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  1. #11
    Photo Engineer's Avatar
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    Yes, a densitometer uses a different light source and collector than a scanner does, and this changes the way it sees light. This is especially true of reflection density.

    PE

  2. #12
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    You must face the fact that a densitometer compares two light intensities, the incident and the transmitted. All it knows about the colors of the two intensities is..? Let us suppose that the sensor of the densitometer responds equally to each wave length of the visible spectrum. Unless your printing material has the same response spectrum, you cannot learn from the densitometer what will be the effect of a certain measured light value on your printing material. You could measure a very bright red light that would have no effect on the paper. Density measured with a very narrow light spectrum may tell you little about the printing density of a negative that appears to the eye to be neutral gray.

    I have a home designed and built easel exposure meter that I can calibrate to read projection density as long as I am measuring a negative developed in a certain developer. If I change to a different developer, especially if I change to a staining developer, I must recalibrate both amplitude and gradient if I expect to be able to predict the density that will be printed at a measurement point. It is especially difficult if I am using VC paper whose contrast varies with color of light. There are many times when a simple test strip is more valuable than any meter.

    You may not get the expected result from anything less than a spectrum analyzer.
    Gadget Gainer

  3. #13
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    I suspect you are making far too many assumptions in your 'experiments'. i.e. are you always using the same wavelength of light for your tests? Or are you using multiples? What control have you designed in for light scattering due to reading a particulate/composite image like film vs. the step tablet? What control have you made to compensate for the difference in light source/transmission between the densitometer and scanner. It seems these are just a few factors that can affect your outcome significantly!

    I'd probably start over making your own step tablet with the same film - that way you can control for any differences in the materials.

  4. #14
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    Well, here is an interesting experiment.

    Read a print with a step scale with a good densitometer.

    Read it again under viewing conditions with a spot photometer calibrated to read the densities the same as if it were the densitometer. In other words, use it as a calibrated eyeball.

    The results are striking and very surprising. Even with the best matched units under identical illuminants, the results are different.

    PE

  5. #15

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    Quote Originally Posted by Photo Engineer View Post
    Well, here is an interesting experiment.

    Read a print with a step scale with a good densitometer.

    Read it again under viewing conditions with a spot photometer calibrated to read the densities the same as if it were the densitometer. In other words, use it as a calibrated eyeball.

    The results are striking and very surprising. Even with the best matched units under identical illuminants, the results are different.

    PE
    Only surprising if you have made bad assumptions without understanding how a densitometer and light meter work.

  6. #16
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    A spot photometer isn't a light meter. They make units that supposedly read like either reflection or transmission densitometers. MacBeth makes a "viewing chamber" to simulate the standard viewing condition or the standard integrating sphere of a densitometer.

    Thus you can effectively duplicate a densitometer in the open air and then measure things visually and with an instrument.

    PE

  7. #17

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    Quote Originally Posted by Photo Engineer View Post
    A spot photometer isn't a light meter. They make units that supposedly read like either reflection or transmission densitometers. MacBeth makes a "viewing chamber" to simulate the standard viewing condition or the standard integrating sphere of a densitometer.

    Thus you can effectively duplicate a densitometer in the open air and then measure things visually and with an instrument.

    PE
    So how does it know what the contrast ratio is and how does that compare to the contrast ratio of the densitometer? That is, a densitomter can be zeroed on its light output so readings are relative to the light output. What do you zero the hand held densitometer on so that you know what the relative brightness is?

  8. #18
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    The Macbeth unit could be coupled to the Macbeth densitometer for calibration. I have to admit that I never calibrated one. I was just given the calibrated unit and taught how to use it in conjunction with the densitometers. We then could correlate what the eye "thought" it saw, vs what the instruments read.

    The calibrated viewing cabinet was about the size of a modern home buffet or about 7 ft high and about 6 feet ling and about 3-4 feet deep. An adjustable overhang allowed for the different lighting conditions to match the densitometer integrating sphere.

    The photometer was mounted at standard viewing distance from a print at an angle of 45 degrees. The densitometer collected light at about 45 degrees as well.

    This data was all suplemented by use of a goniophotometric reading to explain what was going on.

    Basically, a densitometer integrates all reflected or transmitted light, but the eye only sees from one perspective over about 3 degrees to 5 degrees or so. This difference relates to the surface characteristics of the print and causes variations in density as a function of viewing angle that straight densitometry does not pick up even though the instrumentation is identical.

    The same is true of a scanner BTW.

    PE

  9. #19
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    I've thought of a better way to describe this!

    On a sheet of graph paper, imagine a teardrop. The X axis is angle and the Y axis is density. This is a representation of a plot using a goniophotometer. It measures density vs angle of view. Now, imagine a set of teardrops drawn inside of each other and getting smaller. This is the characteristic curve read by a goniophotometer.

    Now, imagine that a scanner looks from the point of the teardrop down. One point source down through the material (and back up if it is a print).

    The eye and the photometer look at a 3 - 5 degree segment of the side of the teardrop at the viewing angle.

    The densitometer is the value obtained by integrating the entire teardrop into a value read through an aperature usually placed at a fixed angle such as at 45 degrees.

    Even if all is calibrated to read all densities as being equal they still get different answers.

    Hope this helps with the above.

    PE

  10. #20
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    I will continue this old thread

    rob champagne posted that interesting link http://www.unblinkingeye.com/Articles/Densi/densi.html

    I guessed that it can be used as correction table between visual and uv densitometers.
    But will it really work that way?

    For example, if I get visual densities for film X developed in Pyrocat-HD and got reading 1.10, can I convert it to UV reading with using that table (for this example, result would be 1.59).

    As the sensitometry has turned out to be not so accurate, is the error caused by using such table for correction tolerable. I don't need really exact readings, just reading for calibrating my N, N- and N+ developments.

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