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randyc1

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I just bought a light meter to measure the light output off PJ screen, but it measures in LUX , how can i easily convert to Lumens ?

Thanks

Harpmaker

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I just bought a light meter to measure the light output off PJ screen, but it measures in LUX , how can i easily convert to Lumens ?

Thanks
You could multiply your Lux reading by 0.09290304 or just use this converter. Now multiply the square feet of your screen by the foot-candle value and you have Lumens.

For example: You are getting a reading of 150 Lux with the light meter at your screen and pointing at the PJ. 150 times 0.09290304 = 13.935 fc. Your screen is 120" diagonal 16:9 ratio which equals 59" x 104". Multiply 59 by 104 to get the screen size in square inches which here is 6136. Divide that value by 144 to get the screen size in square feet which here is 42.611111. The lumens hitting the screen would be 13.935 times 42.11111 which equals 586.81831 Lumens. :nerd:

Yiannis1970

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Or, if you are familiar with meters:

Lumens = Lux x s.m. (screen's width x height)

randyc1

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You could multiply your Lux reading by 0.09290304 or just use this converter. Now multiply the square feet of your screen by the foot-candle value and you have Lumens.

For example: You are getting a reading of 150 Lux with the light meter at your screen and pointing at the PJ. 150 times 0.09290304 = 13.935 fc. Your screen is 120" diagonal 16:9 ratio which equals 59" x 104". Multiply 59 by 104 to get the screen size in square inches which here is 6136. Divide that value by 144 to get the screen size in square feet which here is 42.611111. The lumens hitting the screen would be 13.935 times 42.11111 which equals 586.81831 Lumens. :nerd:
Thanks Harpmaker , i will keep this for reference .

So i put the meter infront of screen facing PJ ,.. what kind of images do i project to see potential light output of PJ ?

Harpmaker

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Thanks Harpmaker , i will keep this for reference .

So i put the meter infront of screen facing PJ ,.. what kind of images do i project to see potential light output of PJ ?
The best image to use is "100 IRE" from a calibration DVD. This is the brightest white signal for video. I forget now if it is the same as a RGB value of 255, 255, 255 or not. I know that "0 IRE" (the darkest black for video) does NOT have a RGB value of 0, 0, 0. Hopefully someone that knows more about this will chime in here.

Yiannis1970

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The best image to use is "100 IRE" from a calibration DVD. This is the brightest white signal for video. I forget now if it is the same as a RGB value of 255, 255, 255 or not. I know that "0 IRE" (the darkest black for video) does NOT have a RGB value of 0, 0, 0. Hopefully someone that knows more about this will chime in here.
Here you go Don...

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mechman

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If you were to measure RGB of a 100 IRE image, and it was to be correct, it should measure 235 235 235. :nerd: :T

Yiannis1970

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Mech, if we are talking RGB then the white pattern is 255, 255, 255. 235 is the white when encoded in 16-235 levels.

mechman

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235 is the white when encoded in 16-235 levels.
I thought that was what we were talking about - video levels. A 100 IRE image measured for RGB from a display that has been properly calibrated should measure 235 235 235. 255 255 255 would be in the whiter than white category for video.

Yiannis1970

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A, ok then...i now understand what you say!

I had the impression that Don was referring to pure RGB values and not video levels.

Harpmaker

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Thanks for the help on this Mech and Yiannis. A lot of people get confused about this topic (maximum White and Black levels). People using a "regular" video source like a DVD player have a dedicated video output of 235 RGB for White and 16 for Black, but many that use a PC as a video source have the 255-0 RGB output and have trouble calibrating their projectors.

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