Switch to English Language Passer en langue française Omschakelen naar Nederlandse Taal Wechseln Sie zu deutschen Sprache Passa alla lingua italiana
Members: 71,550   Posts: 1,573,051   Online: 826
      
Page 2 of 5 FirstFirst 12345 LastLast
Results 11 to 20 of 42
  1. #11
    RalphLambrecht's Avatar
    Join Date
    Sep 2003
    Location
    Central florida,USA
    Shooter
    Multi Format
    Posts
    6,715
    Images
    1
    Quote Originally Posted by srs5694 View Post
    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...
    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'.
    Regards

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

  2. #12

    Join Date
    May 2005
    Location
    Woonsocket, RI USA
    Shooter
    35mm
    Posts
    2,725
    Quote Originally Posted by RalphLambrecht View Post
    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.
    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.

    A strong cyan filter transmits less blue and green than a weak cyan filter, correct?
    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.

    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'.
    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:

    Quote Originally Posted by Wikipedia
    Light is electromagnetic radiation, particularly radiation of a wavelength that is visible to the human eye (about 400–700 nm), or perhaps 380–750 nm.[1] In physics, the term light often refers to electromagnetic radiation of any wavelength, whether visible or not.
    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.

  3. #13
    RalphLambrecht's Avatar
    Join Date
    Sep 2003
    Location
    Central florida,USA
    Shooter
    Multi Format
    Posts
    6,715
    Images
    1
    Quote Originally Posted by srs5694 View Post
    [i]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.
    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.
    Regards

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

  4. #14
    RalphLambrecht's Avatar
    Join Date
    Sep 2003
    Location
    Central florida,USA
    Shooter
    Multi Format
    Posts
    6,715
    Images
    1
    Quote Originally Posted by srs5694 View Post
    [i]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.
    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.
    Regards

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

  5. #15

    Join Date
    May 2005
    Location
    Woonsocket, RI USA
    Shooter
    35mm
    Posts
    2,725
    Quote Originally Posted by RalphLambrecht View Post
    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.
    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.

    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.
    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.

    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.
    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.)

  6. #16
    RalphLambrecht's Avatar
    Join Date
    Sep 2003
    Location
    Central florida,USA
    Shooter
    Multi Format
    Posts
    6,715
    Images
    1
    Quote Originally Posted by srs5694 View Post
    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.
    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.
    Regards

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

  7. #17

    Join Date
    May 2006
    Location
    Aurora, IL
    Shooter
    35mm
    Posts
    1,967
    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.

  8. #18

    Join Date
    Jun 2004
    Location
    Portland, OR
    Shooter
    4x5 Format
    Posts
    3,268
    Quote Originally Posted by srs5694 View Post
    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.
    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...
    Kirk

    For up from the ashes, up from the ashes, grow the roses of success!

  9. #19

    Join Date
    May 2005
    Location
    Woonsocket, RI USA
    Shooter
    35mm
    Posts
    2,725
    Quote Originally Posted by Kirk Keyes View Post
    Quote Originally Posted by srs5694
    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.
    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.
    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.

  10. #20

    Join Date
    Jun 2004
    Location
    Portland, OR
    Shooter
    4x5 Format
    Posts
    3,268
    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.
    Kirk

    For up from the ashes, up from the ashes, grow the roses of success!

Page 2 of 5 FirstFirst 12345 LastLast


 

APUG PARTNERS EQUALLY FUNDING OUR COMMUNITY:



Contact Us  |  Support Us!  |  Advertise  |  Site Terms  |  Archive  —   Search  |  Mobile Device Access  |  RSS  |  Facebook  |  Linkedin