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post #11 of 29 Old 02-24-09, 01:04 PM
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Re: Gain vs energy

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robert_1967 wrote: View Post
Interesting. I use math in my work as Submarine Engineer so I might be able to help you in that area.
wow, that is a job i would find really interesting!
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post #12 of 29 Old 02-24-09, 01:10 PM
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Re: Gain vs energy

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But I am beginning to understand what you are after, and I do believe that you are heading in the correct direction. But I believe that the path you are heading is pointing towards absolute figures. You are correct that if you regard a "cone of light" that is narrow, the "amplitude" i.e. the height of the cone can be very high, even though it actually does not hold as much volume as a more wider but lower cone.

But what I am trying to wrap my brain around is a way to present to the user a set of relative figures that all relates to the famous Magnesium Block. I totally agree that for an absolute quantification of the reflected light you would have to do a so called curve integral. But the normal viewrws do not sit in a circle *wink*. We sit in a row in fron of the screen. And measurements show that there are screens reflecting substantially more light energy "in the relevant line of sight of the observer".
yep, the total light figure is not relevant to the viewers at all.

my only reasoning for looking at these calculations was to show that a screen cannot reflect more than 100% of the light.
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post #13 of 29 Old 02-24-09, 04:37 PM
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Re: Gain vs energy

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So a parameter that I do believe is relevant to add is the fact that various screens may perform different depending of the relative monting hight of the PJ.
That is absolutely correct. All of the screens measured here, except the HP, reflect like a mirror so the brightest view would be a ceiling mounted PJ with the viewer at the same, but opposite, angle of projection. This is the way the data was gathered.

The HP is retro-reflective so the PJ should be at the same level as the viewer to produce the brightest screen.

Sadly, I will have to admit that higher mathematics to me is counting over 20, but I am enjoying watching you guys work on the problem of gain data interpretation; and it is a problem. We can compare our resulting gain curves with those of commercial screens, but even those companies don't really tell you anything other than "higher gain equals a brighter image". I think there is much more info to be gotten from this data. For instance, the SILVER and Silver Fire screens hot-spotted, but none of the other screens did.
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post #14 of 29 Old 02-25-09, 02:32 AM Thread Starter
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Re: Gain vs energy

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yep, the total light figure is not relevant to the viewers at all.

my only reasoning for looking at these calculations was to show that a screen cannot reflect more than 100% of the light.
I apologize for my poor English if I gave anyone the impression that I was promoting the idea of a screen that contradicts the laws of nature. Custard is of course very correct that a screen can never reflect more than the PJ's output. My figures above that indicate something more than 100% is a relative figure when the SUT (Screen Under Test) is compared to the Magnesium block.

My efforts lie in the attempt of augmenting the GAIN parameter to actually include some useful information.

Now to the topic of retroreflectiveness vs (whatever the other one was called).
Assuming that the SUT is homogenous in the X and Y axis. In other words, the screen behaves identically regardless if we rotate it 90 deg or even have it up-side-down. Assuming this, then we can emulate the placement of the PJ by using a PJ but we tilt it 90 deg. Placing the PJ, tilted, on one far side or even outside the width of the SUT will represent a cieling mount. Now using the same equipment you allready have you now can get a curve describing wheter it is a retroreflrctive or other type of screen. If it is retroreflective you should get the peak when you are measuring under the PJ. If it is (the other type) you should get a peak at the proper reflection angle. If you can do some curves I might come up with an idea how to convert it into a factor. Perhaps R-index. R-index may vary from -1 to 1. -1 is an ideal retroreflective screen. 1 is an ideal reflective screen (mirror). 0 would then be the ideal lambertian screen.

A few hypothetical results: (RI = Reflective index)

Black Widow, 0.88:0.87:0.87, RI 0.5
S-I-L-V-E-R, 1.37:1.18:1.01, RI 0.8
Winter Mist 0.94:0.94:0.94, RI 0.1
DL Highpower, 2.69:2.28:1.95, RI -0.7
DL Perlecent , 1.46:1.33:1.22, RI -0.4



Any thoughts on this before I move on to a way of quantifying Hotspotting?
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post #15 of 29 Old 02-25-09, 05:25 AM
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Re: Gain vs energy

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robert_1967 wrote: View Post
I apologize for my poor English if I gave anyone the impression that I was promoting the idea of a screen that contradicts the laws of nature. Custard is of course very correct that a screen can never reflect more than the PJ's output. My figures above that indicate something more than 100% is a relative figure when the SUT (Screen Under Test) is compared to the Magnesium block.
as i have spent over a day trying to calculate and quantify how much light is reflected by MgCo in relation to the dalite hp and other screen samples i would like to show some of my formula.

everyone/anyone - feel free to correct me/help me with this as i probably havn't calculated everything right.


to calculate all gain at one angle

gain=G
angle=A

perimeter of circle = 2 x pi x R


R = sin A x G

where R = radius of circle made by angle A at given gain G

therefore perimeter = 2 x pi x(sin A x G) = total gain at that angle A

to calculate total gain at all angles
total gain at all angles = the sum of 'total gain at angle A' from 0 to 90.


total gain at all angles = the area under curve (INTEGRAL) of x,y graph
where x= angle, y = total gain at angle A

trapeziod rule to calculate area under curve in excel gain spreadsheet.

results abit later
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post #16 of 29 Old 02-25-09, 06:42 AM Thread Starter
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Re: Gain vs energy

You are correct as far as I can see . Very good presentation of your reasoning . I am not contradicting you in any way. I agree that taking all spacial coordinates into account we will get different results. We might even get the result that the pure Magnesium reflects most energy as it is Lambertian in its nature and has almost GAIN 1 at extreme angles, which in line with your own reasoning, would generate a large contribution to the total reflected light.

But other than from a academic point of view, why pursue this line of reasoning? We agree, didn't we , that our target is to come up with a better GAIN figure that gives the user more value and understanding of the relevant screen properties, i.e. in the line of sight of the viewer. Or have I missed something? In that case I blame my juvenile innocence

Last edited by robert_1967; 02-25-09 at 06:54 AM.
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post #17 of 29 Old 02-25-09, 07:05 AM
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Re: Gain vs energy

sorry robert

i'll chuck the readings

to give you an idea mgco3 is coming up tops with the carada for 90 to -90 degrees with an an assumption that the value at 90 degrees is equal to 75 degrees. HP is showing less from what i remember.


i'll stop now. i wont pursue this anymore.
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post #18 of 29 Old 02-25-09, 07:48 AM Thread Starter
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Re: Gain vs energy

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sorry robert

i'll chuck the readings

to give you an idea mgco3 is coming up tops with the carada for 90 to -90 degrees with an an assumption that the value at 90 degrees is equal to 75 degrees. HP is showing less from what i remember.


i'll stop now. i wont pursue this anymore.
No no, you have successfully made it very clear that from a physical and strict scientific point of view, reflected light energy is something other than what the viewer perceives. It was I, who in my eager to simplify the reality put up several boundaries and made false claims about reflected energy that you so correctly reacted on. All credit to you Custard . I bow for your wisdom Its only from discussion and challanged thought we progress. I am ashamed if I have put you off in any way .
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post #19 of 29 Old 02-25-09, 08:59 AM Thread Starter
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Re: Gain vs energy

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mechman wrote: View Post
Wow! This has developed into something way above my pay grade!
LOL .

But really, mech and of course all other out there, what do you think about the HTS suggestion?

GAIN X:Y:Z => X=Peak Gain at 0 degrees, Y=percieved acumulated reflected light from 0-30 degr, Z=percieved acumulated reflected light from 0-45 degr. (all figures related to the magnesium block)

RI => Reflective Index. Range from -1 to +1. -1 equals 100% retroreflective and +1 is a mirror. Zero is a ideal lambertian screen.

Last edited by robert_1967; 02-25-09 at 03:39 PM.
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post #20 of 29 Old 02-25-09, 04:28 PM
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Re: Gain vs energy

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robert_1967 wrote: View Post
LOL .

But really, mech and of course all other out there, what do you think about the HTS suggestion?

GAIN X:Y:Z => X=Peak Gain at 0 degrees, Y=percieved acumulated reflected light from 0-30 degr, Z=percieved acumulated reflected light from 0-45 degr. (all figures related to the magnesium block)

RI => Reflective Index. Range from -1 to +1. -1 equals 100% retroreflective and +1 is a mirror. Zero is a ideal lambertian screen.
First, I am simply in awe of what Robert and Custy are doing here.

I don't mean to "straddle the fence", but sometimes you have to see the big picture before you can examine a portion of it clearly. I think we may really be on to something here that will be a major aid to the entire projection screen industry! This is big stuff!

As it now stands, a screen's gain figure is almost meaningless since all it tells you is maximum gain and gives no hint of other screen attributes; and even a gain-curve won't tell you if a screen is retro-reflective or specular reflective.

Perhaps something that should be taken into account in these proceedings is the "half gain angle", the angle at which the maximum gain figure is reduced to half it's value. If I remember correctly, this is the angle that commercial screen manufacturers use to come up with their viewing cone figure. I don't know if that angle should be taken into account in these formulae or simply given as a separate calculation.

Please keep it up, you guys are doing great!
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