It seems safe to say that "chroma" is the specific terminology of
"saturation" for the CIE L*a*b* space. How would you refer to the distance
between an achromatic color and another color with the same lightness in a
color space like ITP? Achromatic is T,P = 0, 0? CIECAM02 defines
"colorfulness", "saturation" and "chroma" as three different things.
https://en.wikipedia.org/wiki/CIECAM02
Mostly my point is that the term "chroma" does not have a standard
technical meaning and doesn't always refer to CIELa*b* (LCh) definitions.
On Fri, Nov 29, 2019 at 8:59 PM <graxx@xxxxxxxxxxxx> wrote:
Tucker,
Sorry for the confusion, to me, Saturation and Chroma don’t mean the same
thing.
Saturation is a “visual quality”, a “visual term”, if you will. Some use
it to describe the “content of color”, to “verbally quantify” how “rich” or
“vivid” a color appears. I think that’s a fair description? I confess I
don’t use “Saturation” all that much but I can see it does come across in
discussions of gamut mapping.
I would prefer to use the term “Chroma” in this context and in general as
well. I’m sorry if I have not made myself clear, “Chroma” has a clear,
arithmetic definition giving the “distance of the color from the neutral
axis”; the further away a color is from CIE a*, b* = 0.00 0.00, the
“higher” the Chroma. I think we can all agree on that.
I would venture to say that the two Chroma and Saturation clearly have
something in common, if only “semantically” in that both terms describe how
a color is distinct from gray. To me.
Colourfulness is another term used to describe (to some extent) the “same”
phenomenon … I’ll spare you the definition of these terms and I hope you
get a sense that I’m not all too confused in my thinking about these
“visual quantities”.
Thank you so much all for straightening me out 😊
/ Roger
*From:* argyllcms-bounce@xxxxxxxxxxxxx <argyllcms-bounce@xxxxxxxxxxxxx> *On
Behalf Of *Tucker Downs
*Sent:* Friday, November 29, 2019 8:42 PM
*To:* argyllcms@xxxxxxxxxxxxx
*Subject:* [argyllcms] Re: "Luminance", "Saturation", choice of terms are
important too
Luminance, Luminance Factor, Lightness, and Brightness all have different
meanings and are used inconsistently.
Luminance, to me, refers to some integration of a visible spectrum stimuli
multiplied with the photopic function and scaled by the candela constant
and the unit area. Illuminance can be used the same way for point sources
and some solid angle. It's an absolute measure of light.
Luminance Factor is normalized by the "white point" which can be measured
by using a perfect reflector and using this value as a value of 100
luminance factor.
Lightness, to me, refers to the axis in some color space like CIE L*a*b*.
L* is the "lightness axis"
Keep in mind that 100 is not the maximum lightness value or the maximum
luminance factor. Specular reflections are much brighter than a perfect
diffusing reflector. So some specular highlights might have extremely high
luminance value.
Brightness is a psychological percept and is not bound by the above
definitions. Spatial contrast has a large effect on brightness perception,
as does chroma.
Brightness of equal-luminance lights
https://doi.org/10.1364/JOSA.72.001225
Heterochromatic brightness matching with checkerboard patterns
https://doi.org/10.1364/JOSAA.4.000540
Predicting the lightness of chromatic object colors using CIELAB
https://doi.org/10.1002/col.5080160608
Even within some of these papers the authors get things flipped around, so
we can't shouldn't assume that there are clear definitions for these terms.
While that doesn't really matter in casual conversation, in technical
discussion it's important to define your terms in context. I'll point out
that the last paper from Fairchild & Pirrotta uses the term Lightness and
Brightness the same way so even in this context it's hard to be clear.
So with that said, I was a bit lost by something Rodger posted in that
other email thread. Chroma and saturation are not the same thing, can you
clarify? I would expect that saturation refers to the C* axis in LCh while
the term chroma can refer to a specific level as shorthand for saturation
value. I might also use the term chroma axis to refer to saturation.
- Tucker
On Fri, Nov 29, 2019 at 8:21 PM <graxx@xxxxxxxxxxxx> wrote:
Samuel,
Everything is “relative”, at some point, “relative brightness”, “relative
contrast”; our visual system has not equipped us well, humans, for
assessing “absolute levels” of light but only “relative levels”.
I must say that, in my experience, I rarely come across the term
“Luminance” other than in the context of “aperture colors”, to describe
emissive devices. I’m familiar with the term “Luminance factor”, however,
in the context of describing a surface “relative brightness”.
A quick search for “Luminance Contrast” does turn out interesting links,
like some by NASA. One site describes “Luminance contrast » as « The ratio
between the luminance values of adjacent surfaces »… Looks like the term
has found support in architecture?
/ Roger
*From:* argyllcms-bounce@xxxxxxxxxxxxx <argyllcms-bounce@xxxxxxxxxxxxx> *On
Behalf Of *Samuel Chia
*Sent:* Friday, November 29, 2019 6:14 PM
*To:* argyllcms@xxxxxxxxxxxxx
*Subject:* [argyllcms] Re: "Luminance", "Saturation", choice of terms are
important too
Roger,
In the first instance, I am only communicating *what* I perceived with my
sense of vision, *regardless of what I am looking at*. It could be a
monitor, it could be a print. Photons are reaching my eyes and so on and so
forth. I am not concerned about what space the colours are in or linearity
when I am looking at them in this instance, or if a different monitor is
showing me something different. I am just describing what I see, if
something looks brighter or darker, if contrast is reduced or increased.
Of course, thereafter, we can discuss *why* this is what I see, what are
the issues in the system of display that is causing me to see what I see,
and also if I can trust what I see (light source spectrum, visual
illusions, adaptions, surround, metamerism etc.)
https://en.wikipedia.org/wiki/Luminance: Luminance is often used to
characterize emission or reflection from flat, diffuse
<https://en.wikipedia.org/wiki/Diffuse_reflection> surfaces. Luminance
levels indicate how much luminous power
<https://en.wikipedia.org/wiki/Luminous_flux> could be detected by the human
eye <https://en.wikipedia.org/wiki/Human_eye> looking at a particular
surface from a particular angle of view
<https://en.wikipedia.org/wiki/Angle_of_view>. Luminance is thus an
indicator of how bright <https://en.wikipedia.org/wiki/Brightness> the
surface will appear. In this case, the solid angle of interest is the solid
angle subtended by the eye's pupil <https://en.wikipedia.org/wiki/Pupil>.
Whether or not the monitor is accurately displaying the image's colours is
a whole different issue.
Samuel
On Thu, 28 Nov 2019 at 09:38, <graxx@xxxxxxxxxxxx> wrote:
Samuel,
Thanks for taking the time to explain how you use the terms in your
practice.
Allow me – very quickly – when you write “to perceive image colours even
in CIELab space”, this is an interesting “concept”, Samuel. We may both
benefit, here, from further detailing our mutual understanding of the terms
(what you think you are referring to, mentally, and what I think you are
referring to, mentally) regardless of our respective backgrounds or
applications.
Where I begin to differ is the process of “visual adaptation”? I maybe
wrong but, when I look at an image on a monitor, I’m keenly aware that the
colours I see are “mediated” through whatever the monitor white point is. I
always try to convince myself that, if I was to throw the same image on two
monitors, side by side, calibrated to the same specs **except** their
white points, that I would **see** the same colours? Apparently, that has
long been proven experimentally. But I cannot completely discard this
notion in my “model of how I look at colours on a screen”.
I’m sure you mean something very different to me when you write that you
“perceive colours in CIELab space” when you look at your monitor : if you
are, then how can you be seeing “Luminance” which is not linear with
perception?
/ Roger
*From:* argyllcms-bounce@xxxxxxxxxxxxx <argyllcms-bounce@xxxxxxxxxxxxx> *On
Behalf Of *Samuel Chia
*Sent:* Wednesday, November 27, 2019 8:26 PM
*To:* argyllcms@xxxxxxxxxxxxx
*Subject:* [argyllcms] Re: "Luminance", "Saturation", choice of terms are
important too
Roger,
I hear you and agree that language needs to be precise. But to perceive
image colours even in CIELab space, I am looking at it on my monitor or a
print. So that is then luminance contrast that I am experiencing. The
sensation of colour is quite complex so at any given time, one can be
experiencing/imagining both the effects of saturation and/or chroma. That's
why I am ok to use the terms interchangeably unless we begin to talk in
more abstract, specific spaces.
For example, we have talked about the movement of colour in mapping it in
3D space. Graeme is concerned about mathematical spaces because he is
coding Argyll, so he often wants it to be specific. I am a photographer so
I refer to movement in human perception space, which is vague and not well
defined - it just means I think i see a greater or smaller difference. This
has, admittedly, caused confusion. I will try to be better. Thank you.
Regards,
Samuel
On Thu, 28 Nov 2019, 09:11 , <graxx@xxxxxxxxxxxx> wrote:
People,
Just want to say, « Luminance contrast” is an abuse of terms (to me), when
talking about “Lightness mapping” – please. Same with “Saturation”, I
understand it bugs some people, could we phrase ideas in terms of “Chroma
mapping” instead? Or is it really “saturation” we’re after? Luminance is a
measure of “light intensity”, measured in Candelas/sq meter, typically used
on emissive devices such as displays. Anytime we talk about CIE Lab, there
is no “Luminance” involved anymore, it’s “Luminance factor”, if I’m not
mistaking because it’s expressed on a normalized scale between 0 and 100.
CIE “L” stands for “Lightness”.
Sorry, it was meant as a rant… I still enjoy the discussion 😊
/ Roger
