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Thread: How to read "K" value of light???

  1. #31
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    Re: How to read "K" value of light???

    FWIW, Kenko took over the Minolta meter business. The Color Meter IIIF still lives, as the KCM-3100:

    http://www.bhphotovideo.com/c/produc...mperature.html

  2. #32
    Greg Davis's Avatar
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    Re: How to read "K" value of light???

    That would is a really good meter, too. I chose Sekonic because I have their exposure meters and like them, and they gave an educator discount.

  3. #33

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    Re: How to read "K" value of light???

    Quote Originally Posted by Drew Wiley View Post
    Gosh, Stone, I thought you had more class than that! Evvvverybody knows the ONLY color meter that ever existed is the Minolta. They can still be had. But no,
    I'm not selling mine.
    Well there is the Broncolor FCC.

  4. #34
    Resident Heretic Bruce Watson's Avatar
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    Re: How to read "K" value of light???

    Quote Originally Posted by stradibarrius View Post
    I was trying to come up with a way of measuring the temperature of the light.
    In general, you can't. "Color temperature" is about comparing the hue of an ideal black body radiator to the light source you are trying to characterize. In this case, it's trying to compare apples to concrete blocks. Fluorescent light is discontinuous, and the spectrum emitted typically has huge peaks and valleys, not found in the spectrum of an ideal black body radiator. The quality of light between the two sources is incomparable.

    In practice, a color meter basically looks only at the orange-blue axis, and tells you where the light ends up on that axis. Most color meters tell you nothing about the magenta-green axis. Unfortunately, that's what matters most with cheap building fluorescent lights, because they tend to produce the majority of their light via a huge green spike. This is the reason many video cameras show you a sickly green color cast even when you set their "color temperature" to the correct value (say, 6500K for a "daylight" tube).

    For this reason most modern video cameras include a manual white balance function that makes many measurements, including the dreaded magenta-green axis. A good manual white balance made against a calibrated gray target (think 18% Kodak gray) will generally give you fairly accurate color without a noticeable color cast in all but the most challenging lighting.

    As to human clients visiting your shop... showing your wares under color corrected light will make a better impression. There are good fluorescent tubes out there -- KinoFlo True Match. Westcott. Some others. Very little in T5 except for biax bulbs (2G11 base) which are common in cinematography lighting, but not in building lighting.

    Bruce Watson

  5. #35

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    Re: How to read "K" value of light???

    Quote Originally Posted by Bruce Watson View Post
    In general, you can't. "Color temperature" is about comparing the hue of an ideal black body radiator to the light source you are trying to characterize. In this case, it's trying to compare apples to concrete blocks. Fluorescent light is discontinuous, and the spectrum emitted typically has huge peaks and valleys, not found in the spectrum of an ideal black body radiator. The quality of light between the two sources is incomparable.

    In practice, a color meter basically looks only at the orange-blue axis, and tells you where the light ends up on that axis. Most color meters tell you nothing about the magenta-green axis. Unfortunately, that's what matters most with cheap building fluorescent lights, because they tend to produce the majority of their light via a huge green spike. This is the reason many video cameras show you a sickly green color cast even when you set their "color temperature" to the correct value (say, 6500K for a "daylight" tube).

    For this reason most modern video cameras include a manual white balance function that makes many measurements, including the dreaded magenta-green axis. A good manual white balance made against a calibrated gray target (think 18% Kodak gray) will generally give you fairly accurate color without a noticeable color cast in all but the most challenging lighting.

    As to human clients visiting your shop... showing your wares under color corrected light will make a better impression. There are good fluorescent tubes out there -- KinoFlo True Match. Westcott. Some others. Very little in T5 except for biax bulbs (2G11 base) which are common in cinematography lighting, but not in building lighting.
    Bruce is dead on. The Kelvin scale doesn't even really reflect shifts on the magenta/green axis. There are other methods of quantifying, but to get a meter that can read a fluorescent accurately AND communicate better than a simple Kelvin number typically cost a fortune.

    If all you're after is a rough idea, "Pocket Light Meter" for the iPhone includes a Kelvin scale. Not perfectly accurate even for easier to read sources.

    Your best bet is to stick a piece of white poster board under one of these lights in such a way that it is not also lit by any extraneous sources and take a picture with it exposed around zone 6 or 7.

    Bring that shot into lightroom (or whatever you use) and use your eyedropper tool to balance the shot so that the card is true white. Whatever lightroom tells you your balance is your Kelvin rating.

    Mind the handy green magenta slider just below the color temp. (Assuming you are using lightroom) If its strong toward magenta, you lights are green and vice versa.

  6. #36
    Kirk Gittings's Avatar
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    Re: How to read "K" value of light???

    Quote Originally Posted by Peter J. De Smidt View Post
    The "FL" filter isn't very effective anymore, due to the bewildering array of different fluorescent lights. With some commercial interiors I shoot, the color values of the light fixtures will vary all over the place, as the maintenance people apparently buy whatever is cheapest, or whatever they just happen to grab that day.
    Exactly. we used to have cool white and warm white. Now there are dozens of fluorescents. On a DSLR I put it on fluorescent, but include a grey card in the scene which I use to neutralize the color more finally in Lightroom.
    Thanks,
    Kirk

    at age 67
    "The woods are lovely, dark and deep,
    But I have promises to keep,
    And miles to go before I sleep,
    And miles to go before I sleep"

  7. #37
    Kirk Gittings's Avatar
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    Re: How to read "K" value of light???

    The Minolta color meter was never very accurate for fluorescents-I used one for decades for architectural interiors. Every film had a different responce-particularly like from Kodak to Fuji. One had to take what it said and "use your experience" with a particular film then also adjust for long exposure color shift etc. The Minolta color meter took none of that into account nor did it have any memory function. It was mainly about testing and experience. It was tedious. I do not miss it. Digital is a dream compared to transparencies with flourescents.
    Thanks,
    Kirk

    at age 67
    "The woods are lovely, dark and deep,
    But I have promises to keep,
    And miles to go before I sleep,
    And miles to go before I sleep"

  8. #38

    Re: How to read "K" value of light???

    Quote Originally Posted by stradibarrius View Post
    Would any of the common "light" meters be able to give the "K" value of the light in a given scene?
    I have just finished a remodel of my violin shop and the "T5" florescent bulbs give off a strange color cast. I have tried adjusting the WB values of my digital camera to see if I could at least get close.
    I was trying to come up with a way of measuring the temperature of the light.
    All but the cheap off-shore lamps will print the color temperature on the end of the lamp (you probably call them 'light bulbs'). Go up into the luminaire and pull one down. It probably requires a twist of the lamp to get it out. You should be able to see some markings on the lamp near the socket at one end. The color temperature will be printed there.

    However, as has been said, this is only somewhat useful because the green/magenta level will be undetermined by this value, and since the light is a discontinuous spectral distribution, there will still be shortcomings even if you perfectly match up the CT value.


    ---Michael

  9. #39

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    Re: How to read "K" value of light???

    Pretty much any digital camera with manual control has K white balance control where you can preset light temp.
    Set your exposure. If you are combining flash and constant light it is important.
    Guestimate color temp, set it, take pic of white piece of paper. Paper color on preview will be either bluish or redish, based on that select another temp.

    Its longer to write - in few iterations you'll get color temp.

    Works fine with flash (if you can connect sync cable), thungsten, hmi etc combined all together.


    But then again some people have allergy on digital.... even when it helps to take decent film pic

  10. #40

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    Re: How to read "K" value of light???

    Back to many years ago, one of my hats was as an industrial photographer. I used the Minolta II as a starting point but needed experience to get it right. In talking with the plant maintenance people we concluded that all the overhead florescence bulbs were of the GE "cool white" brand, both in the plant and the office area. I asked that they only replace with this same bulb, which they did. This allowed me to use a magenta 40cc filter on camera with daylight transparency film any where in the building. Worked with strobe flash also. For exec portraits, kept them away from windows.

    Tom
    God's gift to you is life, what you do with it is your gift to God

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