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  1. #1

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    Color Temp vs Distance from Light Source?

    This is a mystery I could use some help with. I've also posted this question on other forums to maximize feedback. I wanted to determine the color temp of my diffusion cold light enlarger head, so I got out my Gossen Sixticolor meter (which is designed to sense only blue and red, but not green). In order to first see if the meter would read a “known” color temp accurately, I took readings over the 8x10” diffusion screen on my Macbeth Prooflite which is supposed to be 5000K; readings were 4500K at the center and varied about +/- 500K around the screen. My Gossen Lunasix meter also showed variation in readings around the screen, and I discovered that the color temp decreased where the brightness decreased and it increased where the brightness increased. If the brightness variation is due to voltage variation it would not be too surprising. Then I went to the enlarger and checked color temp at the easel (max aperture, looking toward the lens, room lights off) and discovered that the color temp increased as I moved the meter toward the lens (3000 to 3300, 10%) – obviously not due to voltage variation. Repeating with the lensboard removed for brighter illumination, read 3400K at the easel and about 4000K at the lensboard height, an 18% change. So my question: Why does the color temp change with distance from the light source?

    Any thoughts would REALLY be appreciated.

  2. #2

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    Maybe I'm missing something, but my understanding of basic electromagnetic physics says that shouldn't happen. Is there some filtering medium the light is going through? Are there a lot of tiny particulates in the air? That would scatter the shorter wavelengths and make it tend toward warm.

    Also, could the voltage be fluctuating, leading to your measurements being affected by being taken at different times? I would make the measurements again to control for this.

  3. #3

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    Quote Originally Posted by silveror0 View Post
    Why does the color temp change with distance from the light source?
    I am not a scientist but I don't think that it does.

    If it did I think tha NASA would have found a difference in the colour of planets/stars depending upon where the measurement was taken from e.g. earth based, hubble based or close orbiting satelite. As far as I know, mars, as an example, appears reflects red light when seen from the earth, hubble and on the surface. If it was otherwise the three vantage points would give a different coloured planet.

    It doesn't surprise me that the colour temperature shifts towards the egde of an easel - colour diffraction through the lens and perhaps light refraction from the egdes of a mixing box or negative stage.

    As the meter gets closer to the source the influence of other ambient light may be reduced so giving a different reading to one where there is more ambient light?

    Just my thoughts till S.Hawkins logs on and tells us all different!

    Sim2.

  4. #4
    lxdude's Avatar
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    Quote Originally Posted by Sim2 View Post


    Just my thoughts till S.Hawkins logs on and tells us all different!

    Sim2.
    Is it Sadie Hawkins day already?

  5. #5

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    Two other options. I hope the first is not the case. If your light source moved away really fast, it could cause that. It would also cause a hole in your ceiling. Of course it would have to be really bright because the intensity would decrease quickly as the light moves away.

    The far more likely option is that there is a problem with your meter.

  6. #6
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    For whatever reason, I’ve noticed that color temperature does change with distance from the enlarger light source. I remember having to recalibrate filters (not just the time) whenever I changed the size of the enlargement. When the Durst PCM1001 was introduced, I think the big “selling point” was that the PCM1001 analyzer would adjust filtration based on distance change.

  7. #7

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    Don't know if this is relevant to your setup, but the further away from the source the more chance there is that you are picking up reflected light instead of direct light. Reflected light can have a different frequency distribution than the incoming light. Just think of the difference of bouncing a flash/light off of a gold reflector as opposed to a white/silver one.

  8. #8

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    Just to add a bit more info, I neglected to make it clear that both the enlarger light source and the Macbeth light source are fluorescents. There are no reflective surfaces near the enlarger and no other lights in the vicinity during the measurements (which I tried to keep near the lens axis and away from the fringes of the cone of light). I found a thread on another forum that mentioned how the 2-color Gossen Sixticolor does not handle fluorescents well, OR dim light situations ("dim" was not quantified). In the meantime, I've gone back to the darkroom where I have hanging over the sink a 100W incandescent bulb in reflector (used for viewing prints while in the toner), and I found no change in the color temp reading from the sink up to near the bulb - a distance of about four feet. So I'm beginning to suspect the fluorescents are the root cause of the problem. I've contacted the US distributor for Gossen and explained my problem - will see if I get any feedback. I certainly can't justify the cost of a 3-color meter that does handle fluorescents, considering the few times I'd use a color temp meter. I'll post whatever I learn from them. And thanks for your thoughts so far.

  9. #9
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    Well, first of all, I wonder if the the light is going through any dispersive medium e.g. a lens, a glass sheet, plastic etc. If it is then you could be getting a prismatic effect. And the wavelengths would then be more separated (dispersed) at larger distances.

    Apart from that, I wonder if it is an issue of the meter itself; the meter has fairly large r/g/b sensors, right? And the readings from those are used to deduce the colour temp. So if the meter is placed in such a way as not to have an even distribution of light across all the sensors then the measured colour temp will be incorrect. I'd say that in areas of steep falloff, this may well matter.

    Also there is no guarantee that in areas of steep falloff that the falloff will occur at the same rate across all wavelengths. When people make bulbs balanced for particular colour temps, they don't state (AFAIK) the colour temp distribution. They probably optimize it at the center and that's that. And with fluorescents, I'd certainly not expect a bulb stated to be 5000K to actually be 5000K from on end of the tube to the other. According to my eyeballs, there is always a difference of colour along a fluorescent bulb, with yellowing toward the ends. This effect seems to become more pronounced as the bulb ages.

    ...just some stray thoughts....
    "Only dead fish follow the stream"

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  10. #10

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    As Keith points out, the sensors are probably a cause as well. The Sixticolor is pretty low tech and it uses two CdS cells that are filtered and they drive the needle in the meter to display the color temp. More red light and it swings to lower color temps and more blue light and it goes to higher temps. I have one and I've noticed that if it is not held perpendicular to the light and with equal amouts of illumination across the white disk on the meter, you can get the needle to swing back and forth. More white light over the red sensor and it shows the the temp getting warmer and more white light on the blue side and it goes cooler.

    Another issue is that you may have a green and blue tube in your enlarger - and it's really not going to have a color temp to it. A meter will give a reading, but color temp (when measured by something as simple as a Sixticolor) really needs to be from a black-body radiation source, like the sun or an incandescent source. The Sixticolor will try it's best, but it's really not suited for a non-continuous spectral distribution as found in fluorescents.
    Last edited by Kirk Keyes; 03-05-2010 at 05:16 PM. Click to view previous post history.
    Kirk

    For up from the ashes, up from the ashes, grow the roses of success!

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