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Chromaticity neutrality of WB targets

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Doug Kerr

Guest
The topic of this essay is something that probably has almost no impact on our practical concerns, but I thought it would make good filler for this initial "edition" of the Open Photography Bugle.

It is common, and entirely appropriate, to describe the reflective color of an exposure metering/white balance calibration target in terms of its coordinates in the CIE L*a*b ("CIELAB") color model. (That model was in fact introduced to describe reflective color; it has been "hijacked" to describe the color of light or the color implied for a pixel in a digital image.)

Manufacturers of WB targets, in particular, point with pride to small absolute values of a* and b* for their products as indicative of a very small departure from ideal "chrominance neutrality", and again this is entirely appropriate.

We do, however, get into subtle difficulty when we try to quantify the degree of departure from chromaticity neutrality in terms of the values of the a* and b* coordinates. For example, we might hear, "it would be desirable for a* and b* to not be outside the range ±1.5".

But that doesn't quite describe a range of chromaticity. The reason is that the a*b* plane is essentially a plane of chrominance, not chromaticity.

I won't bore you further here with a discussion of the distinction. Those who are not certain about it, and are interested, might like to read my discussion of it in my tutorial article, "Chromaticity and Chrominance in Color Definition", available here:

http://doug.kerr.home.att.net/pumpkin/index.htm#ChromatcityChrominance

In any event, for a given chromaticity, the chrominance varies with lightness. That is, two colors with the same a* and b* coordinate values, but having different values of L* (lightness), will not have the same chrominance (values of a* and b*).

For example, let's consider a hypothetical WB target whose reflective color measures 75, +2.0, +2.0 under the L*a*b model. That color is not quite chromaticity-neutral. (Its reflectance would be 48.3%, incidentally.)

Now let's look at another target of less lightness - an L* of 42.4 (reflectance of 12.8%, often considered desirable for a target for incident light metering), but the same chromaticity. Its L*a*b specification would be 42.4, +1.3, +1.3.

So if for some reason we had concluded that we needed a target whose a* and b* coordinates were not beyond, for example, ±1.5, then the second target would satisfy that, while the first one would not, despite the fact that they both had the same chromaticity - they were both just as "close to chromaticity neutral".

[This is not at all to suggest that both would be equally attractive as WB calibration targets, owing to the reflectance matter discussed elsewhere in this forum.]

Again, I don't at all claim that this is of any real importance in judging the specifications for WB calibration targets. And I don't mean in any way to demean the presentation by target manufacturers of the a* and b* "bogeys" for their products. It is probably the most practical way to describe what they have - it would be difficult for the average user to interpret a chromaticity-neutrality specification in terms of the CIE x and y coordinates, for example.

But I think it is important to realize that the a* and b* coordinates do not precisely quantitatively describe a chromaticity, nor a departure from "chromaticity neutrality".
 

Michael Tapes

OPF Administrator/Moderator
The topic of this essay is something that probably has almost no impact on our practical concerns, but I thought it would make good filler for this initial "edition" of the Open Photography Bugle.
Well, we certainly agree that this has no impact on our practical concerns of presenting the neutrality of White balance reference products.

Firstly I know of no product, other than WhiBal, that prints specifications in regards to the neutrality of their WB reference card, and also measures every card to certify those specifications on each WhiBal shipped. So is this bad????

I think where your practical premise dissolves, is the reality that the specifications for WhiBal, are based on a specific set of measurement criteria, with an L* spec stated. So simply stated, given that I believe that we must keep the a* and b* channels in Lab mode within +/-1.0 at a L* value of 70-80, don't the subtle differences between chrominance and chromaticity disappear, based on the specified L* value? And if they do not, are not they of no practical concern?

The measurement criteria for the current G6 version of the WhiBal Gray Reference card are:
  • Measured by a Gretag Macbeth SpectroEye Spectrophotometer
  • Setting of D50 light source and 2 degree observer angle
  • L* = >74
  • a* < +/- 0.5
  • b* < +/- 0.5
So speaking for the WhiBal, I do not think there is any doubt of the specificity of what is being specified in terms of extreme neutrality.

Your knowledge of the science on this certainly exceeds mine, but like you opened with, I think given the representations that are made for WhiBal, I see no conflict or issue with the specifications as presented. Perhaps for some other products. I cannot speak for those. I know that ExpoDisc used to measure each "disc", but I cannot speak for them.

I welcome your further thoughts, and of course statements from other companies that might have thoughts on the topic.

BTW...Thanks for getting the forum going to a rip roaring start :>)

.....and I do enjoy learning more of the background science if it leads to better photographic techniques, products, or results, but I think this case is just some interesting background knowledge with no practical application to the subject at hand.

Thanks!

...and of course JHMO YMMV
 
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Doug Kerr

Guest
Michael Tapes said:
Well, we certainly agree that this has no impact on our practical concerns of presenting the neutrality of White balance reference products.
Ah, yes.

Firstly I know of no product, other than WhiBal, that prints specifications in regards to the neutrality of their WB reference card, and also measures every card to certify those specifications on each WhiBal shipped. So is this bad????
I certainly wasn't talking about WhiBal. I don't understand the question.

I think where your practical premise dissolves, is the reality that the specifications for WhiBal, are based on a specific set of measurement criteria, with an L* spec stated. So simply stated, given that I believe that we must keep the a* and b* channels in Lab mode within +/-1.0 at a L* value of 70-80, don't the subtle differences between chrominance and chromaticity disappear, based on the specified L* value? And if they do not, are not they of no practical concern?
Absolutely. If we are talking specifically about, for example, targets with an L* of about 75, then specifying a "bogey" on a* and b* does in fact constitute a bogey for chromaticity.

But if we were comparing a target with an L* of about 75, and a target with an L* of about 45, then the same numerical bogey for a* and b* does not constitute the same bogey on chromaticity. That was my point.

The measurement criteria for the current G6 version of the WhiBal Gray Reference card are:
  • Measured by a Gretag Macbeth SpectroEye Spectrophotometer
  • Setting of D50 light source and 2 degree observer angle
  • L* = >74
  • a* < +/- 0.5
  • b* < +/- 0.5
So speaking for the WhiBal, I do not think there is any doubt of the specificity of what is being specified in terms of extreme neutrality.
It's very specific. And from that data, we could caculate how close to neutral it is. As to "extreme", that is of course relative.

Your knowledge of the science on this certainly exceeds mine, but like you opened with, I think given the representations that are made for WhiBal, I see no conflict or issue with the specifications as presented. Perhaps for some other products. I cannot speak for those. I know that ExpoDisc used to measure each "disc", but I cannot speak for them.
Gee, I don't remember being critical of anybody's specifications.

I welcome your further thoughts, and of course statements from other companies that might have thoughts on the topic.
Or even other people.

BTW...Thanks for getting the forum going to a rip roaring start :>)
Well, I didn't want it to be dull around here!

.....and I do enjoy learning more of the background science if it leads to better photographic techniques, products, or results, but I think this case is just some interesting background knowledge with no practical application to the subject at hand.
Absolutely - just as I said.

Best regards,

Doug
 
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Doug Kerr

Guest
Hi, Michael,

Well, for what it's worth, at the limit of the spec you cited for the WhiBal card on both the a* and b* axes (essentially a worst case), the departure from chromaticity neutrality (in the sense of a*=0, b*=0) would be only about 0.0019 unit on the CIE xy plane (on which we can reckon chromaticity).

That certainly should qualify as "really close".

Best regards,

Doug
 

Asher Kelman

OPF Owner/Editor-in-Chief
Welcome

A huge handshake to welcome you.

I'm pleased that we deal with this in a critical way.

OPF wants to facilitate photographers going from good to excellent. Color management is paramount for professional work.

One can't realize one's vision to a final print if their is no reference to how we perceive color.

White balance is an important factor but not everything. But hardly anything matches this start.

So the presence of a color expert is a huge help to getting photographers tune up for the journey.

Asher
 

Michael Tapes

OPF Administrator/Moderator
Andrew Rodney said:
-->Firstly I know of no product, other than WhiBal, that prints specifications in regards to the neutrality of their WB reference card...

Right here:

http://www.babelcolor.com/main_level/White_Target.htm
Thanks Andrew..

I was not aware that they publish their specs. Good to know and I am glad that they do and that they are real. Every White balance reference should do the same. Not really a reference if we do not know the specs. I would disagree with the use of Pure White (in terms of luminance) for Digital White balance. I believe that it will be hard in many situations not to clip the reference, which would make it useless for White Balance, as you know. I will order one today to get familiar with it, but since it is so well spec'd, I know exactly what I will be getting, which is the point. I like to have one of everything on hand for eval. Thanks for the info, and welcome to the forum. Great to have you here.
 

Andrew Rodney

New member
Despite what Thomas said (and I hold him in the ultimate regard) I had no difficulty using this white tile as a white balance in ACR with my Canon captures. I got no warning or beep and got very nice rendering using it as a white balance source. I did bracket exposures with it and a Macbeth to determine the best "expose to the right" setting. On my 350 Rebel, using a Minolta Flash meter III in incident mode, the "normal" ISO and recommended exposure from the meter produced the best exposure (the white tile was just shy of 252 in all three channels).

Thanks, glad to be here.
 

Michael Tapes

OPF Administrator/Moderator
I look forward to experimenting when the target arrives. I expect it will be of high quality based on my readings on their site. What a pleasure to know what you are getting before you buy it (meaing that they publish specs for their product, as we do, and all should, but few do :>)
 
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Doug Kerr

Guest
Hi, Michael,

Firstly I know of no product, other than WhiBal, that prints specifications . . .
I don't know about "prints", but "Imageguy" provides the L*a*b* coordinates for his Picture Perfect gray cards, but only after asking.

Best regards,

Doug.
 

Michael Tapes

OPF Administrator/Moderator
Doug Kerr said:
Hi, Michael,



I don't know about "prints", but "Imageguy" provides the L*a*b* coordinates for his Picture Perfect gray cards, but only after asking.

Best regards,

Doug.
But how do you know what your card reads unless the card model has a certified spec, meaning that each card is certified to be equal or better than the certified spec?
 
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Doug Kerr

Guest
Hi, Michael,

But how do you know what your card reads unless the card model has a certified spec, meaning that each card is certified to be equal or better than the certified spec?
An excellent point, but the issue to which I was responding is, "what manufacturers 'print' specifications for their gray card products".

The matter of a manufacturer "certifying" that all cards they meet their stated specification (meaning within a stated tolerance), is quite a different layer of the matter.
 
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Doug Kerr

Guest
Hi, Andrew,

Very interesting. Thanks for the reference.

It is interesting that they speak of their target having a very consistent L*a*b* color under different "white" illuminants (certainly important), and then say that this means the target "does not exhibit metamerism".

In fact, the property he describes for his target is a low color inconstancy, a property that is in the same area of interest as metamerism but is not the same thing. For, one thing, we can only make statements about metamerism when we are speaking about how two different surfaces are comparatively perceived under different illuminations. There are no "metamerism-related" proprties that can be quantified for a single surface.

Color constancy (of which color inconstancy is of course the inverse) is the property of a surface exhibiting the same reflective (surface) color under two different illuminations.

In a practical sense, this means that the target measures the same CIE L*a*b color regardless of the illumination under which it is tested (just as "Babel" said). (Note that the instrument that measures the L*a*b color must know what illumination is illuminating the test item - generally, the colorimeter itself supplies that illumination, so it knows!)

This confusing use of the terminlogy is in fact surprising, since the Babel people are clearly quite familiar with these two concepts, as one can see from the notes for their color analysis program. Just an editorial slip, I guess.

The property he descibes for his target can be quantified by the Color Inconstancy Index. His target evidently exhibts a very low Color Inconstancy Index.

A low Color Inconstancy Index generally results from a uniform reflective spectrum across the visble light spectrum.

Best regards,

Doug
 

Danny Pascale

New member
Hello Doug,

I am pleased to join this soon-to-be large crowd of photography enthusiasts!

I will jump right-in and write about some of the comments I saw in this thread.

As mentioned by Michael, there are few companies which advertise detailed specs of their targets. His company and mine do so. This is not as obvious as it may seem. Playing in the extremes, extreme neutrality, extreme whiteness, and extreme blackness, is a tough job. You are often at the limit of instrumental errors, and inter-instrument readings can play tricks on you.

The tendency is to give "indicative" values with no tolerance. Just imagine if a tolerance was given to the ColorChecker target values! Anybody who measured a value slightly off, even if done with an uncalibrated low quality colorimeter, may complain to Gretag. Gretag has never given such a tolerance, but it is still a great, amazingly stable, product after all those years.

Using a white or a grey target is an ongoing debate. I obviously favor the white side instead of the dark side ;-) because I feel it provides better control on the highlights with modern digital cameras. I see people favoring each approach and this is a good sign (let's say both are needed!).

As for the metamerism/color inconstancy issue, well, technically you are correct Doug. However, for marketing purposes, the term "metamerism" is well known by most advanced amateurs and pros. In comparison, "color inconstancy" looks like something related to a digestive problem. The fact that these concepts are closely related made me select the term "metamerism" in order for people to visualize the concept. It is not that people are dumb, but they are not all colorimetry geeks like many of us. Eventually, like for metamerism, color inconstancy will be a familiar term (more a wish than a fact at the moment).

As you mention, I am quite familiar with the two concepts. One of the software tools in the BabelColor program calculates the Color Inconstancy Index (CII) with the latest approved method based on CIECAT02, an encrypted way of saying Commission Internationale de l'Eclairage Chromatic Adaptation Transform 2002! This tool, combined with metamerism index info, also available in the above mentioned software when there are two samples, can help people evaluate how two color samples change appearance and match under various lighting conditions.

His target evidently exhibts a very low Color Inconstancy Index.
Here are some CII values from a randomly selected target (the first one I could grab!)
0.03 CIELAB between D65 (the reference) and D50
0.05 CIELAB between D65 (the reference) and Illuminant A (tungsten lamp)
0.13 CIELAB between D65 (the reference) and F11 (a spiky fluorescent)

As you say, this is a very low CII.

A low Color Inconstancy Index generally results from a uniform reflective spectrum across the visble light spectrum.
I would say "smoothly varying" instead of "uniform" since uniform makes me think of "neutral", but hey, I understand what you meant, which is the idea behind my use of the term "metamerism" in the spec ;-)

Best regards,

Danny

www.BabelColor.com

P.S. Interesting readings on your site.
 
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Doug Kerr

Guest
Hi, Danny,

Welcome aboard, and thanks for coming out to play.

Your observations are well taken.

However, for marketing purposes, the term "metamerism" is well known by most advanced amateurs and pros. In comparison, "color inconstancy" looks like something related to a digestive problem. The fact that these concepts are closely related made me select the term "metamerism" in order for people to visualize the concept.
I understand.

The problem is that,. if as you say, your more sophisitcated customers are familiar with the concept of metamerism, they may be (as I was) trying to figure out what on earth metamerism (or the lack of it) means for a single surface.

But of course maybe they are only familiar with the word!

In any case, it's the paradox we all face in trying to "keep things simple".

My signature line on the dpr forums says: "Make everything as simple as possible - but no simpler."

Thanks for your inputs.

Best regards,

Doug
 

Danny Pascale

New member
Michael,

Thanks!
When do you take the time to sleep with all your activities?


Doug,

As part of a minor site update, a modification was done in the product description page following your remark. It "should" now be acceptable for all publics!


Best regards,

Danny
 
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Doug Kerr

Guest
Hi, Danny,

. . .a modification was done in the product description page . . .
Aw, shucks!

But that is really very apt.

Thanks.

Best regards,

Doug
 
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