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Thread: Darkroom Lighting Color Temp considerations

  1. #31

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    Re: Darkroom Lighting Color Temp considerations

    Quote Originally Posted by StuartR View Post
    CRI is not infallible...
    Yes, a CRI 60 can be better or worse, because a manufacturer can desing the spectrum to have a better measurement.

    ...but a CRI 98 matches the blackbody spectrum in a 98% average in 8 checking points, so it would be really difficult that a CRI 98 source isn't of extreme quality.




    Quote Originally Posted by StuartR View Post
    it is a 1960s standard that applies only to standard pastel like colors, it is not a great measure of accuracy or a lights suitability for use in color critical applications. It is a basically a minimal standard, but the only one we have.
    CRI was revised in 1995, while the most widely used color space today is CIE 1931, since 1931 !! Another popular space is 1976 CIELUV.



    Quote Originally Posted by StuartR View Post
    The colorist mentioned how some high CRI lighting is actually worse than lower CRI in some cases etc.
    It is true that low CRI light sources may match more or less the bayer dyes on the pixels, or the dyes/sensitization in the color films, but a 98 CRI matches (I reiterate) 98% average of the SPD in 8 checking points of the black boby spectrum, so it's extremly difficult you find a flaw.




    Quote Originally Posted by StuartR View Post
    I do not disagree that there are LED's available that are great at color reproduction, but I think your chance of getting one off the shelf is still pretty low unless you specifically seek out particular brands and models, or have the time and ability to test.
    You can get some variable results with a CRI 80 light, but not with a CR 98. Just spend $25 in a +95CRI household LED bulb and judge on your own.

  2. #32
    Drew Wiley
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    Re: Darkroom Lighting Color Temp considerations

    There is a vast difference between interpolating a curve based upon eight spaced readings versus a continuous reading. Those kind of spectrophotometers
    are now less common because they're much more expensive to build and fussier to maintain. But I worked hand-in-hand with the gradual evolution of industrial spectrophotometers right up to their current practical applications, not as an engineer, but as a user involved in feedback to engineers. Sometimes
    I made a game of submitting color samples I knew would throw a curve ball at their latest design. I also taught color matching. At a certain point there is no
    substitute for a trained eye, but I emphasize trained, because its's not all about visual physiology but also about experience and understanding just what to
    look for. Machines are nice for reducing overall eye fatigue, but must be used in relation to trained vision. Note that I'm referring to critical color evaluation,
    not what might be acceptable for display lighting. Of course, one can simply install a selection of lights in the lab to simulate different display conditions,
    and then walk outside to check the color under actual sunlight. That's what I do, in addition to critical light sources in things like the lightbox and retouching
    station. My wife once worked with a six million dollar spectrophotometer that was such a trade secret that her personal lab was behind a timed bank vault
    door. That was in biotech where big big pharmaceutical bucks were on the line at the prototyping phase. No two people in the entire corporation were allowed to know the complete formula behind how that machine worked. But one wannabee industrial spy was caught.

  3. #33

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    Re: Darkroom Lighting Color Temp considerations

    Quote Originally Posted by Drew Wiley View Post
    There is a vast difference between interpolating a curve based upon eight spaced readings versus a continuous reading.
    Drew, if you like HMI:



    ...then you will find that any CRI 98 light source is perfection:



    Just try it...

  4. #34

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    Re: Darkroom Lighting Color Temp considerations

    Thanks Pere. I looked at the online sites of the local lighting stores, and they only have two MR16 LED bulbs available, both CRI 80. I looked for regular bulbs, and they were also CRI 80 (Osram and Phillips). I found some strip lights that were CRI90, but that is the highest I found. I am sure things in mainland Europe and the US are better, and perhaps they have some special lights hiding in the back, but it seems my current best option is to stick with what has proven to work for me, SoLux. By the time the bulbs die, things will probably have gotten better anyway...I should also say that I run a Hahnemühle Certified Studio for digital printing, and the color correct lighting was part of the certification process, and SoLux was approved for that. If I changed anything, I would need to make sure it was still approved. Anyway, that's my situation. LED's are certainly the future (and the present for a lot of applications). I am just glad that fluorescent is going the way of the Dodo!

  5. #35

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    Re: Darkroom Lighting Color Temp considerations

    Quote Originally Posted by StuartR View Post
    I looked at the online sites of the local lighting stores
    Just search CRI 95 led bulb at amazon or ebay

  6. #36
    Drew Wiley
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    Re: Darkroom Lighting Color Temp considerations

    Any real lighting pro can tell you that a lot of the shoot-from-the-hip info on this thread is driven by marketing BS. An 8-point interpolated spectrogram is as
    equally useless for matching off-white wall paint as for subtle skintones in a photographic print. Solux is a reputable company, Phillips is not. And I've talked to GE factory reps in person many times over the years, and 100% of the time they spoke sheer BS. They probably sold used cars on the weekends. Pere- I don't do video, so have no use for either LED panels or HMI, which was once sold for old slow scanning-back digi capture. But if I had to choose, that HMI version you posted does look a lot more usable than the LED one. At least it has a healthy green peak. Fluorescent tubes are still important for many applications; the screw-in ones are another story. Ordinary tungsten should be reinstated; it's only now that there's some official recognition that hot halogens combined with junk floor lamps have caused a lot of fires and cost a lot of lives - something which should have been obvious before they were ever approved as substitutes for screw-in bulbs. On the display end of things, all types of halogen including low-voltage track lighting puts out a lot of UV and is going to fade color prints. Filters over the bulbs help only marginally; and no, inkjet prints are not exempt despite the marketing BS inherent to that particular industry too (they contain quite a few susceptible dyes also).

  7. #37

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    Re: Darkroom Lighting Color Temp considerations

    Quote Originally Posted by Drew Wiley View Post
    An 8-point interpolated spectrogram is as equally useless for matching off-white wall paint as for subtle skintones in a photographic print.
    Drew, here (https://en.wikipedia.org/wiki/Color_rendering_index) it is explained how CRI is calculated. At the bottom there is the "Film and video high-CRI LED lighting incompatibility" section, and why the TLCI standard was generated.

    But something changed in the industry, today a high CRI is a different thing: 98

    If you explore a bit the math involved in the CRI calculation you will find that achieving a 98 rating it only can come from making perfect illuminator. Yes, a CRI 80 allows for "creative" spectrums, but a 98 rating does not allow for inconsistencies, and since 1995 the CRI calculation includes saturated colors, see 1995 TCS colors in the wiki article.

    Well, search datasheets and find a single 98 LED with a not excellent espectrum...


    Regarding HMI, this is an OSRAM brand, the spectrum I posted before is the one of Osram HMI 575W Sel UVS.

    Beyond HMI we have Xenon arc lamps: https://en.wikipedia.org/wiki/Xenon_arc_lamp

    OSRAM also pioneered in the xenon business, it is a good illuminator (spikes are in the IR) that is also used in IMAX theaters. Single problem is that it throws substantial IR/UV. Xenon SPD is not as perfect as a CRI 98 LED but it allows to power an IMAX device.

    This is a tipical xenon SPD, as you can see (400-700nm visible section) it's not as good as any CRI 98 LED you can find:


  8. #38
    Drew Wiley
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    Re: Darkroom Lighting Color Temp considerations

    Pere, I have decades of experience with this subject. Theory is one thing, specific application another.

  9. #39

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    Re: Darkroom Lighting Color Temp considerations

    My rule is "If in doubt, try it out"... As a former interior fotog, there were many different balances on different sources, some were very different on paper, but would balance OK, and some would have little visual difference, but would balance weird on the chrome film...

    Test, test, test, and don't sweat the details until it hits the light box... :-)

    Steve K

  10. #40

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    Re: Darkroom Lighting Color Temp considerations

    Quote Originally Posted by Drew Wiley View Post
    Theory is one thing, specific application another.
    OK, just just test a CRI 98 light source for the specific application, and you'll see. I did it, and I can recommend it.
    Last edited by Pere Casals; Today at 13:08.

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