Light Meter / Screen Gain Meter - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 16 Old 01-10-2020, 08:07 AM - Thread Starter
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Light Meter / Screen Gain Meter

What light meter is needed to accurately measure screen gain?
Where do you buy them?
Will any old light meter work?
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post #2 of 16 Old 01-10-2020, 08:36 AM
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Quote:
Originally Posted by TXFlyGuy View Post
What light meter is needed to accurately measure screen gain?
Where do you buy them?
Will any old light meter work?

Here is the one I have. Inexpensive but quite accurate and easy to use.
https://www.amazon.com/Dr-Meter-LX1330B-Digital-Illuminance-Light/dp/B005A0ETXY/



You simply covert Lux to Fc (Foot Candles) which is the converted to the known comparative gain figure.

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post #3 of 16 Old 01-10-2020, 08:49 AM
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A bit of a tutorial:


Lumens = fc x area of screen (ft 2) ftL = lumens * screen gain / (area of screen in sq. feet not inches)
You superimpose them and you will get FL = Fc x screen gain
Lumens and area of the screen cancel out. - a bit of simple math to illustrate




A fc is 10.76 lux. A fL is 1 fc (assuming unity gain screen)Work back from the formula lumens = fc x area of screen (ft 2)
That gives a output of lumens using the formula ftL = lumens * screen gain / (area of screen in sq. feet not inches)

The formula below is only useful if your initial measurement is in lux


Other ways to put it:


  • If you are using a light meter than reads out in lux, just put the meter at your screen center and aim it at the projector displaying a white full screen pattern filling your screen. Take the reading, multiply it by your screen gain and divide by 10.76. Since my screen gain is 1.0, all I need do is read the meter in lux and divide by 10.76.
  • Most other light meters allow you to read out in lux or fc (footcandles). If you switch to read the value (measured as you describe) in fc, then the lumen output of the projector is (# of fc)x(screen area in sq ft). The ftL coming off the screen is (# of fc)x(screen gain).



Your getting into the fun stuff now......................
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post #4 of 16 Old 01-10-2020, 11:45 AM - Thread Starter
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Okay...but how do I know my screen gain value? It could be .95, .98, or 1.0.

Don't you need the screen gain number in order to work your formulas?
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post #5 of 16 Old 01-10-2020, 01:19 PM
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Quote:
Originally Posted by TXFlyGuy View Post
Okay...but how do I know my screen gain value? It could be .95, .98, or 1.0.

Don't you need the screen gain number in order to work your formulas?
Some screens have been previously tested. What fabric does the screen have?

@MississippiMan what about the LX1010B and URCERI MT-912?

Was trying to determine the gain of the screen here, but the link has a quoted post with the test and the result:
https://www.avsforum.com/forum/139-d...l#post59078916
The lux values are half what they should be. Not sure if I'm doing something wrong.
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post #6 of 16 Old 01-10-2020, 01:52 PM - Thread Starter
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My screen is a "one off" design. Never tested. And that is the reason for the question on light meters / screen gain.
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post #7 of 16 Old 01-10-2020, 01:58 PM
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Originally Posted by TXFlyGuy View Post
My screen is a "one off" design. Never tested. And that is the reason for the question on light meters / screen gain.
That's what I want to find out as well.

What do you mean one off? What brand is it, model?
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post #8 of 16 Old 01-10-2020, 02:41 PM - Thread Starter
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Originally Posted by noob00224 View Post
That's what I want to find out as well.

What do you mean one off? What brand is it, model?

Manufactured by Sherwin-Williams, assembled by Painters United.
Model: Duration - Extra White.
16X9 aspect ratio, 155" diagonal.

A full thread detailing this will be posted very soon.

Last edited by TXFlyGuy; 01-11-2020 at 04:14 AM.
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post #9 of 16 Old 01-10-2020, 03:35 PM - Thread Starter
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post #10 of 16 Old 01-10-2020, 04:06 PM
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Originally Posted by TXFlyGuy View Post
I don't think it's updated.
And there is no substitute for measuring the projector output and screen gain IRL, calculations can be very off.
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post #11 of 16 Old 01-10-2020, 04:34 PM
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Quote:
Originally Posted by TXFlyGuy View Post
My screen is a "one off" design. Never tested. And that is the reason for the question on light meters / screen gain.

Now I and others are happy your happy, but the effort is hardly a "one off design" as any number (ie: 1000s) of White Screen variations have been made, a great many using Paints with sheen tempered with additional Flat or Matte Paints or Polyurethanes. Many, many using SW paints in fact. The application is almost as old as this Forum...and there are about several hundred Threads...and a lot of discourse about the merits of using exactly such combinations. What's Old is New again, but never tiresome...and is made all the more relevant by today's crop of mega-Contrast, Light Cannon Projectors. Before they arrived...Unity or .8-.9 Gain Greys ruled the day.


As mentioned before elsewhere, the Glidden / Diamond Bright White was adjudged the very best (Neutral) and highest Gain (1.3) of all the available White OTS Paints over 5-6 years ago. A solid reason why it figures in every advanced DIY paint formula I've advocated the last 5 years. Before that, it was a Tie between SW-Duration "Extra White" Matte, and Valspar Ultra White Flat Base A...both measured at 1.1 gain

To all of those, any number of Grey Tints and additional ingredients have been added. You know...like the old SS and many of the other ones you were also considering on another Thread.
Consideration should still be give to just what might have been discovered had any of the other well known, exceptional Whites been used. Suppositions on potential performance based on known values are fairly easy to conjure up...and still be pretty accurate.


What it really truly is is a 1st for you and understandably you want to know just how good it is. Will it's measured performance be wholly indicative of it's visual attributes you already have determined to be extremely good? It's the type of information that is always good to know....such tests have always been valuable when discussions come up, helping to sift the wheat from the chaff. Accuracy is paramount though...of course.

However...as good as it may be....and that may well be better than even you realize...it is and still remains a basic White Paint combination of two well known White Paints...sprayed as is always preferred, a combination that aspires to be a excellent, smooth white surface for Projection. No more....but certainly no less than.



But in the end, what you have "discovered" for yourself is all that matters. It's a damnable good feeling to realize you tinkered your way into something special...and telling others only helps make it all seem all the more special.
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post #12 of 16 Old 01-10-2020, 05:09 PM - Thread Starter
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Yes, much "tinkering" with paints.
So, it would be a safe bet that my screen at worst would be a 1.0 gain?
Just wanted to have some solid objective numbers to back up what my eyes tell me.

Yes, basic SW Extra White Matte. With a little Flat thrown in to cut the sheen. I know that a more complex cocktail of paints (SF) may have given a better result. But my goal was a simple OTS formulation.

Tried the PPG Diamond. Eggshell was glossy, like a semi-gloss. And the flat was just as glossy as the Duration Matte. And it was bright white!

Photos coming soon.
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post #13 of 16 Old 01-10-2020, 05:24 PM
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Quote:
Originally Posted by TXFlyGuy View Post
Yes, much "tinkering" with paints.
So, it would be a safe bet that my screen at worst would be a 1.0 gain?
Just wanted to have some solid objective numbers to back up what my eyes tell me.

I would venture to "at worst 1.0 gain" might be a bit understated. Probably closer to 1.1


Quote:
Yes, basic SW Extra White Matte. With a little Flat thrown in to cut the sheen. I know that a more complex cocktail of paints (SF) may have given a better result. But my goal was a simple OTS formulation.

Oh they are completely different animals....and I understand exactly what your desires morphed into...and why. Simple is attractive...so if simple it must be...it must be"simply good". And it seems you nailed down the results you were after.


Quote:
Tried the PPG Diamond. Eggshell was glossy, like a semi-gloss. And the Flat was just as glossy as the Duration Matte. And it was bright white!

.....which is why it is used alongside an appreciable amount of Matte Polyurethane...to mute both the Gloss of the Metallic Paint Bases and temper the high degree of reflectivity of the Mica components. (both Silver and Pearl) I've stated before, it is the achieving ofa balance within a Mix's component properties that is what brings about the best and highest degree of "usable" Performance.


Quote:
Photos coming soon.

Looking forward to the Painting description as much if not more than the imagery....the Latter being wholly possible because of the Former.
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post #14 of 16 Old 01-11-2020, 07:59 AM
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Originally Posted by MississippiMan View Post
Here is the one I have. Inexpensive but quite accurate and easy to use.
https://www.amazon.com/Dr-Meter-LX1330B-Digital-Illuminance-Light/dp/B005A0ETXY/
You simply covert Lux to Fc (Foot Candles) which is the converted to the known comparative gain figure.
It looks like that's the kind of meter used for general area/light brightness and not for measuring reflected brightness of a screen.
I believe a light meter that measures screen gain will also have a field-of-view spec (I think you'll typically want this to be a small, single-digit number for screen-gain).

Here's a better explanation.
Quote:
Originally Posted by NxNW View Post
Sanity check:. What do you mean by "meter"?

Meters that measure light coming *from* a screen have a light gathering objective that looks like a traditional camera lens. Shiny glass. They have names like colorimeter or spectrophotometer.

Meters that measure incident light falling *on* a screen have a white plastic diffuser that is intended to collect ambient light from all directions (and weight it according to a specific cosine function). This kind of meter would include your typical "lux meters"

You can't use an incident light lux meter to measure light coming *from* a screen. I mean you physically can, you'll get a reading, but it can't be used in any kind of formula that was expecting a direct (non diffused) measurement as an input.

Easy $25 DIY black (or any color) ALR paint +$40-$50sprayer screen mix smooth/clean and very easy to learn spraying with little/no mess.
Simple $25-40 DIY black/dark-grey ambient-light rejecting screen, grab two things from a local store..mix..roll..done.
Quick <$250 dedicated black-fabric theater room "A store that sells blinds can help your picture more than a store that sells projectors many times." -bud16415
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post #15 of 16 Old 01-11-2020, 08:49 AM
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Quote:
Originally Posted by Ftoast View Post
It looks like that's the kind of meter used for general area/light brightness and not for measuring reflected brightness of a screen.
I believe a light meter that measures screen gain will also have a field-of-view spec (I think you'll typically want this to be a small, single-digit number for screen-gain).

Here's a better explanation.

That is mostly just Elitist rhetoric. Certainly there are more advanced Colormeters, but they too only provide figures that must be converted to reach a determination of "Gain".
.....and do so for several Hundred dollars more. However if you read closer you'll note that he is specifically referring to non-diffused (direct ) light as comes from a ALR screen...which your screen is decidedly not.



The described Lux to Gain conversions were not / are not made up....they have been used by many members. Yes, you must use it in a reverse manner (the Hand Receptor)

...you can find several Threads on AVS and many more such descriptions on YouTube and Blog Articals on the Web that describe how it is done as well.


It's simply this...when individuals want a full suite of Calibration Software (CalMan) and a Meter designed for that purpose (i1).....they spend capriciously for such...and usually at the insistence of other like-minded / well heeled enthusiasts. Those with simpler needs, and who suffice to use DVD based Calibration Disc methods usually use a Light Meter as previously described to obtain rudimentary Gain figures. Then they can take the Wife out to Dinner/Movie several times over with the money they saved.



Your choice...Go long, or go Short...the variance in the result by going Long will not be worth the dent in your purse unless your innate desire to absolutely know what your PJ / Screen combo is producing before and after calibration is what dictates the gear & method. If that's the case...have at it!

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post #16 of 16 Old 01-11-2020, 10:44 AM
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The described Lux to Gain conversions were not / are not made up....they have been used by many members. Yes, you must use it in a reverse manner (the Hand Receptor)

...you can find several Threads on AVS and many more such descriptions on YouTube and Blog Articals on the Web that describe how it is done as well.
What does reverse manner mean?

I've just started reading about this and not sure where to start. Could you provide some links on how it's done?
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