the ceiling. If you place the PJ high, you will get results that are opposite to the results you want. That's really all I was trying to show.

Regarding fan noise, my PJ is the Epson 1080 Home. My throw is 17 feet. I run the lamp at low power to extend the lamp life and to reduce the fan noise. My screen is 100 inches (diagonal). Your 110-inch screen is 21% larger in area, so you may have to use full power to get the same brightness that I do. If so, I agree that the quieter JVC and Sony PJs might be a better choice.

I don't know who makes the telescoping extension pole. If I run across it again, I will try to remember to PM you.

Hope this has helped.

ETA- I found the link for the pole:

http://www.chiefmfg.com/search_resul...?CategoryID=23
Quote:
Originally Posted by FLBoy

thrang- Light scatters from a screen in all directions. The difference with the HP is that more light scatters towards the PJ. A viewer has a line of sight to a point on the screen. The PJ has a different line of sight to the same point. The angle between the viewer's line of sight and the PJ's line of sight is called the error angle. The smaller the error angle, the higher the screen gain, and the more light gets to the viewer.

What I was testing was the screen gain if a hypothetical viewer--let's call him Mr. Fly--were sitting on the ceiling at 108 inches above the floor and 180 inches from the screen. The screen gain at that point would be representative of the amount of light getting to that point. We would like the screen gain to be low at that point, preferably close to 1.0 or less.

The test shows that with the PJ at 72 inches above the floor, Mr. Fly at 108 has a screen gain that is greater than your human viewer at 37 inches. Bring the PJ down to pedestal height, say 39 inches, and Mr. Fly's screen gain drops while your human viewer's screen gain increases. That's what we want. We want to send the screen's light to the human viewer, not to Mr. Fly on the ceiling. If you place the PJ high, you will get results that are opposite to the results you want. That's really all I was trying to show.

Regarding fan noise, my PJ is the Epson 1080 Home. My throw is 17 feet. I run the lamp at low power to extend the lamp life and to reduce the fan noise. My screen is 100 inches (diagonal). Your 110-inch screen is 21% larger in area, so you may have to use full power to get the same brightness that I do. If so, I agree that the quieter JVC and Sony PJs might be a better choice.

I don't know who makes the telescoping extension pole. If I run across it again, I will try to remember to PM you.

Hope this has helped.

ETA- I found the link for the pole:

http://www.chiefmfg.com/search_resul...?CategoryID=23

Thanks again - I understands what you're saying now

So your Epson is about as close to your heads as mine would be it seems...and in low mode you're not bothered by the noise...

I'll discuss with my calibrator if he believes I'll need to run in low or high lamp mode given my conditions and calculations
Thank you FLBoy!!! This is very helpful to me. High power is out for me unfortunately, but now I know why.
Thanks for the bump, David. I take it you must have a high ceiling-mounted PJ; or is there some other reason you don't think you can use the HP?
This is a really great tool. Now I can tweak positioning of pj and screen together...
Quote:
Originally Posted by maxleung

This is a really great tool. Now I can tweak positioning of pj and screen together...

thanks flboy for the tool. really enjoyed using it (i made a small mistake at the begining and thought the high power was not going to work for me but than fixed it and now i am getiing about 2.5 gain). so i think i am going to order the high power screen. my only concern is that it is going to hurt black levels.

I am thinking about building a small elevator for the projector so i start with the projector in a relatively high position and than lowering it as time goes and bulb dims. are there any cons with positinig the projector high?

Thanks, benthan. Unless you have considerable ambient light sources in your room, black level is determined by the contrast ratio of the projector and by how bright you want your picture. Put a matte white 1.0 gain screen and the HP screen in a dark room with dark walls and ceiling, adjust the same PJ for the same brightness in your whites, and the black levels of the two screens will be identical.

A drawback to positioning the PJ high is that you will diminish the HP's ability to reject off-axis ambient light, which ability is directly proportional to the gain of the screen to projected light. A far better way to regulate picture brightness (including black level) is to position the PJ for maximum HP gain and then control the brightness of the PJ through lens aperture settings, lamp power, picture mode settings, ND filters, or what have you.
Hi again,

I am going to project in a fully dark room. Do i still need to be concerned of any off-axis ambient light?

As far as I know the only way to control the aperture settings on the RS2 (my projector) is through playing with the zoom. I am going to use the max zoom on my current theater so i think I can't get any help from the aperture settings. Am I wrong?

So, it seems that the only way to fight the extra lumens is by using the ND filter. What is your recommendation for a very good ND filter for the RS2?
p.s. is using ND filter going to affect picture quality in any way?

Ben
Quote:
Originally Posted by benthan

Hi again,

I am going to project in a fully dark room. Do i still need to be concerned of any off-axis ambient light?

As far as I know the only way to control the aperture settings on the RS2 (my projector) is through playing with the zoom. I am going to use the max zoom on my current theater so i think I can't get any help from the aperture settings. Am I wrong?

So, it seems that the only way to fight the extra lumens is by using the ND filter. What is your recommendation for a very good ND filter for the RS2?
p.s. is using ND filter going to affect picture quality in any way?

Ben

Not knowing what PJ you had, I was just throwing out a list of several ways of controlling PJ brightness. I can't help you with ND filters, so perhaps someone else who uses one can chime in.

If you have all dark surfaces in your HT, then off-axis ambient light should not be a problem. The only other issue I see with a high PJ mount and the HP is that when you rise from your seat the picture will get much brighter, and vice-versa. I would find that annoying--but that's just me.

I am using the RS10, which does have a variable aperture. I currently think it looks best at an intermediate setting (2) with a 100" HP and with the PJ positioned near eye level at mid-throw (17') and the lamp at low power. I expect I will open the aperture to full brightness after the lamp ages.
FLBoy,

How do you measure a "DIY" Screen's gain without having the initial figures your Calculator requires?

Consider a 225" diagonal screen, getting splashed with a RS-1 (700 lumens) on Normal Lamp, from a Throw of 27'.

Check your PM for a few telling images.

I'm about to embark upon some type of effort to do so...this very Sunday...clueless though I might be at present....but your advice (...or anyone else's...) and some help in resolving this matter would be absolutely welcomed. I'll buy what needs to be bought...just suggest/instruct if you can.

MMan
Quote:
Originally Posted by FLBoy

I am providing calculator versions in both Excel (preferred) and MS Works Spreadsheet (for those who don't have access to Excel).

btw... if anyone doesn't have Excel or MS Works you can download a free program that rivals MS Office: Open Office.
PC and Mac platforms. http://www.openoffice.org/ Their excel equivalent is called Calc but I think you have to download the whole office, just a heads up that that's the program to use once you have installed Open Office.
Quote:
Originally Posted by FLBoy

Thanks, benthan. Unless you have considerable ambient light sources in your room, black level is determined by the contrast ratio of the projector and by how bright you want your picture. Put a matte white 1.0 gain screen and the HP screen in a dark room with dark walls and ceiling, adjust the same PJ for the same brightness in your whites, and the black levels of the two screens will be identical.

Since Bentham turned out to have an RS2 he was going to use at the longest throw this doesn't really apply, but with your RS10 you might even have lower absolute black level with the High Power in the situation above since closing the iris on the RS10 tends to increase the on/off CR. So, lower absolute black for the same white off the screen.

This is without other lighting hurting the on/off CR off the screen enough to make the dimmer mode the one with worse on/off CR off the screen (since with other light sources brighter from the projector helps fight that light to retain on/off CR in the final images).

--Darin
Good point, Darin, and it turns out that is exactly what I am doing. I found my RS10 a bit too bright for my taste with the lens aperture fully open. I use the middle setting for the aperture, which is said to improve the on/off CR compared to the fully open setting. I think the real advantage, though, of the HP in my room with white walls and ceiling is to minimize the damage to my ANSI CR from re-reflected light from the image.
Has anyone been able to use the "All Screen Gain Calculator" with a Carada BW screen?
good stuff
Quote:
Originally Posted by rgathright

Has anyone been able to use the "All Screen Gain Calculator" with a Carada BW screen?

AFAIK, Carada does not publish luminance charts for their screens. A close approximation might be the Stewart StudioTek 130 G3, which is a 1.3 gain angular reflective screen. Here is a link to the luminance chart for that screen. Note that some forum members are of the opinion that the gain of the BW is closer to 1.2 than the advertised 1.4, so take your choice. Hope this helps.
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