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  1. #31
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    What we need is the step wedge on the right and a grating that produces an "image" as high as the step wedge. In simpler terms, the wedge should be about 4x5 and the grating should supply uniform illumination over that area. Otherwise, the result will be useless. Stouffer Iand EK) made wedges with different size steps and in different size sheets. Kodak sheets were up to 8x10, and the smallest step was 0.3 inches (IIRC). Steps were in 0.15, 0.3 or 0.6 increments.

    PE

  2. #32
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    So projecting a nice, big and evenly illuminated spectrum is the crucial thing here.
    If you are the big tree, we are the small axe

  3. #33
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    Wouldn't one with reduced number of steps still be quite useful? Even with just one mid-density, we could measure the relative sensitivities. AFAIK, there should be no contrast difference between wavelengths in a BW material, so density difference at mid-density should be directly comparable to shadow speed at that wavelength. That being said, of course a step wedge helps to find the speed directly without using a densitometer (or scanner).

    I actually was thinking about this project two years ago and have a prism for this purpose. But I think that with a prism, there is a risk of unwanted reflections in the prism itself and in the optical system, too, risking some white light leak-through which would be detrimental for the measurement. I suppose that the diffraction grating has smaller risk if done right, but it is hard verify by eye that you are not having any leak. You should have a sharp-cutting red, green or blue filter and place that filter on a region with supposedly no such wavelength to verify that. So, I'm going the LED way first just because it's so simple; but I'm going to make it small. Maybe I can fit 4-5 steps.

  4. #34
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    You may have a situation where you did not add enough dye. With a limited scale, you would not see the tiny bump of the sensitivity region with that tiny amount of dye added. So, we used a big scale relatively speaking.

    And, a variation in contrast as a function of wavelength is not unusual. After all, you are using white light for exposure and the curve is the integral of all wavelengths but with a spectrosensitometer you actually can see these variations.

    PE

  5. #35
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    OK, I was thinking something like this. Any comments appreciated!

    For situations where more steps are needed, 2 or more subsequent exposures with different exposure times could be used side-to-side.
    Attached Thumbnails Attached Thumbnails spectro.png  
    Last edited by hrst; 06-20-2012 at 01:14 PM. Click to view previous post history.

  6. #36
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    The exposure timer and the time recorder are really awesome; great ideas!

    You mentioned a diffuser, and will this be integrated with a step wedge?

    Presumably the LED will be in a cavity of sorts, and the diffuser placed some distance away; perhaps along the bottom surface where it will make contact with the test film? The "cells" will have to be isolated well of course, to prevent any cross-talk between spectral regions.

    Honestly, it's a brilliant little device you're dreaming up here... LEDs open up so many possibilities.
    If you are the big tree, we are the small axe

  7. #37
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    Quote Originally Posted by hrst View Post
    OK, I was thinking something like this. Any comments appreciated!

    For situations where more steps are needed, 2 or more subsequent exposures with different exposure times could be used side-to-side.
    The scales you show are just fine. You must remember that for negatives you must go from Dmin to Dmax. This, of course, is the raw form of the emulsion.

    If you do not go to Dmax, then you can miss the true extent of sensitivity.

    What you show appears to be a reversal scale.

    PE

  8. #38
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    Quote Originally Posted by holmburgers View Post
    Presumably the LED will be in a cavity of sorts, and the diffuser placed some distance away; perhaps along the bottom surface where it will make contact with the test film? The "cells" will have to be isolated well of course, to prevent any cross-talk between spectral regions.
    Yeah, you got the details perfectly right! Then, the step wedge will be mounted to the diffuser and the step wedge is in direct contact with the film being exposed.

    I'm probably just going to print custom step wedges. Those digital photography people who also occupy our club darkroom, have bought there a $2000 pigment inkjet that is GREAT for printing everything except photographs; for example, PCB exposure masks. I measured a Dmax of 2.8 when printed on a OHP film, and when printed at 1200 dpi, pixels can be seen with a microscope, so this is a great tool for making good enough custom step wedges. (Naturally, a spectral response of the black pigment ink is not guaranteed to be perfectly even, unlike those high-quality and expensive step wedges.)

  9. #39
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    More ideas:

    Making it smaller, simpler & cheaper by leaving out the step wedge (or multiple leds) completely and making the LED spots smaller, and make it blink N times with increasing exposure, so that you move it along the material and get a step wedge-like exposure.

  10. #40
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    Like this.......
    Attached Thumbnails Attached Thumbnails spectro_v2.png  

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