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 Author Topic: Three different color calculators can't be wrong, can they?  (Read 2649 times)
Frans Waterlander
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 « on: August 30, 2013, 06:17:34 PM » Reply

When using three different color calculators to find the CCT (Correlated Color Temperature) for x=0.3545 and y=0.3713, I find basically the same value: 4744.2K, 4743.8K and 4744K. A SoLux data sheet however says that the CCT for those xy coordinates is 4700K. I've asked Kevin McGuire of SoLux for a clarification, but he hasn't responded. Can anybody shine any light on this decrepancy? (pun intended)
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Czornyj
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http://www.pcmus.com/Dive-Photos/SoLux-4700K-50W-Specs.pdf
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Marcin Kałuża
Frans Waterlander
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Cz, what you link to is an old data sheet. In July that sheet was replaced on the SoLux website and it now reads 4700K.
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JRSmit
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Values you present differ by much less than 1 percent a d are about 1 percent of the 4700 mark. What point are you trying to make?
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Jan R. Smit
Czornyj
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Cz, what you link to is an old data sheet. In July that sheet was replaced on the SoLux website and it now reads 4700K.

So it has to be calculated with different CCT formula (like CIEDE2000 instead of CIE 1960 UCS)
 « Last Edit: August 31, 2013, 06:28:12 AM by Czornyj » Logged

Marcin Kałuża
Tim Lookingbill
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Values you present differ by much less than 1 percent a d are about 1 percent of the 4700 mark. What point are you trying to make?

The point that you assume he's making if you carefully examine is really an inverse conical tunnel with no end in sight.

He's posted the exact same tunnel over at Photo.net, skirting into troll territory IMO especially for someone who has written testimony to the value of using Solux lights published on Solux's site.

Maybe he's using reverse psychology complaining about the less than accurate CCT nit-picking of the Solux color temp claims no one seems to have a problem with as a way to name drop Solux as some cool stealth marketing ploy. Not sure.

But I'm pretty sure of one thing and that is I never learn anything I can use to be a better photographer he's posted in the past 5 years as input on both photography sites. YMMV.
 « Last Edit: August 31, 2013, 11:05:29 AM by Tim Lookingbill » Logged
Frans Waterlander
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Jan, different equations used to calculate CCT will give slightly different answers, but they should be within a couple of Ks. So the SoLux numbers are way out of whack and I'm trying to understand why. That's all.
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Frans Waterlander
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Tim, Do you mean to say that you can't help me figure out why measurements that should be within a few Ks are way off?
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Tim Lookingbill
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Tim, Do you mean to say that you can't help me figure out why measurements that should be within a few Ks are way off?

I mean to say I don't know want to know enough about the subject to help anyone because I don't see a problem being off a few Ks that once it's figured out would help anyone including myself become a better photographer and make better looking images.

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Czornyj
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Jan, different equations used to calculate CCT will give slightly different answers, but they should be within a couple of Ks. So the SoLux numbers are way out of whack and I'm trying to understand why. That's all.

x0.345 y0.3713 is a few dE away from Planckian locus, so it can give different CCT values with different formulas.

Here's a measurement of normal halogen light bulb I did with my i1Pro2 - it's much closer to locus, but there's also 15K difference:

White chromaticity coordinates 0.4790, 0.4152
White    Correlated Color Temperature = 2487K, DE 2K to locus =  1.5
White Correlated Daylight Temperature = 2500K, DE 2K to locus = 10.0
White        Visual Color Temperature = 2472K, DE 2K to locus =  1.1
White     Visual Daylight Temperature = 2500K, DE 2K to locus = 10.0
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Marcin Kałuża
digitaldog
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What point are you trying to make?

He hasn't one. Check out if you dare, his post a month or so ago about Solux lights and their reported CCT. Huge rabbit hole that was a major waste of time. If you want this discussion to stop quickly, tell Franz what he wants to hear and we can all go back to something far more useful. Between the series of posts here about Solux CCT values and over on PhotoNet (http://photo.net/digital-darkroom-forum/00bjDL and http://photo.net/digital-darkroom-forum/00bkvN), it should be obvious to anyone Franz is trolling once again.
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Andrew Rodney
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digitaldog
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Jan, different equations used to calculate CCT will give slightly different answers, but they should be within a couple of Ks. So the SoLux numbers are way out of whack and I'm trying to understand why. That's all.

We showed you over a month ago they are wrong and out of whack, you dismissed us. Us being at least 3 independent users, one with a \$13K spectroradiometer that illustrated the Solux values are not to be taken seriously! Get over it.
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Andrew Rodney
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Frans Waterlander
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Cz wrote: "x0.345 y0.3713 is a few dE away from Planckian locus, so it can give different CCT values with different formulas."

The distance to the blackbody locus itself will NOT result in different CCTs. Different algorithms ideally come up with the exact same answer, but depending on the algorithm can be off by a couple of Ks, not 44K or in some other case of SoLux specifications up to 200K.
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Frans Waterlander
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Tim wrote: "You still have not made your case in this regard."
Really? Did I claim that resolving this issue would allow you to make better pictures? I don't think so! I delved deeper into the subject of color temperature, correlated color temperature, ways to calculate both, xy, uv, u'v' color diagrams, etc. and found what I think is a misrepresentation of fact. I'm looking for feedback from people who feel they can contribute on this subject and, I'm afraid, you nor Andrew are particularly helpful.

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digitaldog
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...and found what I think is a misrepresentation of fact. I'm looking for feedback from people who feel they can contribute on this subject and, I'm afraid, you nor Andrew are particularly helpful.

Try the ColorSync list (that list that ignored you last time for good reason) or better, DP Review forums where people will tell you what you want to hear.

Tim, I and other's tried to be helpful, you didn't listen. Certainly there has to be other lists where you haven't wasted people's time you can find to post.
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Andrew Rodney
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http://digitaldog.net/
Czornyj
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The distance to the blackbody locus itself will NOT result in different CCTs. Different algorithms ideally come up with the exact same answer, but depending on the algorithm can be off by a couple of Ks, not 44K or in some other case of SoLux specifications up to 200K.

The farther the distance to locus, the bigger difference with different CCT formulas.

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Marcin Kałuża
Frans Waterlander
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The farther the distance to locus, the bigger difference with different CCT formulas.

SoLux data is off by 6K for one bulb, 44K for another bulb and 213K for yet another one. No differences in formulas and differences in distance to the blackbody locus explains such value, which shouldn't be more than a couple of Ks on average and maybe up to 10K in extreme cases.
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JRSmit
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Waste of time.
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Jan R. Smit
Czornyj
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SoLux data is off by 6K for one bulb, 44K for another bulb and 213K for yet another one. No differences in formulas and differences in distance to the blackbody locus explains such value, which shouldn't be more than a couple of Ks on average and maybe up to 10K in extreme cases.

Apparently there are some CCT formulas you're not aware of:
White chromaticity coordinates 0.3545, 0.3714
White    Correlated Color Temperature = 4745K, DE 2K to locus =  8.3
White Correlated Daylight Temperature = 4749K, DE 2K to locus =  4.5
White        Visual Color Temperature = 4573K, DE 2K to locus =  8.0
White     Visual Daylight Temperature = 4667K, DE 2K to locus =  4.3
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Marcin Kałuża
MarkM
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There is no right or wrong here Frans, there is only how close you think you need to be and how close you think is useful. Both are fair questions and both have been studied for a LONG time when it comes to correlated color temperature. If you're really interested in it, there is a tremendous amount of literature going all the way back to the 1920s.

Originally the idea of CCT was to find the temperature of the Planckian radiator with a perceived color most closely resembling that of a given stimulus. This has been problematic to define experimentally. The CIE came right out and said it:

"The concept of correlated colour temperature used to be based on visual observations. Recent investigations have shown (see Borbely et aI., 2001) that no metrological definition can be based on such perceptual investigations."

So what do you do? The current practice is to use a colorimetric definition based on euclidean distance from the Planckian locus on the uv chromaticity diagram. There are a couple ways to go about this. The easiest is to use C. S. McCamy's simple polynomial approximation which will allow you to calculate the CCT from xy coordinates with a great deal of accuracy. But this accuracy is misleading because it is related to the math, not observation. The formula doesn't care whether you calculate to the nearest 100ºK or to the nearest millionth of a degree.

Another way to do this is based on a binary search through the locus comparing the distance to the stimulus you're testing. You basically start with a temperature, find the the uv coordinates on the Planckian locus and find the distance to your stimulus. Then move to the next value and test again—if the distance shrank you continue, if it grew you move the other way and eventually you narrow in on a value to whatever precision you like. It's computationally easy, but slower than the formula. The method requires that you answer the question how much precision do you want? If you do this with your xy coordinates and decide you only need precision to 100ºK you find the answer is 4700ºk—which is what Solux published. For a lightbulb manufacturer, this is entirely appropriate since you almost never see bulbs specified with any more precision.

And from a colorimetric standpoint it actually might be more precise the appropriate.  Borbély (the same on the CIE cited above), Sámson & Schanda in Wiley's COLOR research and application in 2001 write this about CCT:

Quote
The experimental determination of correlated colour temperature is too uncertain visually to be correctly definable. Thus, we would like to recommend using this term only as a first estimate of the chromaticity of near-white illuminants. For describing lamps and displays, a seven-value scale seems to be adequate
The Concept of Correlated Colour Temperature Revisited

Their seven-value recommendation rounds CCTs to 2700, 3000, 3500, 4200, 5000, 6500, 9500. While you're claiming a discrepancy of 44ºK is a 'misrepresentation of fact' they would have you round up to 5000ºK and be done with it.
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