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,
<https://en.wikipedia.org/wiki/Diffuse_reflection> diffuse surfaces. Luminance
levels indicate how much <https://en.wikipedia.org/wiki/Luminous_flux>
luminous power could be detected by the
<https://en.wikipedia.org/wiki/Human_eye> human eye looking at a particular
surface from a particular <https://en.wikipedia.org/wiki/Angle_of_view> angle
of view. Luminance is thus an indicator of how
<https://en.wikipedia.org/wiki/Brightness> bright the surface will appear. In
this case, the solid angle of interest is the solid angle subtended by the
eye's <https://en.wikipedia.org/wiki/Pupil> 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 <mailto: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 <mailto:argyllcms-bounce@xxxxxxxxxxxxx>
<argyllcms-bounce@xxxxxxxxxxxxx <mailto:argyllcms-bounce@xxxxxxxxxxxxx> > On
Behalf Of Samuel Chia
Sent: Wednesday, November 27, 2019 8:26 PM
To: argyllcms@xxxxxxxxxxxxx <mailto: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 <mailto: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
