[argyllcms] Re: CCT of a Speedlight
- From: "Wire ~" <wire@xxxxxxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Mon, 12 Jul 2021 15:35:38 -0700
On Sat, Jul 10, 2021 at 1:03 PM ternaryd <ternaryd@xxxxxxxxx> wrote:
On Sat, 10 Jul 2021 10:46:02 -0700
Ben Goren <ben@xxxxxxxxxxxxxxxx> wrote:
I’ve found CCT to not be all that useful in
photography. Its best use is in marketing of
consumer household lightbulbs, where it gives
an easy-to-understand expression of what sort
of “look" / “mood” the light will create.
I thought to have understood that CCT is only
half of one story, the "tint" being the other
half. And that the color rendering of an
illumination would be a second story. D50 is a
computed spectrum, not a CCT, I know, but it is
identified by a CCT (5003), and xenon discharge
light is said to be quite close to daylight.
Also argyll computes the Duv to the closest
D-spectrum. Or did I get this wrong?
Re the story of CCT and "tint", anyone, please correct me is this is wrong:
• CCT (K) by definition is the Plankian (black-body) locus.
• Duv ("tint") measures are relative to CCT along correlate lines.
• D65 is 6504 K +0.0032 Duv
• The 'uv' pertains to the CIE 1960 UCS graph, the horseshoe diagram that
MacAdams vertically squashed to reduce non-uniformity of differences.
• The omission of a Duv measure for a CCT generally implies a Duv of 0 but
literature varies (more below).
• The daylight locus is a constant Duv offset from the black-body. Common
spectrum locus charts include the daylight locus, but it's also just as
common to regard only a few daylight points are interesting: D50, D55, D65.
• CCT K measures in context of imaging are presumed to be on the black-body
locus (assume Duv = 0) unless otherwise mentioned.
• In consumer lighting, CCT includes a stated or implied emitter
technology, e.g. florescent, with spectral traits that may obviate the Duv
measure; these traits are commonly acknowledged, if only implicitly, by the
illumination technology under consideration. Therefore, lamp CCT includes
an implicit proviso of the electro-chemical behavior of tech under
consideration. In case of this thread topic, it's xenon electronic flash.
In such a case, CIE coordinates are much more than advisory in context, but
a specific tech may do all kinds of stuff that the CIE doesn't account for.
Literature for LED lighting seems to be changing thinking, maybe because
its performance is defined in terms of CIE.
Argyll's "visual" temps with dE'00 offsets are a nice way of transcending
the arcane dimensions of Duv and focusing on white perceptual tracking.
However, I imagine that if you ask a lighting technician what he thinks
about the "visual" measure, he'll probably have no idea what you're talking
about, unless if he's up to speed on LED tech.
I take Ben Goren's points about the fuzziness of desirable camera behavior
very well, because he keeps in mind a distinction of artistic intentions
from quantitative measures while avoiding disservice to either point of
view.
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