From: "Ron Bean", INTERNET:rbean@execpc.com
Date: Fri, Dec 31, 1999, 7:16 PM
RE: Color Temperature and Metamerism
Applied Color Theory - https://www.ledet.com/margulis
I've been reading two of the books Dan recommended a while back:
"Principles of Color Reproduction" by Yule, and "Principles of
Color Technology" by Billmeyer & Saltzman. They're both excellent
(BTW you can skip the math and still get a lot out of them).
I learned a couple of things that I haven't seen elsewhere, and I
thought I'd mention them here.
Billmeyer & Saltzman's discussion of metamerism is especially
good-- they talk about "invariant" vs "conditional" (metameric)
matches between two colors, as a result of the spectral content
of the light vs the spectral reflectance of the pigments. In the
past I've heard that metamers are colors that match under 5000K
lighting but not under some other light source, but they talk
about it in terms of *any two light sources*.
I had gotten the idea that 5000K lighting produces more
"accurate" color, but I think they would prefer to say it's more
*standard* rather than more accurate. They mention the idea of a
"color rendering index" as being *relative to some standard*.
5000K is more "accurate" in the sense that it's the most common
standard. But you could pick some other standard, if you were
designing something to be viewed under a specific light source.
In fact, I recently saw a color swatch form used by a silkscreen
printer that includes a space to specify which light source is to
be used for each job (including "customer supplied light source").
I've seen books that implied (or even stated directly) that all
light sources have a color temperature, but that's clearly not
the case. Color temperature refers specifically to black body
radiation, and therefore only applies to light sources that
behave like black bodies at some temperature. Daylight,
incandescent light, and arc lights are reasonable (though not
exact) approximations of black body radiation, but most
garden-variety flourescents and gas-discharge lamps (ie, most
commercial lighting) are not, unless they've been specifically
engineered to behave that way.
Flourescents can have wierd peaks and dips in their spectral
graphs (in addition to the mercury lines), depending on the
phosphors they use. They show the spectral power graph of a type
of "5000K" flourescent called a "prime color" lamp that has
several sharp discontinuities in it, which is not what you'd see
with a true 5000K light source. And they point out that this
spectral content must be accounted for when making calculations
to be used with this light source.
Yule also has a comment about cheap flourescents with oddball
spectral contents:
"Unfortunately, the old type of flourescent tube is still used
where merchandise is displayed, so that a printed package
originally evaluated under the standard light source looks very
different to the consumer. In particular, the reds appear dull
and brownish. But if the package were printed to look
satisfactory in flourescent light, its color would appear
exaggerated in daylight or tungsten light. It is to be hoped that
the old type of flourescent tube will eventually become obsolete,
for this appears to be the only satisfactory way out of the
difficulty."
That was in 1967, and they haven't gone away yet...

-------------------------------------------------------------------------------
From: Chris Murphy, INTERNET:lists@colorremedies.com
Date: Sat, Jan 1, 2000, 2:02 PM
RE: Re: Color Temperature and Metamerism> In the
>past I've heard that metamers are colors that match under 5000K
>lighting but not under some other light source, but they talk
>about it in terms of *any two light sources*.
Metamers are color samples that match under one kind of illuminant but
not under another because their spectral properties are different. Under
one kind of lighting the differences may cancel each other out, or just
not be affected enough for a visible difference to occur. In other words,
you can have metamers that don't match under 5000K but do match under
tungsten for example.>I had gotten the idea that 5000K lighting produces more
>"accurate" color, but I think they would prefer to say it's more
>*standard* rather than more accurate.
Yes, but I would add to this. 5000K is rather vague. The standard
illuminant for graphic arts is actually D50, not 5000K. They aren't the
same. D50 is an illuminant, 5000K is not. D50 has a very specific
"spectral power distribution" which is a fancy way of describing what
amounts of each visible wavelength that light source outputs.
Some problems with ANSI's "standard lighting source" or whatever they
refer to it as, and the ICC specification is that the luminosity is not
specified. The amount of light absolutely affects perception of a color.
So it's possible to specify D50 in two locations yet get different
results because of luminosity.
There is also a problem with CIE colorimetry. That is, CIE is the
International Commission on Illumination and among many other things,
they define standard illuminants such as D50, D65, A, B, F2, F7, F8
(many, many others). The problem is that they never intended CIE
colorimetry to apply to cross media applications. So D50 for a monitor
isn't the same as D50 from a viewing booth on paper.> They mention the idea of a
>"color rendering index" as being *relative to some standard*.
The standard illuminant to which the light source is attempting to
simulate. What is the standard illuminant 5000K fluorescents are trying
to simulate? I don't know but I find it difficult to believe it's D50
given the CRI numbers they advertise. Daylight fluorescents (such as
Phillips TL-90 and GE's Chroma 50) both are more similar to the F8
illuminant than they are D50. They really aren't the best solution for
standard lighting (D50 in the U.S.), and in some application (such as
photography) they can be problematic.>5000K is more "accurate" in the sense that it's the most common
>standard. But you could pick some other standard, if you were
>designing something to be viewed under a specific light source.
Sure. One of the benefits of spectral measurement data only profiles
(i.e. the profile contains only measured data, not some profile
manufacturer's interpretation of them) is that the appearance of specific
RGB, CMYK (or whatever primary system is being used) file can be
predicted under any of the standard illuminants (or for which a custom
illuminants spectral power distribution has been defined).
So for Women's Day magazine, the publisher's marketing people might
conclude most readers of the magazine view under warm halogen lighting.
They could then create separations that account for that lighting
condition. Readers now see lipstick ads with closer matches to the actual
color of the product. This can be a big deal.
A large format printer might decide to make separations to compensate for
use of metal halide or mercury vapor light sources used in trade shows.
It is a common complaint in the trade show industry that what they saw at
the printer to approve the booth signs looks different in the trade room.
Too late once it gets there.> but most
>garden-variety flourescents and gas-discharge lamps (ie, most
>commercial lighting) are not, unless they've been specifically
>engineered to behave that way.
Fluorescent bulbs contain mercury. Excited mercury is how the phosphor
coating fluoresces and emits light - it's the actual inner surface
coating where the bulk of the light comes from. However, the mercury
itself gives fluorescent bulbs very spiky distribution; that is,
fluorescent lights give off massive emission at some wavelengths of the
visible spectrum.
For analog proof to press sheet, since they are made of ink pigments,
they are not metamers so metamerism doesn't occur even though 5000K
fluorescent isn't really very close to D50. But with the increase of
proofs that don't use actual ink pigments, this will become an increasing
problem because inkjet proofs (for example) and press sheets ARE
metamers. A D50 light source will be more crucial in cases where proofs
not using ink pigments will be used as contract proofs.>That was in 1967, and they haven't gone away yet...
Of course not. They're making too much money; especially on "full
spectrum" bulbs. Guess what? A candle is also a full spectrum light
source, so that's one of those B.S. marketing terms I was referring to
before. I'm waiting for everyone to have a hissy fit over this too. It
would be very worthwhile - get rid of your fluorescents as you have the
opportunity. Besides, the mercury content makes them unfriendly to the
environment. Some states now have specific laws regarding their disposal
because of this.Chris Murphy-------------------------------------------------------------------------------
From: "Ron Bean", INTERNET:rbean@execpc.com
Date: Tue, Jan 4, 2000, 11:32 PM
RE: Re: Color Temperature and MetamerismChris Murphy <lists@colorremedies.com> writes:
>5000K is rather vague. The standard
>illuminant for graphic arts is actually D50, not 5000K. They aren't the
>same. D50 is an illuminant, 5000K is not. D50 has a very specific
>"spectral power distribution" which is a fancy way of describing what
>amounts of each visible wavelength that light source outputs.
A "black body" heated to 5000K should also have a very specific
spectral power distribution (if you don't believe me, ask a
physicist). If that's not the way it's used in the graphic arts,
then I think they're mis-using the terminology. BTW the same math
describes radiant heating; it's just in a different part of the
spectrum.
>What is the standard illuminant 5000K fluorescents are trying
>to simulate? I don't know but I find it difficult to believe it's D50
>given the CRI numbers they advertise.
I think the problem is trying to construct a lamp that
behaves like a theoretical ideal, instead of the other way
around. As a practical matter, it would have made more sense to
define the standard in terms of the behavior of a specific type
of lamp. If someone used a different type of lamp, that would be
non-standard by definition.
That way you'd *know* that everyone was using the same thing,
instead of having everyone approach the ideal from different
directions, and in ways that don't match each other.
In other words, it would be a real-world standard, instead of a
theoretical standard that everyone approaches but nobody actually
meets.
Maybe that's what happens when standards are set by theoreticians
rather than engineers...-------------------------------------------------------------------------------
From: Chris Murphy, INTERNET:lists@colorremedies.com
Date: Wed, Jan 5, 2000, 7:29 PM
RE: Re: Color Temperature and Metamerism>A "black body" heated to 5000K should also have a very specific
>spectral power distribution
Of course it does. You're saying a black body at 5000K. 5000K by itself
isn't very helpful and it's not the same thing as D50 or daylight 5000K.>I think the problem is trying to construct a lamp that
>behaves like a theoretical ideal, instead of the other way
>around.
Illuminant E is the theoretical ideal. D50 isn't a theoretical ideal, it
just happens to be the standard. The problem is fluorescent technology
isn't suitable for simulating an illuminant with a smooth spectral power
distribution like D50. Halogen can do this with a special filament and
reflector, and such a bulb exists:
www.soluxtli.com>Maybe that's what happens when standards are set by theoreticians
>rather than engineers...
I don't know the history of where D50 came to be or why ANSI selected it
as the standard. If I had to guess, it would be alone the lines of the
human visual system being well adapted to sunlight more than an
artificial light source, and they basically had to pick a standard
illuminant that didn't have a color cast (like most fluorescent, all
incandescent, most tungsten, halogen, and all metal halid, mercury vapor,
and sodium I forget what based lights. Those are poor lighting choices.
Incandescent is arguably the most common but it's extremely red and has
very little purple/blue emission. So basing the standard on a bulb
instead of a bulb on a standard I think is backward because with the
exception of the SoLux bulb I mention above, there isn't one suitable in
terms of consistency (between bulbs and as the bulb ages), and reasonably
neutral and a defined illuminant.
In order to get a true D65 you need to include a certain amount of UV
radiation whereas D50 is very weak in UV. Getting UV emission in a light
source that is ALSO well behaved (smooth spectral power distribution
instead of spiky) is exceptionally difficult and I'm not aware of it
being done or worked on. If you want UV, you usually end up with some
fluorescent bulb designed for that purpose and it also has very spiky
"other" emission other than UV which those buying the bulb don't care
about. In the graphic arts, we would care about that behavior.
The point of bringing up this info about D65 is that it represents
something not attainable at this point for a well behaved artificial
lighting source. D50 is something that is attainable to within a very
decent tolerance of true D50, and it isn't fluorescent technology.Chris Murphy-------------------------------------------------------------------------------
From: "Ron Bean", INTERNET:rbean@execpc.com
Date: Wed, Jan 5, 2000, 10:32 PM
RE: Re: Color Temperature and MetamerismChris Murphy <lists@colorremedies.com> writes:
>You're saying a black body at 5000K. 5000K by itself
>isn't very helpful and it's not the same thing as D50 or daylight 5000K.
Which is why it's not helpful to assign a "color temperature" to
something that is not black body radiation. It's sloppy
terminology, and it confuses the issue by collapsing it into a
single term that doesn't describe what's happening.
It would be much better to skip that and talk about the spectral
content of the actual light source (my original point was that
various textbooks describe this stuff in a way that is
misleading).
>>I think the problem is trying to construct a lamp that
>>behaves like a theoretical ideal, instead of the other way
>>around.
>D50 isn't a theoretical ideal, it just happens to be the
>standard.
So, how *exactly* do you build a D50 light source, and why isn't
it defined in terms of something everyone builds the same way?
>...they basically had to pick a standard
>illuminant that didn't have a color cast (like most fluorescent, all
>incandescent, most tungsten, halogen, and all metal halid, mercury vapor,
>and sodium I forget what based lights. Those are poor lighting choices.
IMHO the best lighting choice would be the one that the product
will actually be viewed under (if you know what that is). Your
trade show example is a good one; I would argue that D50 is a
poor choice in that situation.
>Incandescent is arguably the most common...
I'm not so sure about that these days. I have about half
incandescents and half flourescents at home. Commercial buildings
are overwhelmingly flourescent.
The real problem is that incandescents have too much red, and
flourescents usually have too little, so there's no way a single
standard can accomodate them both.
>So basing the standard on a bulb
>instead of a bulb on a standard I think is backward because with the
>exception of the SoLux bulb I mention above, there isn't one suitable in
>terms of consistency (between bulbs and as the bulb ages), and reasonably
>neutral and a defined illuminant.
The problem is that you need a standard that you can hold two
objects under to see if they match. That means it has to be a
physical device. Yes, it will age, and you have to deal with
that somehow. But first you have to agree on *how* to build it,
so you don't have radically different light sources claiming to
be standard. If you can't do that, then it's unsolvable.
In fact, if everyone used identical so-called "5000K" flourescents,
then that would be a defacto standard, even though it's technically
wrong. If they all use *different* so-called "5000K" flourescents,
then it's hardly worth having a standard in the first place.
You could still have other standards for other purposes, but when
the question is "do they match", it has to be under an existing
light source (however imperfect), and one that the people who buy
the lightbulbs can agree on.
>D50 is something that is attainable to within a very
>decent tolerance of true D50, and it isn't fluorescent technology.
So if it were defined as something other than flourescent, then
flourescents would clearly be wrong, by definition, and there
would be no way for anyone to claim otherwise.

-------------------------------------------------------------------------------
From: Chris Murphy, INTERNET:lists@colorremedies.com
Date: Thu, Jan 6, 2000, 1:44 AM
RE: Re: Color Temperature and Metamerism>Which is why it's not helpful to assign a "color temperature" to
>something that is not black body radiation. It's sloppy
>terminology,
Yes exactly.>It would be much better to skip that and talk about the spectral
>content of the actual light source
Yes exactly. That is the definition of an illuminant. An illuminant is a
defined spectral power distribution. So an incandescent lightbulb in my
home is a light source, but it corresponds to illuminant A as defined by
the CIE. Illuminant A has a specific spectral power distribution.

>So, how *exactly* do you build a D50 light source, and why isn't
>it defined in terms of something everyone builds the same way?
Spectral power distribution curves do not define HOW to make a light
source produce a given spectral power distribution :) they simply define
the AMOUNTS (usualy in % reflectance or normalized output) of each
wavelength in the visible spectrum, usually in 10nm increments.
To build such an artificial light source takes a lot of R&D, lots of
trial and error, and a good spectroradiometer to ascertain the spectral
power distribution of your light source. Once you determine the spectral
power distribution for that light source, you can call it an illuminant
(light source + spectral power distribution curve = illuminant).
Once you do this, you can patent it, and someone has done this and they
have patented it.

>IMHO the best lighting choice would be the one that the product
>will actually be viewed under (if you know what that is). Your
>trade show example is a good one; I would argue that D50 is a
>poor choice in that situation.
I disagree. I think the issue is that it would be difficult to get enough
"D50" type lights in such a room to provide adequate lighting. D50 is the
single standard. People don't follow the single illuminant standard for
lighting conditions so I don't see how having more than one is going to
help matter.
Despite that, I'm not saying I do not support multiple illuminant
lighting condition standards. For trade shows there would likely be three
standards, mercury (or mercury blended), metal halide, and one other I'm
not able to remember at the moment. Anyway, you could have to make custom
separations for each of these lighting conditions in order to get the
color you are expecting.
I guarantee you that you're going to have a problemm making separations
that compensate for non-standard (non-D50) lighting without using some
kind of color management system to assist you. All proofing systems are
based on D50, or D65, so conventional proofing systems aren't going to
even help you. Plus, the service provider of these products will need to
invest in multiple proofing "booths" or areas in order to provide
suitable lighting for each standard you decide on.>I'm not so sure about that these days. I have about half
>incandescents and half flourescents at home. Commercial buildings
>are overwhelmingly flourescent.
I have compact fluorescent outside, and primarily incandescent indoors
with a couple of halogen lights. Even if it turns out the situation is
mixed, you realize that for distributed advertising, you really can't
target a standard lighting condition. Even within fluorescent lighting
there are more than 12 different standard illuminants defined by the CIE,
probably more independent (non-standard) fluorescent based illuminants
exist as well.>The real problem is that incandescents have too much red, and
>flourescents usually have too little, so there's no way a single
>standard can accomodate them both.
Yes. Fluorescents also have massive (MASSIVE) spikes in orange-yellow and
another at yellow-green. I'm sure there's another but I don't have any
spectral power distribution curves for any fluorescent bulbs since I'm at
Macworld at the moment.

>The problem is that you need a standard that you can hold two
>objects under to see if they match.
If it's an analog proof and a press sheet, both of which contain actual
press ink pigments (i.e. they are NOT metamers) they will look the same
and MATCH under all lighting conditions. They won't look correct (per the
standard), but they will look wrong in the same way.
With the increasing use of proofs that do not use actual press ink
pigments, they are metamers. The farther you deviate from D50 which is
what separations are based on (Photoshop separation tables, curves, and
ICC Profiles all assume D50 lighting), the bigger the potential different
between press sheet and proof under non-D50 lighting conditions.
>Yes, it will age, and you have to deal with
>that somehow.
The SoLux bulbs essentially die before their color temperature (spectra
power distribution) is significantly atered (like maybe 30 to 50 K).
Fluorescents can last a very long time well after they are no longer
qualified for operation, as much as 500K. Most wait until they flicker
before they replace their lights.> But first you have to agree on *how* to build it,
>so you don't have radically different light sources claiming to
>be standard. If you can't do that, then it's unsolvable.
They don't claim to be the standard. No light claims to have a CRI of
100. SoLux is the closest it gets at 99+.>In fact, if everyone used identical so-called "5000K" flourescents,
>then that would be a defacto standard, even though it's technically
>wrong. If they all use *different* so-called "5000K" flourescents,
>then it's hardly worth having a standard in the first place.
Then the companies aren't differentiating themselves in their own minds.
Why don't printers follow standards exactly and force their presses to
behave exacly alike?

Chris Murphy-------------------------------------------------------------------------------
From: "Ron Bean", INTERNET:rbean@execpc.com
Date: Sun, Jan 9, 2000, 1:10 PM
RE: Re: Color Temperature and Metamerism
Chris Murphy <lists@colorremedies.com> writes:
[lotsa stuff I basically agree with, just a few comments...]
>To build such an artificial light source takes a lot of R&D, lots of
>trial and error, and a good spectroradiometer to ascertain the spectral
>power distribution of your light source.
It would take a lot less trial and error if the standard were
based on an existing light source.
>People don't follow the single illuminant standard for
>lighting conditions so I don't see how having more than one is going to
>help matter.
They don't follow the standard because it was too hard to make a
lightbulb that follows it closely. Now that the SoLux bulbs are
available, it's a lot easier.
Unfortunately, people are used to using the flourescents, and
they're not likely to change unless they become dissatisfied with
them for some reason.
>Despite that, I'm not saying I do not support multiple illuminant
>lighting condition standards...
>I guarantee you that you're going to have a problemm making separations
>that compensate for non-standard (non-D50) lighting without using some
>kind of color management system to assist you.
Well, we have those now...
>Plus, the service provider of these products will need to
>invest in multiple proofing "booths" or areas in order to provide
>suitable lighting for each standard you decide on.
Probably one booth with several light sources.
[BTW Billmeyer & Saltzman claim you should specify the entire
booth, not just the lightbulbs. That sounds like overkill to me,
but they know more about color than I do.]
>Even if it turns out the situation is
>mixed, you realize that for distributed advertising, you really can't
>target a standard lighting condition.
Yeah, that's the paradox. Even if you get a perfect color match,
it's never going to be viewed under those lighting conditions
again.
>With the increasing use of proofs that do not use actual press ink
>pigments, they are metamers. The farther you deviate from D50 which is
>what separations are based on (Photoshop separation tables, curves, and
>ICC Profiles all assume D50 lighting), the bigger the potential different
>between press sheet and proof under non-D50 lighting conditions.
Which is why the separations should be based on whatever they're
using in the pressroom. If that's the SoLux bulbs, no problem.
If it's the usual flourescents, most likely the printer will
blame the designer for any problems.
-------------------------------------------------------------------------------
From: Chris Murphy, INTERNET:lists@colorremedies.com
Date: Sun, Jan 9, 2000, 7:41 PM
RE: Re: Color Temperature and Metamerism>It would take a lot less trial and error if the standard were
>based on an existing light source.
I'd like to hear what existing light source that is NOT PATENTED is also
suitable as a proofing light source.>They don't follow the standard because it was too hard to make a
>lightbulb that follows it closely. Now that the SoLux bulbs are
>available, it's a lot easier.
Before SoLux there was 5000K fluorescent; even when those were available
people were either uninformed or ignored the standard lighting condition
and still complained about color, and could not believe lighting could
make that much difference.
The point of this is that if one standard is impossible for a majority of
people to follow, multiple standards will be also.Chris Murphy-------------------------------------------------------------------------------
From: Chris Murphy, INTERNET:lists@colorremedies.com
Date: Mon, Jan 10, 2000, 1:50 AM
RE: Re: Color Temperature and Metamerism>If it's patented, you license it.
I would not support making a patented technology the standard. It would
be unfair for ANSI to say, "this company gets to win the lottery.">Since this would generate quite a bit of business for the patent
>holder, they'd probably give you a pretty good rate (and patents
>do expire).
Why would they give a good rate? If ANSI says, "Joe Blow's technology is
the standard, anyone who wants to make it has to go to them," Joe Blow
has every incentive to ramp up production and maintain a monopology so no
one can compete. That's how markets work. The only time you get a good
rate is when there is competition.>It's not *that* unusual for standards to specify patented
>technology. If you want to build a standard 56K modem, you have
>to license technologies from several companies.
yes and remember what it was like when company A couldn't use company's
b's technology or vice versa. It basically took 2 years for them to
figure out they needed to co-develop and patent the v.90. There are v.90
modem choices today low cost because more than one company has licensing
authority for v.90. Your lighting example is nothing like this.> The standard way
>of measuring octane for gasoline uses a special test engine made
>by a particular company.
There are other formulas for computing octane. That's competition. They
do not generate the same results, but they still compete. If this one
company abuses their patent, the state or federal agencies could mandate
their own standard for sake of consistent terminology for consumers.

>So what's your solution for making people follow the standard?
Send Christmas cards to all their customers telling them that their
vendor doesn't use standard lighting. I don't have all the answers.
Ultimately it comes down to education, so I'm doing what I can to educate
people on the problem. There are limits to what any one individual can do.

Chris Murphy

-------------------------------------------------------------------------------
From: "Ron Bean", INTERNET:rbean@execpc.com
Date: Sun, Jan 9, 2000, 10:31 PM
RE: Re: Color Temperature and MetamerismChris Murphy <lists@colorremedies.com> writes:
>>It would take a lot less trial and error if the standard were
>>based on an existing light source.
>
>I'd like to hear what existing light source that is NOT PATENTED is also
>suitable as a proofing light source.
If it's patented, you license it.
Since this would generate quite a bit of business for the patent
holder, they'd probably give you a pretty good rate (and patents
do expire).
It's not *that* unusual for standards to specify patented
technology. If you want to build a standard 56K modem, you have
to license technologies from several companies. The standard way
of measuring octane for gasoline uses a special test engine made
by a particular company.
>The point of this is that if one standard is impossible for a majority of
>people to follow, multiple standards will be also.
So what's your solution for making people follow the standard?
I don't think there's a technological solution to that one.
But I'd like to think there might be a non-technological one.

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