Re: uv and measurements.
Re: uv and measurements.
- Subject: Re: uv and measurements.
- From: "Chris McFarling" <email@hidden>
- Date: Wed, 23 Dec 2009 11:11:08 -0500
- Importance: Normal
Yes confusion is rampant not only in regard to this topic but to color
management in general. On one hand these products and technologies are
marketed in such a way as to make a color managed workflow easy. But for
anyone who has tried to implement a full blown color managed workflow you
know that it is nowhere near easy. There are so many caveats that you need a
consultant or someone with nearly consultant level knowledge to do it right
and maintain it I think, for anything but the most simplest of workflows.
Anyway, speaking of confusion, I'm not clear on what you mean by "legacy
light source unspecified" when referring to the M0 condition. First of all
which X-Rite spectro models fall into this category? Would this be devices
such as EyeOne, DTP70, DTP41? Obviously the light source of these devices is
known. Does that mean that the amount of UV in the light source is unknown?
Likewise which devices would fall into the M2 category? Any of those same
devices with a UV filter on them?
You mentioned that no one currently makes an M1 device but then you
recommended using an M1 device if making a measurement that is intended to
represent the visual process. That's a bit confusing as well. What should we
take from that?
Thanks,
Chris McFarling
--------------------------------------------------
From: <email@hidden>
Sent: Wednesday, December 23, 2009 9:57 AM
To: <email@hidden>
Subject: uv and measurements.
The whole topic of UV in measurements is currently being examined with
care from many different standards bodies. There is a new form of
standardization within ISO that basically describes four standard
measurement conditions:
1: M0 legacy light source unspecified
2: M1 D50 illuminant or illuminant condition where the ratio of UV to
visible matches D50
3: M2 UV cut
4: M3 Polarized.
The reason that M1 is being advocated by organizations such as TC130 is
that it finally specifies the amount of UV that is applied to object being
measured. One key component of M1 is the ratiometric consideration of UV
to Visible with respect to D50. This specification assumes that the
fluoresence component is independent of absolute amplitude. I built a lab
instrument to investigate this and I found that reciprocity generally
holds true, so the specification does make sense.
The average user has absolutely no control of the UV in the light booth,
the instrument, or the nature of the Optical Brighteners in the paper. I ,
personally, would never recommend using an M0 device for general profile
generation, but many very good profiles have been generated using such
instruments. The interaction of the source and the paper is totally
unspecified. Many engineers will say that a tungsten bulb will put out
enough UV to excite the OBA in the paper. This is completely true, but the
amount of UV in an M0 instrument is very variable and it is a very strong
function of the regulation of the lamp. MO devices have two unspecified
sources of potential error: the UV content of the source and the amount of
OBA in the paper.
Why use UV cut (M2)? In theory UV cut should give the media independent
readings and provide better correspondence between instruments. M2 is best
suited for applications that involve quality control and measurements of
ink on paper without regard to the substrate, per se. It is important to
note that if one is converting a spectral measurement to a Status Density
measurement, the legacy M0 condition should be used. The status density
specifications generally demanded a tungsten based illuminant.
To the best of my knowledge, no one actually builds an M1 instrument. It
is not trivial because the characterization of the UV is not easily
accomplished. A filter that would map a typical tungsten source to d50
that maintains the ratio demanded by the M1 specification would reduce the
total output of the source by a factor of roughly 375. A traditional white
LED has virtually no UV component, so an led based instrument would
require compound illumination.
Measurements of many popular light booths show that there has been
virtually no compliance with the UV component of d50. This fact supports
use of M0 or M2, because there is no control of UV in the booths, why
would M1 do any better? The old MacBeth SP3 booths actually do quite will
with filtered tungsten and custom filters, but they require large amounts
of power to achieve that goal (remember that factor of 375X loss of total
energy).
So what would I recommend:
If you are making a measurement that is intended to represent the visual
process, the M1 condition should be used, but M0 has shown great utility
albeit with a high degree of inter- instrument variablity. Keep in mind
that the variability in light booth output swamps any of the differences
you would see between M0 instruments. For this reason, one should expect a
lot of push back from manufacturers against M1 until manufactures of light
booths take the UV component seriously as well.
If you are making a measurement that is to be transmitted to another
facility for confirmation, the M2 (UV cut) should be used because it
minimizes the differences in UV content in illumination between
instruments and it makes no assumption about viewing conditions. One could
make the argument that because the UV content in most viewing environments
is so out of control, making a profile with UV cut should be OK because
the effect of UV is totally out of the profile maker's control.
I believe that we (X-rite) offer a visual solution which allows inclusion
of the UV component in a light booth to be applied to profile. This
process links the profile to the viewing booth which is important in some
situations, but keep in mind that this making a profile both instrument
dependent and light booth dependent.
There is no easy answer to the UV issues because they are so totally out
of control in the market place. There are also different requirements,
some require UV and some need to avoid the effect of UV. With this in
mind, it should come as no surprise that the consumer is confused.
Regards,
Tom Lianza
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