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Thread: Color seperation filter used in Drum Scanners

  1. #1

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    Color seperation filter used in Drum Scanners

    I am interested in construction of drum scanners and any information relating to colour filters used between the light source and PMT. i.e. the beam splitter or dichroic mirrors?

    I found a few manuals online, but they don't any specs for the filters used. I am interested in there spectral characteristics specifically.

  2. #2
    8x10, 5x7, 4x5, et al Leigh's Avatar
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    Re: Color seperation filter used in Drum Scanners

    Look on EdmundOptics.com

    - Leigh
    If you believe you can, or you believe you can't... you're right.

  3. #3
    Drew Wiley
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    Re: Color seperation filter used in Drum Scanners

    Well, don't just look at the Edmunds site, but order a textbook from them on this subject. It's fairly complex. I have a huge old OCL (Optical Coatings Lab) catalog that contained a tremendous amount of detailed information on dichroic filters; they also had superb technical assistance over the phone back then, when they were the biggest coating lab in the world. Now all split up, elsewhere. Spectral characteristics shift according to angle of incidence and temperature. It's an entire engineering field in its own right. There is an optical engineer who contributes to this forum as Nodda Duma.

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    Re: Color seperation filter used in Drum Scanners

    Quote Originally Posted by Leigh View Post
    Look on EdmundOptics.com
    - Leigh
    Thanks I had a look, I tried a few searches but could not find any parts that might for any specific drum scanners.


    Quote Originally Posted by Drew Wiley View Post
    Well, don't just look at the Edmunds site, but order a textbook from them on this subject. It's fairly complex. I have a huge old OCL (Optical Coatings Lab) catalog that contained a tremendous amount of detailed information on dichroic filters; they also had superb technical assistance over the phone back then, when they were the biggest coating lab in the world. Now all split up, elsewhere. Spectral characteristics shift according to angle of incidence and temperature. It's an entire engineering field in its own right. There is an optical engineer who contributes to this forum as Nodda Duma.
    I understand the topic well enough. What I am looking for is some of the details of the ACTUAL filters or dichroic mirrors units used in some of these machines. Any actual specs for real parts are of interest.

  5. #5
    Resident Heretic Bruce Watson's Avatar
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    Re: Color seperation filter used in Drum Scanners

    Quote Originally Posted by Ted Baker View Post
    Thanks I had a look, I tried a few searches but could not find any parts that might for any specific drum scanners.
    That's understandable since most manufacturing of drum scanners ended in the mid-late 1990s. You are extremely unlikely to find any third party parts for a drum scanner.

    Quote Originally Posted by Ted Baker View Post
    I understand the topic well enough. What I am looking for is some of the details of the ACTUAL filters or dichroic mirrors units used in some of these machines. Any actual specs for real parts are of interest.
    If that's what you need, you should talk to Aztek. They still make a few drum scanners a year (so rumor says), and have at least some inventory of parts they use to repair Aztek / Howtek drum scanners. It's possible that Dainippon Screen is still viable. Maybe ICG. But Aztek is more or less the "last man standing" in the drum scanner biz.

    Bruce Watson

  6. #6
    Drew Wiley
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    Re: Color seperation filter used in Drum Scanners

    What makes you think scanners are all the same in that respect? Anyone seriously in the game would have ordered up a custom coating run to their specifications. You can't just go out and grab a filter. If you want "real parts" you'll probably have to cannibalize them from "real machines", just like everyone else who now has to do to keep these kinds of machines going, or else find someone in the business of providing parts via that same method.

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    Re: Color seperation filter used in Drum Scanners

    Quote Originally Posted by Drew Wiley View Post
    What makes you think scanners are all the same in that respect?
    Not necessarily the same, but not that different either as they are all trying to solve the SAME PROBLEMS... There is always something to learn...

    Plus you can contrast how they attempted to solve certain problems compared to CCD technologies.

    These drum scanners in this thread appear to have very similar spectral responses: https://www.largeformatphotography.i...-Drum-Scanners

  8. #8
    Drew Wiley
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    Re: Color seperation filter used in Drum Scanners

    A former member with a lot of experience is Jim Browning, who still owns patents on big laser printers. He might still be listed. Otherwise, contact him at Digital Mask.

  9. #9

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    Re: Color seperation filter used in Drum Scanners

    Most drums use an hallogen lamp (of about 55w) and 3 dichroic mirrors to split the 3 beams.

    This arrangement has an advantage, the 3 beams are mostly proportional so if the lamp changes a bit its power then the 3 beams will change by similar amount, not exactly the same because a shift in power may change a bit the effective kelvin.

    The splits should be similar than those in dichroic color heads of enlargers or those in 3 CCD cameras.

    Some manufacturers do state that they trim the band of each beam with additional interference filters, this is a dichroic mirror working as a filter. I guess that the cut-off nm are undisclosed, one may need an spectrometer to sample each channel. I guess that a blind region between 2 channels may help a consistent separation across several films.

    IMHO, it is mostly irrelevant what specific spectral nature drums have. In theory, independently from scanner illumination, and for the same film, the output from two scanners can be perfectly matched with a 3D LUT. This is because scene spectral information has ben reduced to a 3 dimensional space.

    Color layers of a film cannot have arbitrary shapes, for each silver level after first developer there is a resulting spectral shape in that layer(after color developer), this is what allows a 3D LUT to contain all color matching information between two systems.

  10. #10
    Drew Wiley
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    Re: Color seperation filter used in Drum Scanners

    Specific coatings can be employed to fine-tune such things, Pere. And then you've got the issue of patents. Workarounds in some manner or another either had to come into play between different manufacturers, or licensing was involved. And there was unquestionably an evolution within the prime time frame of scanners themselves. And there are various kinds of color film, some from obsolete processes quite different from what you describe, as well as other media, that get scanned as well. You're oversimplifying all this. That's why contacting an actual engineer involved in the specifics would be in order.

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