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Calculate brightness

post #1 of 15
Thread Starter 
I have been considering doing a 140" wide scope screen with .8 gain (enlightor 4k), but I have no idea how many lumens are needed to light this screen up to around 20ft-l.

Can someone work me through the math assuming an A lens is used?
post #2 of 15
The basic formulas are:

Screen brightness = PJ Lumens / Screen Area in ft2 * screen gain

Or, for 16ftL target:

PJ lumens needed = 16 * Screen Area (ft2) / Screen gain

There are other factors to consider when designing a room, but that will get you in the ballpark.

Don't use mfgr specs, except for Runco which is usually very close. And many PJ's output varies by throw position within it's lens range. Work with a good dealer (like me smile.gif ) who will be sure you aren't making a mistake in your choices.
post #3 of 15
Let me be the first to concur with Scott. I might add never ever scrimp on light output of the projector; you will have buyers remorse until you buy another projector. Also keep in mind the diminution of lamp performance over it's life. Bottom line: buy a projector with enough headroom to get your target fL even after say six months or more of use.

Art
post #4 of 15
How far should you hold the meter away from the screen to get an accurate reading?
post #5 of 15
How many Foot candles should you get between 12-16fc on the screen??
post #6 of 15
Quote:
How far should you hold the meter away from the screen to get an accurate reading?
If you mean an illuminance meter (e.g. Minolta T-10, CA-813, etc.) then wherever you hold it must correlate with the virtual screen size at that point in space relative to the PJ's beam size. As you hold closer to the PJ, the virtual screen size will get smaller. So generally, unless you have a tripod and know what you are doing, best to hold at the screen, literally, so you know what area to use in the calculation.

Note that if you use a luminance meter (e.g Minolta LS110) it measures off the screen so something has to be illuminated in order to measure and it measures the screen gain. Due to the gain involved, this type meter is very sensitive to viewing angles in both planes, so not as good for comparisons IMO unless you have a exact from and to location that is 100% repeatable. And you have to guess at what the screen gain really is at that specific angle of incidence in order to back into Lumens. For lumen determination, an illuminance meter is best, it takes the screen gain out of the measurement.
Edited by GetGray - 9/30/13 at 5:48am
post #7 of 15
Quote:
Originally Posted by michaeldip View Post

How many Foot candles should you get between 12-16fc on the screen??
This is a personal preference. I prefer brighter but the ubiquitous number thrown around here is a target of 16ftL. (ftL = ftc * screen gain). That number has some basis in what theaters use to set their projectors up, but I think that's with a open frame? (no film). Anyway, 16ftL is the minimum I shoot for with some bulb aging for the majority of my customers. Personally I prefer 25+.
post #8 of 15
Quote:
Originally Posted by RapalloAV View Post

How far should you hold the meter away from the screen to get an accurate reading?

It shouldn't matter, unless you are creating a shadow, or the edges of the screen fall within the sensor area.

If you take a flashlight and move it back from the wall, the wall will get 4x darker every time you double the distance.

If you set the flashlight (or projector) in the same spot, the wall will stay the same brightness when you back away from the wall. The reason for that is that the area of the wall is compressing into a smaller angle of your view at the same rate that the brightness falls off. It cancels out.

Angle affects brightness with high gain screens. With perfect diffuse (gain 1.0) screens, the gain shouldn't matter. Actually, with gain 1.0, each point on the surface emits light with a cosine distribution, but as the angle increases, the surface compresses in your view, conveniently canceling out to produce constant brightness from all angles.
Edited by rcohen - 9/30/13 at 7:29am
post #9 of 15
Quote:
Originally Posted by rcohen View Post

Quote:
Originally Posted by RapalloAV View Post

How far should you hold the meter away from the screen to get an accurate reading?

It shouldn't matter, unless you are creating a shadow, or the edges of the screen fall within the sensor area.

Position does matter. If you are taking the reading to calculate lumens.
post #10 of 15
in foot candle what is a good number to get ???if screen is 1.3gain
post #11 of 15
"good" is a relative, personal preference and their or other factors to consider when choosing a target.

If a target was 16ftL and you don't have light walls causing room reflections in turn causing errors, and you are not considering bulb aging or are measuring an aged bulb already, and the screen really is 1.3, etc., etc., then a target reading might be 12.3 fc
post #12 of 15
Quote:
Originally Posted by GetGray View Post

This is a personal preference. I prefer brighter but the ubiquitous number thrown around here is a target of 16ftL. (ftL = ftc * screen gain). That number has some basis in what theaters use to set their projectors up, but I think that's with a open frame? (no film). Anyway, 16ftL is the minimum I shoot for with some bulb aging for the majority of my customers. Personally I prefer 25+.

Looks like the number should be 14ftL. The DCI Spec references SMPTE RP 431, which is discussed here:
http://dcinemacerts.net/dcinematools.com/images/tech_upload/Color_Processing_for_Digital_Cinema_4-_Measurements_and_Tolerances.pdf

100% white should be 14ftL +/- 1 ftL for Reviewing rooms and +/- 3ftL for theaters.

But yeah, it's a matter of preference.
post #13 of 15
Do you point it at the screen or at the projector?
post #14 of 15
Depends on what type meter you are using.
post #15 of 15
Running studio tek 130 105inch 2:35:1black room
Jvc x70 high lamp iris -12 getting 13fc meter just off the screen facing projector
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