This is a followup to my blog "A spectrophotometric romance". Be forewarned, this post is rated PG, for Print
Geek. And it will be only just a little bit silly.
This post is necessary in order to
consolidate comments, questions, and answers that have appeared in many
different places. To date (Monday, Feb 18) the post has been viewed 753 times.
Since there are only 587 certifiable print geeks in the world, this blog post
is the Justin Beiber YouTube video of the print geek crowd.
There have been 76 comments in four
LinkedIn discussion groups and on the blog. I have had email and phone
conversations about the post with nine people, none of whom are in my immediate
family. The interested folks include manufacturers of spectrophotometers and
reference materials, printers, physicists, color scientists, people in
standards groups, and designers.
Netflix called me the other day to
complain that my email traffic was starting to bog down streaming video speed
in Los Angeles, the greater New York metropolitan area, and one neighborhood in
Scranton, PA. So please forgive me if I left out one of your astute comments.
Can
I read a full copy of the TAGA paper?
Sure. Here is a link to the full paper: Evaluation of Reference Materials for Calibration ofSpectrophotometers. The
paper was presented at the 2013 TAGA conference in Portland, OR.
Now for the obligatory plug. TAGA is
great. I try not to miss the TAGA conference. It is one of the only conferences where you can learn
about new technologies in all parts of the printing industry. You will hear
from academics and professionals alike, without the sales hype of most
conferences. The next TAGA conference will be held in Fort Worth in March 2014. I hope to see
you there. If you do, then buy me a beer.
Do you want a second opinion about
TAGA? Here is what Sandy Hubbard had to
say about this year’s TAGA conference: TAGA,
You’re It!
What
is going on in the industry to try to fix the problem of inter-instrument
agreement?
Idealliance (in the US) and Fogra (in
Europe) have assembled groups of experts from all the graphic arts
spectrophotometer manufacturers to work on the issue. I was cautiously
optimistic at the start, and am cautiously a bit more optimistic after
attending the first meeting. I attended the US meeting. I was impressed that everyone
in the room was sincere in their desire to find agreement on how to find
agreement. I didn’t hear any finger-pointing or blustering about why one company
provided the one true measurement system and everyone else is wrong.
Of course, we spent the first six hours
complaining about how spectrophotometer owners use their delicate instruments to
hammer in nails and drop ketchup and fired onions into the optics. But we got
past that soon enough, and started complaining about cops who give out speeding
tickets and about politicians.
The ISO standards group that covers color
measurement for the graphics arts (TC130) has also made some recent changes
that will eventually improve inter-instrument agreement. See the heading “What is
being courageously changed?” below.
Why
do the instruments disagree?
Speaking of finger-pointing, why can’t the
various spectrophotometer designers just build their instruments the correct
way? Then they all would agree, right?
That all makes sense, but the difficulty
is that there are many different ways to build a spectrophotometer “correctly”.
The different correct ways may not give the same results. You may think that I
am just copping out, since I represent an instrument manufacturer, but listen
up. I have a parable for you.
Tevye (from Fiddler on the Roof) is in the
marketplace talking politics. One person speaks of the great new world that the
tsar is bringing. Tevye looks at this man and says, “You’re right”. The second
man disagrees, and says that the tsar is bringing ruin to the country. Tevye looks
at this man and says, “You’re right”. The third man is beside himself. “Tevye,
how can they both be right!?” Tevye ponders a moment, and then says, “You’re also right!” If two instruments disagree, it is totally possible for both of them to
be right.
Here is the short list of things that can
go cause disagreement.
Fluorescence – Recently, optical brighteners have
become ubiquitous. They are added to paper to make the paper appear brighter.
The trouble is, the amount of whitening that happens depends on the amount of
UV content in the light in the spectro. If two spectros emit different amounts
of UV, they will excite the optical brighteners by different amounts, and the
spectros will disagree.
I think we should point the finger at the
darn paper companies who put the optical brighteners in the paper. Or maybe at
the designers who like the paper that is whiter.
White
level – Believe it or
not, the various standards labs can’t quite agree on what “white” is. If you
send a white tile to be measured by several of them, you may see a 1 ΔE of
difference in L*. This is a clear case of “you’re right”. I think we should
point the finger at the standards labs.
Lateral
diffusion – A spectro
illuminates a small area of the sample, and collects the light reflected by
another small area. One would think that these should be the same size, this
would cause a problem. If the sample has some translucence (it’s cloudy), then light
will scatter outside the aperture. The smaller the aperture, the bigger the
relative effect. Now get this… a few of the BCRA II tiles have bunches of
lateral diffusion.
I think we should point the finger at the print
buyers who force us to measure colorbar patches that are 1.5 mm tall. Or the
paper companies. You would think they could make each sheet on a paper roll
just a millimeter or two longer to make room for decent sized color patches.
By the way, someone wrote a paper about lateral diffusion a long time ago. The guy’s probably in a
nursing home in Milwaukee singing Those
Were the Days.
Linearity – One would expect that a decent engineer
would be able to design a spectro that is linear. Of course, there is a little
problem that detectors are nonlinear when they come close to saturation. By the
way, that’s where the detector has its best noise immunity.
And there is the little problem that
transistors and all that stuff like to be nonlinear down where the signal levels
are smallest. Oh, and the issue that it is hard to devise a set of samples to
calibrate nonlinearity with. One is left with assembling a series of gray
samples with different reflectance values, and having a national standards lab
measure them.
Did I mention that the national standards
labs are ghastly expensive when you ask them to measure something? I think I’ll open my own national standards
lab. Or just blame the stupid laws of physics.
Black
level – Theoretically two
spectrophotometers could disagree on the reflectance of a perfectly black
object. I think they probably do disagree just a tiny bit, but I don’t have
enough data to verify this.
Wavelength
shift – Just like two
spectros might not agree on what 50% reflectance is, they might disagree on
where 500 nm is. Thankfully, we have physical standards, such as mercury-argon
lamps, that we can use that will emit at certain precise wavelengths. But… it
would be nice if there were a few more emissions lines. Stupid laws of physics,
anyway.
Spectral
bandpass – Don’t even get
me started on this one!
Goniophotometry – This one is a favorite of mine, simply
because of all the cool words. Besides “goniophotometry”, there is “indicatrix”,
and the acronym “BDRF”.
The basic idea is that the amount of light
reflected from a surface depends on the angle that the light hits and upon the
angle that it is viewed. In practical terms, a 0/45 spectrophotometer that
collects all the light emitted between 40 and 50 degrees might not agree with
one that collects all the light emitted between 43 and 47 degrees.
When you see me at TAGA 2014, buy me a
beer and we can talk about the birds and the goniophotometers.
By the way, someone once wrote a TAGA paper
about the goniophotometry of printing ink. The writing in the paper shows many of
the early warning signs for dementia.
So many possible issues! In the words of
Rabbi Seymour Goldstein, “Oy vey, mein kapella! This is so meshuga, it gives me
such tsuris!”
Also in the words of Rabbi Seymour,
“Grant me the courage to change those
things that I can change,
The serenity to accept those that cannot
be changed,
The wisdom to know the difference,
And a little duct tape and shoe polish to
fill in the gaps.”
What
is being courageously changed?
There are a number of things that have
changed or are changing to try to improve inter-instrument agreement.
One concrete decision that came out of the
Idealliance meeting (in the US) was that we all agreed to trace our “white”
back to the same national standards lab. We decided to go with the US-based standards
lab NIST. I don’t know if this was discussed at the European meeting, but
hopefully we can all decide on a single national lab.
The ISO standards group that works in the
graphic arts color measurement arena (TC 130) has recently made some important
changes that have already, or will, improve inter-instrument agreement.
Most notable is the “M0 / M1 change”,
which addresses the optical brightener issue. Spectrophotometers with the M1
designation will agree in UV content so that optically brightened papers will
be measured the same. For more information, have a look at some other
wonderfully written articles: “The problem with optical brighteners”, and “New lighting conditions”.
Another change (one that is interesting
only to the print uber-geeks) was made to ISO 13655, having to do with wavelength
bandwidth. Interested in more information? Buy me a couple of beers and we’ll
talk. Buy me a few more beers after that, and we can go sing Don’t Stop Believin’ at a karaoke bar.
These changes to the standards are
essential, but they take a while to filter down. The really odd thing is that a
change to the standards rarely changes existing equipment, you know? For some
reason, I need to send my old spectro in to get it updated, or maybe buy a new
one, for gosh sakes.
What
serenity is required?
Now it is the print buyer’s turn to get
the pointy finger.
There is a rule of thumb in the
statistical processing world. The accuracy of an instrument is only allowed to
take up 30% of a tolerance window. Thus, if the tolerance window for a certain
printed color is 2.0 ΔE, then my instrument must be accurate to within 0.6 ΔE.
If you go beyond that, it just makes things worse.
The reality is unfortunately that two
instruments of different make and model will likely not agree to within 0.6 ΔE.
Setting tolerances closer than that will inevitably just make people want to
drink more beer.
Anyone
have a spare cup of wisdom?
Wisdom… I wish I was smart enough to know
what to say about this.
Hopefully my TAGA paper has contributed to
the collective wisdom. My message was to know when a courageous attempt to make
two instruments agree should have been tempered with a bit of serenity.
Duct
tape and shoe polish
Another discussion item in the US inter-instrument
agreement consortium was the development of a test strip with a selection of
colored patches that could be measured by different instruments. After
measurement, something could be done with the readings.
We discussed several options about what could be done:
1) The measurements taken with a single
instrument could be compared over time to verify that the instrument has not
changed,
2) One instrument could be compared with
another to see if they measure substantially the same,
3) An instrument could be compared with
measurements from a national standards lab (or perhaps a secondary lab) to
assess the accuracy of the spectrophotometer, or
4) The set of samples could be used to
standardize one instrument to another -- that is to provide a way of converting
one instrument so as to match another.
The four potential goals are successively
more complicated and expensive. In particular, obtaining measurements from a
national standards lab would cost thousands of dollars, so it is not cost
effective for most users. The fourth option may require a color scientist/math
guy, so might only be an option for the spectrophotometer companies that can
afford to hire color scientist/math guys.
Colligation
I am not done playing with duct tape. I am in the process of cogitating on colligation. The ancient Greek, Ptolemy developed a set of equations that could be used to predict the positions of the planets at any time. The equations were based on a lot of wrong assumptions like the Earth is in the center, and all the rest of the celestial bodies move in circles that revolved around circles. The model worked, at least to an extent.
I am not done playing with duct tape. I am in the process of cogitating on colligation. The ancient Greek, Ptolemy developed a set of equations that could be used to predict the positions of the planets at any time. The equations were based on a lot of wrong assumptions like the Earth is in the center, and all the rest of the celestial bodies move in circles that revolved around circles. The model worked, at least to an extent.
A millennium or so later, Copernicus decided
that the Sun belonged at the center. (I hope everyone celebrated his birthday yesterday?) Then Kepler came along and decided that
ellipses made the whole thing simpler, and then Newton provided the big
colligation. The inverse square law of gravity was the grand unifying theory
that explained the whole enchilada.
My goal is to colligate. I have gone
through the Ptolemaic thing in my TAGA paper, and shown that things go subtly
awry when you have the wrong starting assumptions. I have done the Keplerian
thing and did regression with (what might be) the right assumptions. It’s time
to roll up the sleeves and do the Newtonian thing.
I plan to collect a bunch of spectros,
enough to represent the range of physical differences that we see out in the
field. I plan to collect a bunch of sample sets. I have the ones that I used
for the TAGA paper, but I will gather more. Some new samples are already on a
plane, headed to my office.
Then I plan to systematically go through
specifically designed sample sets to try to ascertain what differences exist
between the real world instruments. For example, I will measure a set of white
and gray samples with varying levels of gloss to see if there is a
goniophotometric difference – a difference in gloss rejection. I will measure a
series of white to gray samples to ascertain whether the typical instruments in
the field differ in their view of linearity.
And so on.
I am not sure where the colligation will
lead. It could be that it will show that courage will be required, and there
will be another round of “gosh darn it, we need to tighten up specs on
something or other”. Or it could be that more duct tape is in order, and a
simple standardization, carefully applied, can make an appreciable improvement.
I am still cogitating on this inter-instrument
colligation. I welcome any suggestions or offers of help. At least, up to the
point where I spend 23 hours and 56 minutes a day responding to emails.
