[argyllcms] Re: question about i1 studio, M2 and spotread
- From: Graeme Gill <graeme@xxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Thu, 31 Oct 2019 10:55:08 +1100
Mete Balci wrote:
Hi,
My question is: i1 studio is using M2 measurement mode and I believe the
standard illuminant for M2 is not specified, am I wrong on this ?
M2 is a measurement condition, related to the actual light source
used to make the spectral measurement. It is the "UV filtered" type
of light source. All reflective spectral instruments divide the
raw spectral readings by the light source spectrum, so they return
the reflectance at each wavelength (i.e. 0 to 100%). The white tile
calibration is all about getting a reference for that division.
The nature of the light source has implications in relation to fluorescent
effects of (typically) the paper, but doesn't affect the reflectance measurement
directly. Standard XYZ calculations are then based on a theoretical D50
light source being used on the measured spectral reflectance.
If it is
not specified, how does spotread calculate the color values ?
Multiply the reflectance value by the standard D50 spectrum then integrate
by the standard 1931 CIE CMF's (Color Matching Functions).
Are they
reported directly by i1 display, so actually the illuminant used for the
measurement is propreitary ?
It's proprietary in the sense that X-Rite doesn't publish it, but anyone
can measure it from a device, and it's encoded in the instrument calibration
information that's stored in the instrument EEPROM. Part of the factory
calibration is setting all this stuff up.
Does -i illum parameter (default D50) have effect on this ?
Yes, it sets the illuminant used to compute the XYZ values. So you
can compute the XYZ under different illuminants, once you know
the reflectance values.
Its default is
D50, but again if M2 does not specify a standard illuminant, how to specify
one here ? I see there is an option of D50M2, does it mean M2 is always
used with D50 ?
There's really no point in playing with M0, M1, M2 etc. on a ColorMunki/Studio
since it doesn't have UV, and can't compute FWA, so simulation of D50, A and UV
cut Ultra Violet/Fluorescent responses isn't possible. You're stuck with
M2, because that's all the instrument can do.
(Ideally Argyll would know this about the measurement and issue a warning if
you try and compute M0 or M1 from a UV cut instrument, but that's not
something
I've hard coded into things.)
Cheers,
Graeme Gill.
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