How to calibrate and measure ft lamberts? - AVS Forum | Home Theater Discussions And Reviews
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Old 03-14-2012, 01:10 AM - Thread Starter
EJ
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Aside from an isf calibration, how do I measure the correct amount of light I'm throwing? First, I have a 1.0 gain screen. What image do I project, a 100 ire full-frame white field? Then, do I get a light meter and try to get close to the 16 ft lamberts recommend?

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Old 03-14-2012, 06:29 AM
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Old 03-14-2012, 06:39 AM
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Yep, use the instructions in the link given, or buy one of the versions of Calman or Chromapure with the i1 Display meter and the software will give you a ftL reading off your screen.

(I assumed by "aside from an ISF calibration" you meant paying someone else to come in and calibrate. )
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Old 03-14-2012, 10:35 AM
 
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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.
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Old 03-14-2012, 06:22 PM
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Quote:
Originally Posted by AV Science Sales 4 View Post

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 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).
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Old 06-11-2015, 02:29 PM
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I need some help from you people in figuring out the correct conversion from foot lambert to the equivalent lux.
Some say that 1 foot lambert is equivalent to 1 foot candle, others that 1 candela per square foot is Pi of 1 foot lambert.
1 foot candle is shown to be equivalent to 1 lumen per square foot and, since there're 10.76 square foot in 1 square meter, then 1 lumen per square foot is 10.76 lumen per square meter, which is equivalent to lux.
So, if I start from 14 foot lambert, I have to divide by Pi and multiply by 10.76 to obtain the lux equivalent of circa 48.
Is it correct?

Last edited by Optimus_Fine; 06-12-2015 at 09:40 AM.
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Old 06-12-2015, 04:57 AM
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My research indicates what millerwill posted above, that on a unity gain screen, 1 Foot Candle = 1 Foot Lambert. Understand that they are different measurements though, fc is a measure of illuminance, ie the light illuminating a surface. fL is a unit of luminance, ie the light reflecting off a surface. If you know the real gain of your screen, then ftL = fc * gain, just beware that gain specification is usually not equal to the actual gain of the screen.

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Old 06-12-2015, 10:03 AM
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Problem is that I have a display and a lux meter, so I must know the exact ratio between foot lambert and lux. Some websites say that to convert candela per square meter to lux one must multiply by Pi, but if I measure maximum luminance possible (clipping whites), the value shown at the center of the screen is 380 lux, which, divided by Pi, would be 120 nit. But the same display has been measured at well over 300 nit in some websites.
So either a lux and a nit have the same energy or colorimeters give bogus numbers for luminance with 200% margin of error.
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Old 06-12-2015, 10:21 AM
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Quote:
Originally Posted by hifiaudio2 View Post
Yep, use the instructions in the link given, or buy one of the versions of Calman or Chromapure with the i1 Display meter and the software will give you a ftL reading off your screen.

(I assumed by "aside from an ISF calibration" you meant paying someone else to come in and calibrate. )

My AEMC CA813 light meter does a better job - the i1 Display meter isn't as good for measuring foot lamberts. And, I compared my CA813 with a less expensive Dr Meter yesterday - the Dr measured much higher. I still think the CA813 is a better light meter.

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Old 06-12-2015, 11:01 AM
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Quote:
Originally Posted by Optimus_Fine View Post
Problem is that I have a display and a lux meter, so I must know the exact ratio between foot lambert and lux. Some websites say that to convert candela per square meter to lux one must multiply by Pi, but if I measure maximum luminance possible (clipping whites), the value shown at the center of the screen is 380 lux, which, divided by Pi, would be 120 nit. But the same display has been measured at well over 300 nit in some websites.
A fc is 10.76 lux. A fL is 1 fc (assuming unity gain screen). So your fL is 10.76 lux * gain.

See what an anamorphoscopic lens can do, see movies the way they were meant to be seen
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Old 06-13-2015, 03:23 AM
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I've found the definition of foot-lambert:
Quote:
foot-lambert
noun
1. a former unit of luminance equal to the luminance of a surface emitting or reflecting 1 lumen per square foot. A completely reflecting surface illuminated by 1 foot-candle has a luminance of 1 foot-lambert ft-L





Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
So, 14 foot-lambert is the luminance of 14 lumen per square foot, which multiplied by the ratio to a square meter, gives circa 151 lux.
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Old 06-16-2015, 02:04 PM
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I've found out that, to meaure 48 cd/m2 in lux from a direct view screen, one must divide 48 cd/m2 by the distance squared from screen to photo diode facing the screen (for an easy conversion) of 1 meter which gives 48 lux, then multiply by the screen area in m2 of the 100% white window and that will be the lux reading the meter must report.

Last edited by Optimus_Fine; 06-16-2015 at 02:11 PM.
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