difference in LED or Dichro filters

Discussion in 'Enlarging' started by John Koehrer, Mar 20, 2009.

  1. John Koehrer

    John Koehrer Subscriber

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    Curiosity strikes!
    When split grade printing with a color head the recommended filtration is max Y & C. When constructing a head, mention is made of Blue and Green LEDs. Why the difference?
     
  2. RH Designs

    RH Designs Advertiser Advertiser

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    VC papers are sensitive only to blue and green light. A yellow filter removes blue light, leaving green (and the rest) so is equivalent to a green LED, and magenta removes green leaving blue. One is an additive system (LED), the other subtractive, but the result is similar.
     
  3. John Koehrer

    John Koehrer Subscriber

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    It had to be that simple didn't it.
    Thank You
     
  4. Bob F.

    Bob F. Member

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    Added point: green light activates the lower contrast component and blue light the harder contrast component of the emulsion. Hence, a mixture of the two provides the variability of the contrast from hard to soft.
     
    Last edited by a moderator: Mar 20, 2009
  5. MattKing

    MattKing Subscriber

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    If you turn your cyan filter to max, you can pretend you are using an additive system :smile:.

    I sometimes do this, when I need longer exposure times.

    Matt
     
  6. RalphLambrecht

    RalphLambrecht Subscriber

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    ?

    I believe, all this does is add density. Cyan filtration removes equal amounts of green and blue light.
     
  7. nworth

    nworth Subscriber

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    An ideal cyan filter (there is no such thing) will have no effect. It transmits all the blue and green but blocks the red. In general, the magenta (for blue) and yellow (for green) controls on the dichro head are all you need to control VC paper contrast.

    For LED heads it's a different story. They use red, green, and blue lights in an additive fashion, but the controls may be set up to imitate the familiar magenta, yellow, and cyan. In that case, the cyan control sets the brightness of both the blue and green lamps, the yellow control sets the brightness of the green and red lamps, and magenta sets the red and blue lamps. You should be able to use the yellow and magenta controls pretty much like the dichro controls to change contrast, but the cyan control will add exposure to both the high and low contrast emulsions. In any case, the light would be different than what you get from a dichro head, so some calibration will be needed.
     
  8. MattKing

    MattKing Subscriber

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    Actually Ralph, the cyan filter removes the complements to green and blue - it removes magenta and yellow.

    But you are right, in combination with the magenta and yellow filters it does add neutral density.

    Oh, and "Way Beyond Monochrome" is one of my favorite references (along with the tables from your website) :smile:.

    Matt
     
  9. srs5694

    srs5694 Member

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    Some basic color theory:

    Visible light is often described as consisting of three primary colors: red, green, and blue. In reality, of course, it's a continuum of wavelengths; red, green, and blue are simply labels we apply to particular ranges of wavelengths. The RGB distinction is good enough for many purposes, though.

    The primary colors can be combined (added) in various ways:

    • red + green = yellow
    • red + blue = magenta
    • green + blue = cyan
    • red + green + blue = white

    Note that these are additive colors. You'd get these colors if you had LEDs of these colors (or other light sources with appropriate filters in front of them) and mixed the light from these LEDs. This is how TVs and computer monitors work, incidentally. What gets confusing, and has resulted in some incorrect information posted above, is that filters work on a subtractive system. In a subtractive system, you start with white light and can subtract colors, which produces the color name we apply to the filter:

    • white - red = cyan
    • white - green = magenta
    • white - blue = yellow
    • white - red - green = blue
    • white - red - blue = green
    • white - green - blue = red

    When using an additive enlarger (something with RGB LEDs or other light sources with RGB filters), the mixture of red, green, and blue lights is handled more-or-less directly by controlling the relative brightness of these three colors. When using most enlargers with cyan, magenta, and yellow filters, a white light source's red, green, and blue wavelengths are blocked by the cyan, magenta, and blue filters, respectively. The two systems are theoretically identical, although of course imperfections in the filters and the spectral sensitivity curves of the papers may result in some deviations from a theoretical ideal. For instance, theoretically adding cyan filtration will have absolutely no effect on most B&W enlarging papers, since cyan filtration removes red light (not green and blue light, as incorrectly stated in one earlier post), and B&W papers are insensitive to red light. In practice there may be some small effect of cyan filtration because the filter might block a little green and/or blue light or because the paper may have a small sensitivity to the red light that's blocked by the filter.

    FWIW, I own a Philips PCS130 enlarger with PCS150 color controller. This combination uses three halogen bulbs with red, green, and blue filters in front of them. The color controller lets me adjust the brightness of the three primary colors independently of one another. This works differently from the system that nworth describes ("the cyan control sets the brightness of both the blue and green lamps...", etc.); the "red/cyan" control adjusts the brightness of the red lamp, the "green/magenta" control adjusts the brightness of the green lamp, and the "blue/yellow" control adjusts the brightness of the blue lamp. I found that using this system makes it easier for me to understand how the emulsions respond -- but I'm not sure if that's really the system or if how it all works just finally "clicked" with me at the time I bought this enlarger.
     
  10. RalphLambrecht

    RalphLambrecht Subscriber

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    You, nworth and srs5694 are right, of course. The cyan filter removes red light and transmits blue and green. However, my logic was as follows:

    If B&W paper is only sensitive to blue and green light, and dialing-in cyan requires longer exposure times without changing the contrast, then cyan must remove blue and green light at equal amounts. A strong cyan filter transmits less blue and green than a weak cyan filter, correct?
     
  11. RalphLambrecht

    RalphLambrecht Subscriber

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    Nice write up. Thanks for taking the time. It was helpful and highlighted my mistake, of which there are many.

    To be picky, I believe the term 'visible light' is tautological. As I have been taught, 'light' is defined as the visible range of electromagnetic radiation (roughly the wavelength from 400-700 nm). Consequently, there is no such thing as ultraviolet or infrared light but only ultraviolet or infrared radiation. Light is visible by definition. Saying 'visible light' is like saying 'they arrived one after the other in succession'.
     
  12. srs5694

    srs5694 Member

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    If your premise is true, then the conclusion does follow logically. As my own enlarger uses an additive system, I can't test whether the premise is true or not -- and of course, even if it's true for one enlarger, it might not be true for another. If it is true for a given enlarger and its cyan filter, them my comment is that it's probably a rather poor cyan filter if it removes significant amounts of blue and green light -- that, in addition to the red light that the filter necessarily removes, would give the cyan filter a significant neutral density (ND) component.

    OTOH, perhaps some cyan filters are deliberately designed with significant ND components out of necessity or to simplify color balancing on color papers. If somebody with a subtractive color enlarger would care to run some tests to get at least a rough quantification of this effect, that would be useful information in this thread.

    FWIW, with my Philips PCS130/PCS150 with additive color head, dialing red light (cyan filtration equivalent) in or out makes absolutely no discernible difference in prints, or at least didn't the time I tried it. I probably tested this with Agfa MCP310 RC paper, but I may have tried it with other paper(s) instead of or in addition to the Agfa. My conclusion is that my enlarger's red filter removes nearly all of the green and blue light from the light source, and the paper(s) I used wasn't/weren't very sensitive to red light.

    That depends on what you mean by "a strong... filter," I suppose. This gets into the more complex reality of light wavelengths, vs. the simple red-green-blue distinction I outlined earlier. For instance, a filter might pass just a small range of wavelengths in, say, the red region, centered around the peak sensitivity of color papers' red-sensitive layers; or it might pass a much wider range of wavelengths, possibly even including green and blue light. A cyan filter specifically, of course, must pass both green and blue light, but it could do so by passing a wide range of green and blue wavelengths, or a narrow range of green and a narrow range of blue; and it might block some of the light even from the regions it's supposed to be passing. I don't know enough about how filters in subtractive color enlargers are designed to say what's common.

    The way I would use the phrase "a strong... filter" would be in reference to a filter that passes most of the light in a fairly narrow range but little or no light outside that range, so "a strong cyan filter" would pass most of the light in the blue-green range but little or no light in the red range. If a cyan filter blocks significant amounts of green and blue light, then it's got a significant ND component.

    What you're saying makes sense; however, some Googling suggests that the phrase "visible light" is quite common, even on sites with some authority, such as a NASA page on the spectrum and a page on types of light at Georgia State University. The Wikipedia entry on light begins:

    So it looks to me as if some people in some fields might use the term "light" to refer exclusively to visible wavelengths, but this practice isn't universal.
     
  13. RalphLambrecht

    RalphLambrecht Subscriber

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    You are correct. It isn't universal, but vage terms creeping into common language does not mean that they have to be used in a technical discussion either.

    Light is the visible portion of electromagnetic radiation. I guess, sometime ago someone said 'visible light' instead of 'visible spectrum' or 'UV light' instead of 'UV radiation'. Everyone is free to call it as they like, of course, but I prefer and propose to call it as intended to avoid confusion.
     
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  15. RalphLambrecht

    RalphLambrecht Subscriber

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    Absolutely correct, but I think we are forgot the impact of this fundamental statement in the previous filter discussion.

    A filter (cyan or otherwise) filters all light, because it does not have 100% transmittance for any wavelength of light. However, a color filter is useful, because it is bias. Cyan for example, filters mostly red and little green and blue, hence its inherent color. The important point for us is that cyan filters 'filter' some green and blue light too, and not in insignificant amounts, as the previous text indicates. In fact, to compensate for the unintended filtration, filters need exposure compensation factors, and if applied, the full color bias can be taken advantage of.

    The cyan filter, as in most color enlargers, has a variable strength, which can be dialed-in, typically with a numeric setting from 0-130 (Durst). This is what I meant with 'weak' and 'strong' filtration. The difference between the min and the max setting is significant to print exposure (at least a stop or more). This proves, strong cyan filtration filters almost all red but also significant amounts of green and blue. A weak cyan filtration does not. Consequently, a variable cyan filter can be used to control the amount of green and blue light reaching the paper. The effect is similar to neutral-density filter, because a variable cyan filter 'filters' green and blue (on top of red) at equal amounts, and that was my original point.

    I hope this explains why I said a cyan filter would filter green and blue (besides its main task of red filtration, which I forgot to spell out). The variable cyan filter in many enlargers do enough of it, for us to make use of them as neutral density filters if needed.
     
  16. srs5694

    srs5694 Member

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    I cited several references from at least somewhat authoritative sources that use "light" to refer to all wavelengths, not just the visible ones. If you've got sources that contradict that, then please post them. Otherwise, to my mind, it's you (about whom I know nothing) vs. NASA and the Georgia State University Department of Physics and Astronomy. No offense intended, but in that contest, I'll accept NASA's and GSU's use of the term "light" as being more authoritative, although I recognize that the pages I cited seem intended for general public consumption rather than for use by scientists.

    That's what I was referring to earlier as ND filtration -- filtering some green and blue in addition to red (for a cyan filter). The question in my mind is how much ND filtration exists in a typical color head's filters. As my own enlarger uses additive (RGB) filters rather than subtractive (CMY) filters, I can't answer that question experimentally.

    I understand your meaning, then. I thought you were referring to a filter (you did write "...a strong cyan filter..."), not the dialed-in filtration on a color head. In that context, dialing in more cyan filtration should increase whatever ND effect exists on the green and blue wavelengths. I personally wouldn't want to rely on this or promote its use, though. For one thing, AFAIK color heads aren't marked in ways that make it clear how much ND filtration you're getting from each color channel, so you'd need to run tests to calibrate your filtration. For another thing, if ND filtration varies from one color enlarger's filters to another, any specific values you learn for one enlarger might not apply to another. At the extreme there are additive enlargers, such as the LED heads mentioned in the title of this thread or the Philips Tri-One enlargers. Dialing in "cyan filtration" on these just reduces the red light intensity, so there's no ND filtration going on. If you run into an enlarger that has a cyan filter that blocks more green than blue (or vice-versa), you'd also run into contrast shifts if you filter in this way.

    If you want ND filtration, a better approach is to increase all three filters by a fixed amount. My understanding is that most color enlargers' filters are marked in Kodak units, in which a value of 30 is equal to one stop of exposure. Thus, increasing all three channels' filtration by 30 will reduce exposure by one stop. (In the case of an additive enlarger, you'd only need to adjust the green and blue channels, but for a subtractive enlarger, the ND component of the cyan filter could be significant enough to require its use, too.) So for instance if you were using a C/M/Y filtration of 0/50/20 and you wanted to reduce the light by one stop, you'd change it to 30/80/50. Adjusting all three filters in this way should work with any color enlarger, provided that its filters aren't faded. (If the scale on the color dials isn't in Kodak units, you'd need to adjust the values appropriately.)
     
  17. RalphLambrecht

    RalphLambrecht Subscriber

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    Let's stick to the scientific definition of 'light' and some logic, and let's forget the big names for a moment. We both could cite many sources where the term 'light' is used and just as many where the term 'visible light' is used. The internet is full of both, and even some dictionaries show both terms. I don't disagree that big names use the term 'visible light' in casual descriptions. The term is used all over, but that doesn't make it scientifically correct. I'm merely saying that is a sloppy and description, best left to general conversation.

    One of the best sources for optical fundamentals is 'The Principals of Optics' by Hardy and Perrin. On page one, they define light as:
    'The term 'light' is used to describe radiant energy that is capable of affecting our sense of sight.'

    'Physiscs' 2nd Edition by Tipler states:
    'Electromagnetic radiation with wavelengths in the range of about 400 to 700 nm, to which the eye is sensitive, is called light.'
    (note: no mention of 'visible light', and the sentence shows how silly that would be)

    The Oxford Dictionary defines 'light' as:
    'The natural agent that stimulates sight and makes things visible.'

    Webster's simplified definition surprised me:
    'Something that makes vision possible'

    Nevertheless, they all agree that light and vision are inseparable connected. If light is visible radiation, then there is no such thing as 'invisible light' and also no need for the term 'visible light', since all light is visible by definition.

    By the way, if you look at Hardy and Perrin, then the term 'UV light' or 'infrared light' is also wrong, because they don't affect our sense of sight. I'm sure that you find many reputable sources who got this one wrong too.

    The above error is the reason for my insistence. The danger is not in calling 'light' needlessly 'invisible light'. The danger is to make a grammatical extrapolation from it and wrongly referring to 'UV light' instead of 'UV radiation'. I don't reject trustworthy references, but they still have to withstand the challenge of logic, NASA included.

    However, you are correct that the term 'visible light' is very common and that I lost the fight against the mutilation of language and NASA. Oh, well.
     
  18. Chan Tran

    Chan Tran Member

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    LED's have a much narrow band than a filtered light source, so using LED would work but the response may not be the same.
     
  19. Kirk Keyes

    Kirk Keyes Member

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    Or, for B&W printing, you could just us 0/80/50 as the cyan filter will have little affect on a paper that has little red sensitivity.

    Printing on color reversal paper, it's just as you say.

    And, a photon is a photon, the Universe around. Some are visible, some are invisible, but they are all just photons. The speed of light is not in respect to the wavelength of the light, i.e. it's not only the speed of visible light, the speed of UV radiation, the speed of IR radiation, the speed of microwaves, the speed of x-rays. They are all photons, and all light, and it's simply the speed of light.

    I think Einstein would have differentiated this one if he had found it made a difference. He was a pretty smart guy, afterall...
     
  20. srs5694

    srs5694 Member

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    That's the more important of the recent things this thread is about, though -- empirically, how much of an effect will that cyan filter have on B&W exposure? If the filters were theoretically perfect, the cyan filter wouldn't have any effect at all. RalphLambrecht is claiming that they have a significant effect, although he's not provided detailed data. My own enlarger is a Philips PCS130/PCS150 with additive color, so I have no cyan filter with which to test. If the cyan filter has a non-trivial effect, then you would need to add that 30 cyan to the 30 magenta and 30 yellow to get a full 1-stop reduction in exposure.
     
  21. Kirk Keyes

    Kirk Keyes Member

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    Right - it's all about how pure the cyan filter is in any particular enlarger. An ideal cyan filter will have no effect on typical B&W papers. A poorly made one may have a huge effect. Conversely, an enlarger with an ideal yellow and an ideal magenta filter will be all you need to get 1 stop of ND when using B&W paper that is insensitive to red light.
     
  22. MattKing

    MattKing Subscriber

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    To add one stop neutral density, I put the extra 30 cyan of filtration in, because it just feels better to do so :smile:.

    It might be reasonable to assume, as well, that the magenta, cyan and yellow filters we are using would be similar in their imperfections, so using all three may be more consistent.

    Of course, my preference for all three may have something to do with the colour printing I used to do.

    Matt
     
  23. RalphLambrecht

    RalphLambrecht Subscriber

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    Expose a sheet to a mid gray without any filter, then dial in some cyan, and print again. You'll find out how significant the effect of cyan filtration is. Filters have density. C60 has more density than C90. That's why the cyan filter isused by some as a neutral-density filter. No further data required!
     
  24. RalphLambrecht

    RalphLambrecht Subscriber

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    Kirk

    What do you think should happen when I change the cyan filtration from C30 to C60? Nothing? Not so, the print gets lighter, because the filter is getting denser.

    It has nothing to do with pure, poor or ideal. A filter does exactly that, it filters. Color filters are bias and filter more of one wavelength than of another, but there is no miracle sharp cut-off, and there is no ideal cyan filter. A cyan filter filters all wavelengths of light, just a lot more red than any other wavelength. That's why all filters have filter factors. The denser the cyan filter gets, the lighter the print will get. To avoid that, apply the filter factor or make use of the effect, and use it as a neutral density filter. This doesn't mean it's a poor filter. It means, it's a filter.

    This is easy to test in 5 minutes in the darkroom.
     
  25. RalphLambrecht

    RalphLambrecht Subscriber

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    Kirk

    Please try that. C/M/Y 0/80/50 vs 30/80/50

    You will find that tey are different. The C30 component acts as a neutral density filter and lightens the print. A 60/80/50 filtration will be lighter yet, because there is even more neutral density filtration. They are not bad filters, they are just filters. That's how they work.

    A suggestion was made to dial in all three at once to get a neutral filtration as in C/M/Y 30/30/30. I agree with that. It works for color, because they cancel each other out. It works for B&W, because M and Y cancel each other out and cyan has a neutral effect anyway. If cyan has no effect, 0/30/30 should do the same as 30/30/30, and there should be no need for the cyan component. Of course, that is not so.

    If Einstein could have tested his theories in the darkroom; he would have done so. Some things are easier to try than to talk about.
     
  26. BetterSense

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    I'm a materials scientist and I work with electrical engineers, chemists, and materials scientists on a daily basis, and I read and write technical research papers every day. Regardless of any dictionary definition, I can tell you that in the scientific community, the fact is that it is universally understood that "light" does not necessarily refer to visible light. In technical literature and in person, I hear the word "light" used to refer to radio waves, microwaves, infrared lasers, and xrays and gamma rays, blackbody radiation, and basically any electromagnetic radiation. I opine that the above quoted terminological opinion is quite incorrect and should be abandoned in the interest of understandability. There may be a lay usage of the word 'light' just as there is a lay usage of the word 'work' but in both cases I have to consider the respective definitions currently used uniformly by the scientific community as the ones to be touted about as "most correct".

    The quite distinct concept of 'visible light' is arrived at by integrating the luminance ("light") over one of various well-argued-over luminance functions that supposedly simulate the average human eye's sensitivity curve, to arrive at an artificial picture of "visible light". There is as parallel situation with sound. Scientists refer to everything from infrasonic waves in the earth's crust to far, far ultrasonic lattice vibrations in crystals as 'sound', quite apart from any individual's ability to hear it. If you hang around semiconductor scientists much, you will eventually hear of "phonons" which are quantum "particles" of sound which are shed to crystal lattices during energy transitions! I'm sure that won't fit in the groove of an LP.