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  1. #11
    RalphLambrecht's Avatar
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    Quote Originally Posted by Steve Smith View Post
    It doesn't stop some people asking where they can get replacement filters from as theirs have 'faded' though!


    Steve.
    Steve

    My understanding is that dichroic filters are interference filters that selectively reflect the unwanted portion of the spectrum, passing only the desired color. Since the unwanted energy is reflected, and not absorbed by the filter, dichroic filters do not fade.

    http://en.wikipedia.org/wiki/Dichroic_filters

    However, if something is wrong with your filtes, It's probably easier to get a new (used) dichroic head than replacing the filters.

    If that is not successful, you can try here:

    http://www.dichroicfilters.co.uk/
    Regards

    Ralph W. Lambrecht
    www.darkroomagic.comrorrlambrec@ymail.com[/URL]
    www.waybeyondmonochrome.com

  2. #12
    onepuff's Avatar
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    Quote Originally Posted by onepuff View Post
    HID lamps are not suitable for enlarging as they are not "full spectrum" lamps. They emit light at a number of wavelengths which when combined give light at a given colour temperature but have significant gaps in their spectrum so will not render correct exposure results when using photographic paper.
    Unfortunately, fluorescent lamps present a similar problem to HID lamps in that the light they emit is composed of several discrete wavebands. Fluorescents use a mixture of phosphors, each emitting a wavelength of light so they do not have a continuous emission spectrum. This is also the reason they tend to have a low CRI, even lower than most metal halide lamps.
    " ... a cook who relies on nothing but a sharp knife has no guarantee of producing excellent dishes." - Yoshihisa Maitani

  3. #13
    onepuff's Avatar
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    Quote Originally Posted by RalphLambrecht View Post
    Steve

    My understanding is that dichroic filters are interference filters that selectively reflect the unwanted portion of the spectrum, passing only the desired color. Since the unwanted energy is reflected, and not absorbed by the filter, dichroic filters do not fade.

    http://en.wikipedia.org/wiki/Dichroic_filters

    However, if something is wrong with your filtes, It's probably easier to get a new (used) dichroic head than replacing the filters.

    If that is not successful, you can try here:

    http://www.dichroicfilters.co.uk/
    There are a number of companies in the UK who supply dichroic filters and these can even be cut to order to suit a specific shape or size requirement. I have dealt with UQG before and they seem to be a good company. Others you may consider are http://www.comaroptics.com/ and http://www.knightoptical.co.uk/. There are more out there too.
    " ... a cook who relies on nothing but a sharp knife has no guarantee of producing excellent dishes." - Yoshihisa Maitani

  4. #14
    Nicholas Lindan's Avatar
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    Quote Originally Posted by ic-racer View Post
    dichroic filters are ... Heat and fade proof
    Well, almost...

    A dichroic filter is a resonant cavity for light. It resonates at the pass/block wavelength - but also at sub-multiples. A bit like a musical instrument that produces not only the fundamental tone but also higher octave harmonics. A filter that passes at 700nM (deep red) will also pass at 350 nM (deep purple/near UV). This isn't much of a problem for most photo processes - though dichroics would make lousy safelight filters.

    To block the higher harmonics the filters incorporate an absorbing filter. This absorbing filter is subject to fade just like any other colored filter.

    The resonant cavity of a dichroic is made of very very very thin layers of metal and conductive metal oxides - so thin the metal is transparent. These coatings are subject to abrasion and corrosion if the protective over-coat degrades. Close proximity to a halogen lamp is a good degrader.

    Though they last a lot longer than wratten filters in the same duty, they are still subject to fade and degradation. It is not unheard of for them to need replacement. There is no need for precautionary replacement. When they fail it is quite apparent with blistering, peeling and mottling. Storage in high humidity environments, like the 10 years an abandoned enlarger can spend in the corner of a garage, can destroy the filters - but by then most of the enlarger will also be quite rusty and the lens covered in mold.
    DARKROOM AUTOMATION
    f-Stop Timers - Enlarging Meters
    http://www.darkroomautomation.com/da-main.htm

  5. #15

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    For a B&W multi contrast head would they use a green and blue dichroic filters.
    ?

  6. #16
    Jon Shiu's Avatar
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    Quote Originally Posted by mike c View Post
    For a B&W multi contrast head would they use a green and blue dichroic filters.
    ?
    They use same filters as color head, ie yellow, magenta, cyan.

    Jon
    Mendocino Coast Black and White Photography: www.jonshiu.com

  7. #17

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    Thanks Jon,I have a old chromega head that needs its gel filters replaced and was thinking of using dichroic filters to replace the gel filters.

    Mike

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