Heat Filters

Discussion in 'Enlarging' started by Ray Rogers, Jul 14, 2010.

  1. Ray Rogers

    Ray Rogers Member

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    Does any one have a 301A filter or information on it or similar items?
    What is the cheapest heat filter around?
    Does anyone have any data on any filter of this general type?

    (If the 301A is technically different, thats OK; I am interested in both)

    Thanks,

    Ray
     
  2. Neal

    Neal Subscriber

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  3. Ray Rogers

    Ray Rogers Member

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    Neal,

    Thanks... that was helpful.

    It was pointed out to me that my post was less than clear... I should clarify that filters for enlargers might be different from the 301A type filter, and that filters for other purposes (eg. conversion, duplication and/or projection) might be of interest to me as well.

    My hope was to get more info... esp. exact transmission data,
    and locate an inexpensive source as well.
     
  4. Monophoto

    Monophoto Member

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    My first enlarger was a Durst F60 (way back in 1977). Shortly after getting it, I read somewhere that it would be useful to have a head-absorbing glass. Considering the design of the Durst, that didn't make a lot of sense, but logic didn't stop me, and I ordered the heat absorbing glass offered by Durst as an accessory for the F60. That was in the good old days when you could actually order an odd-ball part at a local camera store.

    Anyway, when it came, it struck me that it didn't appear to be anything more than an unusually thick piece of glass - about 3/16" thick. It may have had some unusual spectral absorption characteristic, but that certainly was not apparent to this observer. Instead, it seemed to me that the shear bulk of the glass was the factor that made it heat absorbing.

    So that leads me to wonder if all that is really required is thick glass.

    More recently, I was in a local glass shop and noticed that they offered glass cut to specification in a variety of thicknesses.
     
  5. Kirk Keyes

    Kirk Keyes Member

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    Ray, if enlarging isn't your application. what is? THat might help us figure out what your looking for.
     
  6. Photo Engineer

    Photo Engineer Subscriber

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    Generally, heat absorbing glasses, to be really good, are slightly thicker than ordinary "window" glass and appear decidedly cyanish when viewed on edge compared to normal glass.

    PE
     
  7. wiltw

    wiltw Subscriber

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    From Kodak...

    Optical Printers
    If your equipment holds filters between the light source and the original transparency/neg, use KODAK Color Printing
    Filters (Acetate) for color-balance adjustments. Use KODAK WRATTEN Gelatin Filter / Color Compensating if
    you must place the filters between the lens and the duplicating film/print paper. Also use a heat-absorbing glass, an
    ultraviolet-absorbing filter (such as KODAK WRATTEN Gelatin Filter No. 2B or a KODAK Acetate Filter / Color
    Printing CP2B).
     
  8. Ray Rogers

    Ray Rogers Member

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    Hi Kirk.

    Yes I understand. I was quite sleepy when I first posted this but essentially
    I am looking for the transmission spectra of different types of glass filters.

    I know that the publication
    Special Filters from Kodak for Technical Appications
    would provide much information but cannot locate it.

    Thanks
     
  9. Ray Rogers

    Ray Rogers Member

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    Physics 101

    Yes they all do seem to be rather thickish...
    I think that they actually are compounded differently, however.

    This does bring up an interesting question regarding how filters work,
    particularily about what role their thickness and reppitition might play in their behavior.

    For simplicity, all examples here assume 100% transmission for all other wavelengths:

    1.
    If filter "007" (which is 1 mm thick)
    reduces the amount of wavelength "Agent X" by 50%

    How much would the same filter reduce that wavelength by
    if the filter were twice (ie 2 mm) as thick ?

    And if that same filter was 2 meters thick?

    2.
    If the 1mm thick filter above reduces a certain wavelength by 50%
    does this mean that a second filter (of the same type & thickness)
    placed in the same light pathway will remove an additonial 50%?

    thereby outputting only 25% of the orignial wavelength?

    3.
    Same question as #2, but now the filters are different in composition:

    Assume you have two different filters, "007" and "86".
    Both reduce the transmission of wavelength "X" by 50 %

    Can you reduce the amount of wavelength "X" by 50% twice
    (once for each filter) to get a final of 25% ?

    ---

    This may be very basic for some of you,
    but I think how filters add and subtract (?) can be confusing for many people.

    Many People
     
  10. Ian C

    Ian C Member

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    Percentage of Heat Transmitted by Multiple Sheets of HA Glass

    From what I’ve read HA glass is an alloy of glass that contains an iron compound that absorbs heat from the light passing through.

    The thicker the HA glass, the more heat is absorbed. For that reason, two sheets in series can absorb more than one.

    HA glass also gets hot quickly and might need to be cooled by a sufficient forced air flow to draw off the heat and limit its temperature. Usually 300F(149C) is the limit stated in the data I’ve read.

    Suppose that the heat absorbed by each sheet of HA glass is a%. For n identical sheets of HA glass in series the percent of the original heat transmitted through the system is

    T = (1 – a)^n

    The percentage of heat absorbed is

    A = 1 – (1- a)^n

    Example: Consider HA glass that absorbs 40% of the heat passing through it.

    Then a = .40

    If you had n such identical HA glass sheets in series then the heat transmitted is

    T = (1 - .40)^n = (.60)^n

    On sheet would transmit 60% of the heat.

    Two sheets would transmit (.60)^2 = .36 or 36% of the original heat.

    Three sheets would transmit 21.6%, and so forth.

    But each sheet also absorbs some visible light as well. I verified this with a light meter on the baseboard of my Beseler 23C II with and without the HA glass. This loss isn’t much but is measurable with a regular light meter. The amount of heat present at the HA glass on my 23C modest, so a cooling fan isn’t needed. The bulb is only 75 watts.
     
  11. Kirk Keyes

    Kirk Keyes Member

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    TO add to what Ian said, here's a nice graph from Schott showing the IR transmittance of one of their IR absorbing glass:
    http://www.us.schott.com/borofloat/english/download/transmit_ir_range.pdf?highlighted_text=IR

    Keep in mind the graph is in transmittance. If it was in absorbance, then you could more easily see that a sheet twice as thick had twice the absorbance.

    Ray - Your questions #2 and #3 are more easily seen when using absorbance. That's why most all lab tests that use spectrophotometery use absorbance rather than transmittance, as the math can be done by using simple addition or subtraction.

    Keep in mind the Beer-Lambert Law (aka Beer's Law):
    http://en.wikipedia.org/wiki/Beer–Lambert_law
    It tells us that if the filter (or cuvette) thickness is doubled, then the absorbance is doubled.
     
  12. Kirk Keyes

    Kirk Keyes Member

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    Absorbance = -log (percent transmittance/100)

    The abs of a 50% transmittance filter is 0.30.

    The abs of a 25% transmittance filter is 0.60.

    Adding two 50% transmittance filters, each with an absorbance of 0.30, gives a filter with an absorbance of 0.60.

    That is, 0.30 + 0.30 = 0.60, or the same as a 25% transmittance filter.
     
  13. Kirk Keyes

    Kirk Keyes Member

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  14. Ray Rogers

    Ray Rogers Member

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    Ian, Kirk,

    Thanks!