Calculating lumens - AVS Forum
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post #1 of 11 Old 10-09-2012, 07:37 PM - Thread Starter
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I was calibrating my the W7000 last Friday and I got kind of confused on calculating lumens. I have the equation and I wanted to know if its the correct one.

(lux * sqft of screen) / 10.76 = lumens

It seemed weird to me because from the same distance on my 100in screen I get

(455 * 29.6) / 10.76 = 1250

Then, with the 120in screen, I get (455 * 42) / 10.76 = 1776

Did I get the equation wrong?
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post #2 of 11 Old 10-09-2012, 09:01 PM
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what are you using to measure 455 lux on both screens?

typically with a colorimeter or spectro you're going to be measuring fL or cd/m

For lumens using fL is simplier

fL * sqft = lumens.

You should be getting a lower fL reading on the larger screen.

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post #3 of 11 Old 10-09-2012, 11:03 PM - Thread Starter
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Quote:
Originally Posted by sotti View Post

what are you using to measure 455 lux on both screens?
typically with a colorimeter or spectro you're going to be measuring fL or cd/m
For lumens using fL is simplier
fL * sqft = lumens.
You should be getting a lower fL reading on the larger screen.

A light meter
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post #4 of 11 Old 10-09-2012, 11:06 PM - Thread Starter
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Do someone know if I have the right equation? I don't really want to set up my equipment to get the ftL.
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post #5 of 11 Old 10-09-2012, 11:09 PM
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The conversion from lux to fL is lux/10.76 = fL, so yes you have the right formula.

The part that isn't right is that you're measuring the same light output on a screen that has 30% more area. Something is wrong there.

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post #6 of 11 Old 10-09-2012, 11:27 PM - Thread Starter
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Quote:
Originally Posted by sotti View Post

The conversion from lux to fL is lux/10.76 = fL, so yes you have the right formula.
The part that isn't right is that you're measuring the same light output on a screen that has 30% more area. Something is wrong there.

I'm gonna measure again. I am trying to sell my screen so I can get a bigger screen. Going from 120in to 160in.
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post #7 of 11 Old 10-09-2012, 11:32 PM
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I suppose the other thing to consider is the gain of the screens.

your actual lumens would be actualLumens =(measuredLumens/screen gain)

160 inch is considerably bigger, even if you get the same gain screen the picture would be noticeably dimmer.

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post #8 of 11 Old 10-09-2012, 11:42 PM - Thread Starter
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Quote:
Originally Posted by sotti View Post

I suppose the other thing to consider is the gain of the screens.
your actual lumens would be actualLumens =(measuredLumens/screen gain)
160 inch is considerably bigger, even if you get the same gain screen the picture would be noticeably dimmer.

I read the wrong lux reading. I knew something was wrong. I just did the readings again. I'm using a HP screen close to eye level, so I should be ok with the brightness. I got a lux reading of 455 with my 120in screen and I max the zoom out and that gave me 265 for the 160in screen. Thanks for the help
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post #9 of 11 Old 10-10-2012, 05:55 PM
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Really, 10 feet isn't wide enough? You could just move closer and not change the screen size and keep the bright image you have now.

For example, if you sit 13.3 feet from the 120" screen, you have a viewing angle of 41.2 degrees

If you change to a 160 inch screen and stay at 13.3 feet viewing distance, your new viewing angle is 53.2 degrees.

You can achieve the same 52.3 degree viewing angle with the 120" screen if you simply sit 10 feet from the screen. And sound quality is likely to be better when you are 10 feet from the screen also as direct sound from the main speakers and center channel will be a bit more prominent (compared to refleted sound energy that is delayed and "stirred" considerably compared to the direct sound from the speakers). No cash outlay for a new screen and a high probability of better sound quality in the bargain - just move 3.3 feet closer. Of course, that may not be possible in some rooms (risers or other features, etc). I use a wider viewing angle than both of these examples with a much smaller screen... ON PURPOSE. Because of the sound quality issue. The impact of viewing the movie on a smaller screen with similar viewing angles is exactly the same as the impact you get from a larger screen with more distant viewing seat because the viewing angle determines how large you perceive the images, not the size of the screen.

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post #10 of 11 Old 10-11-2012, 03:56 AM - Thread Starter
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Quote:
Originally Posted by Doug Blackburn View Post

Really, 10 feet isn't wide enough? You could just move closer and not change the screen size and keep the bright image you have now.
For example, if you sit 13.3 feet from the 120" screen, you have a viewing angle of 41.2 degrees
If you change to a 160 inch screen and stay at 13.3 feet viewing distance, your new viewing angle is 53.2 degrees.
You can achieve the same 52.3 degree viewing angle with the 120" screen if you simply sit 10 feet from the screen. And sound quality is likely to be better when you are 10 feet from the screen also as direct sound from the main speakers and center channel will be a bit more prominent (compared to refleted sound energy that is delayed and "stirred" considerably compared to the direct sound from the speakers). No cash outlay for a new screen and a high probability of better sound quality in the bargain - just move 3.3 feet closer. Of course, that may not be possible in some rooms (risers or other features, etc). I use a wider viewing angle than both of these examples with a much smaller screen... ON PURPOSE. Because of the sound quality issue. The impact of viewing the movie on a smaller screen with similar viewing angles is exactly the same as the impact you get from a larger screen with more distant viewing seat because the viewing angle determines how large you perceive the images, not the size of the screen.

I would still want a bigger screen. But I will have to sell 2 screens first. Doesn't seem like its happening anytime soon. But I'm satisfied with just using my 120in. There's no big rush actually.
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post #11 of 11 Old 10-15-2012, 03:30 PM
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For what it is worth, the calibration industry is semi self regulated with foundations in the USA which intern follows imperial measurement, typically fl =foot lambert and hence the domination of the term fl used in calibration software.

The international SI unit is cd/m^2

Lumen (lm) is also an SI unit, and so is Lux (lx)

1Lux = 1 lumen/m^2 (abreviated as 1 lx =1 lm/m^2)
(m^2 = metre square)

1Lux = 1 cd/m^2

Lux is a measure of intensity as perceived by the eye

The difference between Lumen and Lux is that Lux takes into account the spread of luminous flux over an area such as a screen.
Lumen is output from the projector, Lux is what you see as a whole.

The standard method for screen measurement is based upon luminous flux per solid angle, candela per area, which brings us back to footlamberts (fl) for US motion picture industry or SI-unit(international standard) candela per square metre (cd/m^2)

For home theatre it is typical to find (10fl)(34cd/m^2)~(14fl)(48cd/m^2) range, even as low as (7fl)(24cd/m^2) when bulbs age.

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