Can I use lux meter to get ftL of a plasma? - AVS Forum
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post #1 of 5 Old 01-07-2014, 12:44 AM - Thread Starter
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Sorry if this is a repost. I post this in VT60 forum, and then I forget I probably should post this here as this is more about calibration...

I just got a VT60 today, and I just want to check and make sure everything is OK so I don't have to return it. one thing I want to check is the ftL. It does not look too bright, so I took out my lux meter and check reading.

anyone can confirm if my math is correct or not?
I displayed a 100% (with PLUGE) pattern using DVE HD Basic disc, and using my LUX meter, I measure about 120lux for 50% contrast, and 140lux for 60%, and 180 for 80%.
I am not sure how to convert from LUX to ftL, but after some search, seems like LUX is for lumiance (emitting), and ftL is for illuminace (reflecting). Since there is no screen to reflect, I just assume there is no loss of reflection, and thus, 1 lux / 3.426 = 1 ftL, is this correct?

So, 50% contrast --> 120 lux = 35 ftL
60% contrast --> 140 lux = 41ftL
and 80% contrast --> 180 lux = 52ftL.

I just want to know if this sounds right or not. I can probably take out my i1 Display Pro to save me from doing the math, but my lux meter is more easily accessible vs the i1 Display PRO and also need to setup CALMAN, which I haven't done for a long while.

Thanks!
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post #2 of 5 Old 12-09-2014, 09:12 AM
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This is the right idea but the equation is wrong

lux divided by pi (3.14159) gives you candela per square metre (cd/m2)

120 lux = 38 cd/m2 = 11 ftL
140 lux = 13 ftL
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post #3 of 5 Old 12-09-2014, 10:50 AM
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Lux is a measure of illuminance, and ftl is a measure of luminance.

I think you can only do the pi conversion when the surface being measured is perfectly diffuse.
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post #4 of 5 Old 12-09-2014, 03:52 PM
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Quote:
Originally Posted by fight4yu View Post
I just assume there is no loss of reflection, and thus, 1 lux / 3.426 = 1 ftL, is this correct?
I know ftL are still very popular in the U.S.A., but really you are better off getting into the habit of using modern scientific units when you are able, ie. cd/m^2.

Modern standards use the Scientific units, and there is one less conversion factor needed (m^2 <-> ft^2) when converting between other radiometric and photometric units, since they are all SI - i.e. wavelength, color temperature, Illuminance (lux) etc.

See http://en.wikipedia.org/wiki/Light
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post #5 of 5 Old 12-09-2014, 06:41 PM
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yea, I can't stand ftl

The following is my current understanding of luminance and illuminance measurements - I might be off in some details:

The geometry of measuring luminance means that it's a distance independent quantity. In other words, it is a quantity that does not change over distance. So a luminance meter is essentially measuring the luminance of the display, even if you take that measurement from a distance (so long as the entire display is subtended by the measuring angle).

When you measure illuminance, you're measuring the luminous flux per unit area. The area in question is the sensor in the instrument. Thus, the farther away you hold the instrument, the smaller the illuminance.

A luminance meter uses optics so that only a narrow solid angle of light is being measured. See the first diagram on this page. The important thing to note here (and this is the distance independence idea) is that as the instrument moves further away from the light source, a larger area of the light source will be channeled into the measurement angle. This will perfectly counterbalance the attenuation of light that is due to the increased distance. Note that the sensor is actually measuring illuminance, but it can calculate luminance by dividing the measured illuminance by the measuring angle. And, assuming the luminance of the light source doesn't change appreciably over that small measuring angle, it will give a good approximation to the actual luminance.

An illuminance meter essentially does the same thing, but without optics, and measures over 180 degrees. Because of this, unless you have the meter literally right up against the screen, a substantial portion of the measured light will be coming from beyond the display being measured. This is fine, if all you are interested in is the illuminance at any given point in the room (due to all light sources). More importantly, even if you do have the illuminance meter right up against the screen, the illuminance being measured is due to light coming from more than just a small angle. And unless your display has PERFECT viewing angles (i.e. it is a perfect lambertian radiator, with equal luminance in every direction), then dividing the measured illumiance by 180 is only going to give you an "average" luminance over all these angles.

(note: technically, we should be speaking in solid angle and in terms of the surface of the unit sphere: i.e. steradians). But it's simpler to sometimes explain in terms of degrees in two dimensions.

Last edited by spacediver; 12-09-2014 at 11:09 PM.
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