On Dec 4, 2008, at 5:56 02AM, Graeme Gill wrote:
Timothy Hattenberger wrote:I plugged in a recently re-certified Eye One and argyll recognized it fine. I then calibrate on the white tile, and proceed to setting the monitor luminance. I just calibrated this display last evening with an I12 and set it to ~124 cd/m2, but the I1 Pro measured it at approximately ~178cd/m2. So I used the EyeOne match software with the I1 Pro to see what was going on, and that measured properly (~125 cd/m2).Which version of Argyll are you using ?
V1.0.3
I think the problem is that when I do a calibration of the I1 Pro in EyeOne Match, it turns the light on and takes a reflectance measurement of the white tile. The light does NOT come on when calibrating using Argyll, which I think I can convince myself would lead to the artificially high measurements.You are doing an emissive measurement, and since there is no absolute emissive reference available, the only calibration possible is black level calibration. The light will only come on when doing a reflective calibration, where there is an absolute reference available (the white tile). EyeOne Match is doing an (unnecessary) reflective calibration, along with the dark calibration performed for both emissive and reflective measurement, Argyll is not.
Right I understand that it's emissive, and there's not absolute standard, but was not that sure of Gretag's calibration procedure. So this makes sense.
I tried calibrating on a dark surface and also with no calibration (-N) and got the same results. Any ideas? I pasted the dispcal command below and also the data Argyll reads for the instrument upon dispcal initialization.Getting the absolute calibration right is tricky. It depends a lot on the geometry (the direction of the light being measured, and the sensitivity of the instrument to light from different directions).
I remember doing an experiment before measuring these LCDs with a PR650 across the max tilt angles of the display and it not varying that significantly. I suspect that then that if there's any sensitivity to angle it may be with the instruments (radiometer vs. irradiometer. )
I've seen different instruments disagree by += 50% when measuring the same source of light. Certainly my i1d2 and i1pro's don't really agree with each other, even when using Gretags drivers (see below).
True, but when you measure the same display with the i1pro using Argyll and then i1pro using EyeOne Match it appears they give you <1% difference. But when I do I was getting the >40% error.
The disparity might well be greater on LCD displays, since LCDs have a much narrower emission angle, and the i1pro is much more directional than the i1d2.
So if we're calibrating/profiling a bunch of displays in our facility using Argyll, then we should pick one instrument or the other because there's really no good absolute luminance with these instruments? I guess I could come up with a fudge factor and normalize both instruments to the PR650. Unless of course we're using the EyeOne Match software, which somehow appears to give the same result. Which is right? Any ideas?
The current Argyll drivers return results that are (as far as I can tell) identical in absolute calibration to the results using the Gretag/X-Rite drivers, so there's not much more I can do but rely on their factory calibration. It's not clear to me whether EyeOne Match is second guessing the factory calibration (it may be, particularly if it marries a particular instrument to a particular display), and/or whether you are using an older version of Argyll. If the former is the case, then they will be in trouble if X-Rite get their act together andstart calibrating their i1d2's and i1pro's against the same standard :-)
I will try to call XRITE, but it will be tricky to navigate this since Argyll is not supported. Any other ideas?
With the instruments I have, measuring the same spot on my CRT using the Argyll drivers: i1pro rev D: 58.20 i1pro rev A: 52.70 i2 disp 2 41.01 DTP94 42.11 Using the gretag drivers: i1pro rev D: 57.72 i1pro rev A: 52.37 i2 disp 2 41.38 (Note that my display drifted a couple of cd/m2 over the course of the measurements.) Using an LCD display: Using the Argyll drivers: i1pro rev D: 258.56 i1pro rev A: 243.40 i2 disp 2 186.13 DTP94 186.74 Using the gretag drivers: i1pro rev D: 262.70 i1pro rev A: 244.34 i2 disp 2 184.42 [ I did come across an interesting bug in investigating this, andit's a bug I suspect that the Gretag/X-Rite driver suffers from as well:The threshold that is used to detect when the sensor is over range (saturated) is when an average of 10 spectral bands are over a threshold of sensor full scale. In the case of peaky light sources (such as displays), it'sperfectly possible for a measurement to be under this number, and yet for the measurement numbers to be quite measurably affected. So it turns out that my automatic switch of display integration time from 2.0 seconds to 1.0 second for bright displays isn't happeningwhen it should, and measurements of display brightness's around 275 cd/m^2 or over are being distorted. I will fix this by setting a zero tolerancefor over-range, and allowing for 3 different display measurement integration times in the future. ] Graeme Gill.
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