Compressing distance for measuring low-level light. - AVS Forum
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post #1 of 14 Old 10-02-2005, 11:25 AM - Thread Starter
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Ok, we all know what the heck I am talking about. Back in January I applied the technique of reducing the projector-meter distance by a factor of ten in attempts to get 100x for light to the detector.

Now, one of the weaknesses in inexpensive detectors is their inability to accurately measure low-light conditions. Has anyone compared a 'cheap' detector using the distance-compression method vs. static-distance and an expensive detector?

I can speak for a lot of po-folks that the difference between spending $250 and +$3,000 is more than a night in the dog house.

When will D* stop pushing HD-Lite while charging us for full HD? Digital input on a CRT is a reality, not a possibility.
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post #2 of 14 Old 10-03-2005, 09:04 PM - Thread Starter
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Am I to believe that not a single person that afforded a more expensive unit even attempted to see what the trivial results might have ben from the el'cheapo unit?

Or, are there a bunch of people too afraid of revealing the results? :D

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post #3 of 14 Old 10-03-2005, 11:38 PM
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Not many of us who have the "cheapo" versions have the cash for the ultra-pricey pro instruments. :)

Check out umr's Accucal website though - you might find it interesting - he does compare an Eye-One with another higher-end instrument. However, no mention of distance to projector though.
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post #4 of 14 Old 10-03-2005, 11:52 PM
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I use the Spyder2 with a 2' image while the expensive guys brag about being able to pick up 0.0001ftL off the screen - meanwhile I happily measure 3500:1 calibrated contrast with 1ftL blacks! Of course I started calibrating back when the ColorFacts Spyder1 was the only choice...so you are saying you just this year figured this out? Spyder2 is much better at blacks - but the projectors contrast was not that good anyways when the Spyder1 was around. I never suffer dark reading issues.
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post #5 of 14 Old 10-04-2005, 05:53 AM - Thread Starter
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KRAS (Kevin?), do you find it better/easier to skrink the image to 2' than move the Spyder2 closer to the projector? Also, what s/w are you using with the Spyder2 (its OEM or something else)?

Max, I hadn't considered us in our po-folks limitations, but then again I was only consider those with more means.

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post #6 of 14 Old 10-04-2005, 06:10 AM
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Quote:
Originally Posted by maxleung
Not many of us who have the "cheapo" versions have the cash for the ultra-pricey pro instruments. :)

Check out umr's Accucal website though - you might find it interesting - he does compare an Eye-One with another higher-end instrument. However, no mention of distance to projector though.
None of the examples shown are front projector applications so the distance is very small from each display.

I typically measure front projectors within one or two feet of the projector with the EyeOne. Using the EyeOne with the diffuser at this distance with an Optoma H78 the limit would be about 3500:1 contrast ratio based on previous readings. With an AEMC CA813 the limit would be around 350,000 :1 contrast ratio in the same application. The last H78 I calibrated measured at 1500:1.

I have not compared these measurements to a high dollar Minolta which is why I did not reply to this thread. Any instrument that is reading fL or cd/m2 on illumination measurements without a diffuser is not designed for that application.
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post #7 of 14 Old 10-04-2005, 06:54 AM - Thread Starter
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Jeff, I presumed you were measuring On/Off contrast, correct? If so, why the diffuser?

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post #8 of 14 Old 10-04-2005, 09:47 AM
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Quote:
Originally Posted by TheFerret
Jeff, I presumed you were measuring On/Off contrast, correct? If so, why the diffuser?
I was measuring on/off contrast from the projector directly.

A diffuser (cosine-corrector) is a requirement for proper measurement of illumination (directly from the projector). Measuring reflected light (luminance) does not require the diffuser because the reading is intended to be of a specific spot area and directional. While illumination measurements are intended to cover a 180 degree view.

To understand this think of the diffuser as a sample conditioning requirement of the light sensor. Every light meter intended for illumination use has a diffuser as a sample conditioner for the sensor. Stated more scientifically, "The cosine corrector is a sampling optic designed to gather light over 180 degrees, thus eliminating optical interface problems associated with the light collection sampling geometry inherent to other sampling devices." Another indication that a light meter is intended for this type of measurement is the unit of measure will be in footcandles or lux.
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post #9 of 14 Old 10-05-2005, 07:34 AM - Thread Starter
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Is the diffuser something one would not want, then, if they intended to measure ANSI contrast? I mean, if the collect for a 180º solid angle I wonder if edging-effects for a smaller-size dark area within an ANSI CR pattern might be more harmful and less virtuous.

I suppose this might be better explain in knowing when not to use a diffuser.

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post #10 of 14 Old 10-05-2005, 07:54 AM
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Ferret,

I believe you are missing the point. The diffuser is for measuring a light source directly. It does not depend on the test pattern or desired result. Not using a diffuser in this application is an incorrect use of the instrument and will likely result in significant errors.

The sensor size and pattern size will effect how well you can measure the ANSI pattern. Many diffusers are small enough that this is not a factor, but you should observe how the diffuser hits the checkerboard pattern.
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post #11 of 14 Old 10-05-2005, 08:02 AM
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Ferret,

The VESA standard for checkerboard luminance and contrast does specify a luminance measurement. This does indicate a luminance light meter should be used if you are following the standard. Unfortunately, as discussed previously very low black level sensitivity is a problem with luminance measurements. This is why people are using an illuminance measurement device. Once you begin measuring illuminance a diffuser is required.

I would purchase the VESA Flat Panel Display Measurements Standard Version 2.0 if you want to improve your understanding of the problems associated with measuring a displays performance.
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post #12 of 14 Old 10-05-2005, 11:10 AM - Thread Starter
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Thanks for the explanation, Jeff. And what is the VESA Flat Panel Display Measurements Standard Version 2.0 costing these days? Lemme go look ...

... wait a sceond ... Flat Panel Display?

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post #13 of 14 Old 10-05-2005, 12:54 PM
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It applies to projection systems as well. It is a very comprehensive document and will run about $60 delivered.
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post #14 of 14 Old 10-05-2005, 01:20 PM
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TheFerret

I use ColorFacts with the Milori Spyder2 (now released as the DataColor Spyder2). The Spyder has always had a diffuser on it and designed to measure the projector - the new DataColor Spyder2 adds a lightpipe LCD filter attachment for panels - as the diffusor was causing problems there (LCD panels change color at angles). What you have to remember is that with diffusors - all visible LED/LCD need to be turned off - so make sure you are behind the stand with your laptop. I close my laptop lid while I take a reading. Those Blue LED that is the trend on power buttons are rather nasty!

See Ursas thread about the SpyderTV - and don't ask what the slight Spyder2 diffs are in the various packages as everyone seems confused. Mine is a Milori bastard prototype that used their Optimagery design/driver with the Spyder2 guts in the case - I am sure production models vary slightly. The advantage of the Milori version is it was hand calibrated to 0.001xy readings - I doubt this happens on the SpyderTV.

I use a Manfrotto by Bogen light stand - works great - a nice Italian black with brass fittings that easily demonstrate what a DLP rainbow is. :D Light stands are better for this than tripods - I can go 12' high and do projectors on cathedral ceilings. When I do stuff in my office - I just put the projector on a table and have the sensor about a meter away. Using a poster board - I can measure the image size.


The only problem with the light stand is you need an angle attachment to make sure you are looking at the projector (as no diffusor is perfect!). The old Milori attachment was just folded metal - it is easy to bend. Dunno what the new DataColor Spyder2 uses.
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