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  1. #11
    hrst's Avatar
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    Quote Originally Posted by onepuff View Post
    You may well encounter problems using LEDs with VC paper as they are not "full spectrum" lamps.
    Why would this cause problems? Please elaborate.

    Silver halide are sensitized to two wavelength regions, blue and green, and you need to have those, and you will most probably have those. If you mean that sensitization dye absorption spectra would mismatch with LED spectra, I really doubt that. Neither are that sharp.

    Using blue/green led arrays is naturally the easiest way compared to white-light source and dichroic filters. Mechanics are ruled out, it's electrically controlled, all you need to do is mix the light but that will be as easy or easier than in dichroic filter heads.

    In fact it may work well enough even without mixing/diffusing but I would make sure it is mixed so that blue and green comes from same (all) directions.

    Furthermore, with LED arrays it is possible to adjust separately exposure and also contrast at different locations! I have been thinking of for example 8x8 matrix for 6x6 film. This would work additionally to manual dodge&burn.

    vedmak, have a go at it and report back how it works!

  2. #12
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    Quote Originally Posted by hrst View Post
    Why would this cause problems? Please elaborate.

    Silver halide are sensitized to two wavelength regions, blue and green, and you need to have those, and you will most probably have those. If you mean that sensitization dye absorption spectra would mismatch with LED spectra, I really doubt that. Neither are that sharp.

    Using blue/green led arrays is naturally the easiest way compared to white-light source and dichroic filters. Mechanics are ruled out, it's electrically controlled, all you need to do is mix the light but that will be as easy or easier than in dichroic filter heads.

    In fact it may work well enough even without mixing/diffusing but I would make sure it is mixed so that blue and green comes from same (all) directions.

    Furthermore, with LED arrays it is possible to adjust separately exposure and also contrast at different locations! I have been thinking of for example 8x8 matrix for 6x6 film. This would work additionally to manual dodge&burn.

    vedmak, have a go at it and report back how it works!
    Discrete single chip LEDs emit light at a single wavelength while "white" LEDs contain 3 chips (usually chip on board these days) which when combined give white light at a specific colour temperature depending on the emitted wavelengths of the chips used and how much each chip is driven. This allows the colour temperature to be tuned. Colour changing LEDs use 3 chips also in red green and blue and are pulsed on and off by a controller to vary the brightness of each of the chips to mix the desired colour.

    There are two problems when using LED light sources for photographic printing. The first is That the actual emitted wavelength from single colour LEDs is precisely that - one wavelength. Incandescent and halogen lamps produce light at many wavelengths and the better lamps have a flatter spectral curve giving a higher CRI. Filtration is used in enlargers to allow a band of a number of wavelengths of this spectrum to pass and photographic papers have a spectral response curve which will vary depending on the part of the spectrum received. VC papers produce differing contrast dependant on the part of the spectrum they receive. LEDs only produce a very narrow part of the spectrum so the paper's response will only be to this narrow part of the spectrum when exposed to light from an LED. This means that you will not get the same response from the paper as you would expect from a light source with a wider spectral curve combined with filtration with a wider band pass. The paper will only give the response within its spectral curve to the single wavelength it receives. This leads us to the second problem - colour mixing. If you have two un-mixed discrete single colour LEDs they will produce 2 wavelengths so the paper will respond to both of these and give a combination of the contrast you would expect from both wavelengths within its response curve. To get a single new wavelength of light from the combination the light needs to be collected via a lens system (the design would depend on the emission pattern from each chip) and needs to be mixed within a mixing rod or tube. To vary the contrast of the paper however you need to be able to tune the colour of the light (as you would using filters). To do this with LEDs means pulsing the LEDs to vary the brightness and this consequently varies the exposure time required. It is like trying to expose with a lamp which flickers at a varying rate. To overcome this difficulty it may be possible to build a separate LED module for each desired colour of light but this would be expensive or you could measure the output from the mixer for each desired wavelength and vary the exposure to suit.

    I hope I have clarified what I believe are the difficulties of using LEDs as a light source for printing. I believe it is much easier to start with a broad spectrum lamp then use known filters to achieve the same result. As I said though - it does no harm to experiment.
    " ... a cook who relies on nothing but a sharp knife has no guarantee of producing excellent dishes." - Yoshihisa Maitani

  3. #13
    Lee L's Avatar
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    Here is konakoa's page on LED enlarger heads, with examples of prints made with a blue/green LED light source.

    http://www.deadbread.com/crumbs/23c.html

    Lee

  4. #14
    Steve Smith's Avatar
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    I have successfully converted a DeVere 5x4 enlarger to LED using the RGB PCB from an LED stage light like this: http://www.soundandlights.eu/large_p...LED_Par_64.jpg

    Using just the blue LEDs will give grade 3.5, possibly 4. Someone here has managed grade 5 by including some UV LEDs into his design.

    EDIT: The 'someone' I am referring to is in the link Lee posted above).


    Here is my initial post: http://www.apug.org/forums/forum43/6...-source-2.html

    And here is a post with some sample images: http://www.apug.org/forums/forum43/7...ht-source.html

    Quote Originally Posted by onepuff View Post
    There are two problems when using LED light sources for photographic printing.
    I believe that results trump theory every time.

    Quote Originally Posted by onepuff View Post
    To do this with LEDs means pulsing the LEDs to vary the brightness and this consequently varies the exposure time required. It is like trying to expose with a lamp which flickers at a varying rate.
    Pulse Width Modulation is a very common way of controlling LEDs and is what I intend to do with mine. The frequency can be in the tens of kHz and isn't really the same as flickering. However, limiting the current with a series resistor will work just as well. No need to over-complicate matters.

    Quote Originally Posted by onepuff View Post
    it does no harm to experiment.
    Indeed!


    Steve.
    Last edited by Steve Smith; 04-16-2011 at 09:11 AM. Click to view previous post history.
    "People who say things won't work are a dime a dozen. People who figure out how to make things work are worth a fortune" - Dave Rat.

  5. #15
    RH Designs's Avatar
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    Most blue LEDs are not "blue enough" to match the sensitivity of the hard emulsion on VC paper. That said, there are LEDs available with the right spectrum. If you choose the LEDs carefully you can achieve a contrast range much wider than that available using white light and filters. I have obtained an ISO(R) range of 45-200 from Ilford Multigrade 4 RC using just blue and green LEDs of the right wavelength - this is quite a lot wider than with under-lens or dichroic filters. You don't need UV LEDs, just the correct wavelength blue ones. The bad news at present is that LEDs of that wavelength are not available in high-power versions, so you need rather a lot of them!

    Onepuff: it is not true that LEDS emit only a single wavelength, a glance at the data sheet of any LED will show you that. Also, many white LEDs use a phosphor illuminated by a blue chip to emit white light, they are not a combination of RGB chips.
    Regards,
    Richard.

    RH Designs - My Photography

  6. #16
    Steve Smith's Avatar
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    Quote Originally Posted by RH Designs View Post
    Also, many white LEDs use a phosphor illuminated by a blue chip to emit white light, they are not a combination of RGB chips.
    As a user of many white LEDs (at work) I would say that most white LEDs are modified blue types.

    Have a look here: http://www.designledproducts.com/ to see the clever sort of things being done with LED lighting now (I'm not giving away any secrets though as we are subject to a confidentiality agreement).

    Whilst white LEDs might not have a full, even spectrum, they are full enough that we can filter them with translucent coloured inks to give red, green, blue, orange and violet (see applications in the link above).

    One of the salesmen/applications engineers from the company I linked to is also a photographer who has recently bought himself an enlarger. I know that he has also become a member of APUG. Perhaps if he sees this, he could add some further information.


    Steve.
    Last edited by Steve Smith; 04-16-2011 at 09:16 AM. Click to view previous post history.
    "People who say things won't work are a dime a dozen. People who figure out how to make things work are worth a fortune" - Dave Rat.

  7. #17
    Mainecoonmaniac's Avatar
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    Does any old timers remember the Minolta 45A? It uses xenon tubes instead of LEDs. I'm wondering varying the exposure of the different colored LEDs to change contrast?

    http://item.mobileweb.ebay.com/viewi...d=220748638246

  8. #18
    hrst's Avatar
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    Quote Originally Posted by onepuff View Post
    Discrete single chip LEDs emit light at a single wavelength . . . That the actual emitted wavelength from single colour LEDs is precisely that - one wavelength.
    No. LEDs are far from monochromatic. For example, take a look at LED datasheets. They show the spectra. Typically, the bandwidth measured at 0.5*intensity is something around 20 - 70 nm. This is far from "one wavelength".

    Furthermore, sensitizing dyes are not sensitive only to one wavelength - they also have a broader sensitivity spectrum.

    This is why I don't believe the mismatch would be a real problem.

    If the wavelengths (LED emit maximum vs. sensitizing dye absorption maximum) do not coincidence perfectly, it shouldn't cause any harm, just some loss in sensitivity, thus need for more light as some of it is wasted. If the mismatch is severe, it would prohibit the extreme contrast grades (00 and 5, for example), but I think this won't be a problem. The papers are anyhow designed for tungsten light sources that are very low in low-wavelength blues, thus I'd guess the sensitization goes to cyanish blue. And, I would give an educated guess that the green used in power LEDs is spot-on or very near.
    Last edited by hrst; 04-16-2011 at 07:36 PM. Click to view previous post history.

  9. #19

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    Quote Originally Posted by silveror0 View Post
    konakoa,
    Can you please enlighten me a bit regarding "white" LEDs? What is the color temp of "white"? The reason I ask is that after reading Ilford's website info on the lightsource for their VC papers, they say that their filters will work OK with any incandescent source. Incandescents are about +/- 2700 deg-K. Incandescents are also going to become harder and harder to find in the near future, or so I hear from retailers who are stocked to the ceilings with CF bulbs as the current trend.
    Silveror0, the white LEDs I used are rated at 4500K. The reason I choose that version was that they were the brightest available of all the white LEDs. It takes a lot of LEDs to get the equivalent brightness of a single fluorescent or tungsten enlarger lamp.

    4500K does work well and I can print at all of the contrast grades with it. Yet as you noted VC filters are designed with much warmer tungsten lamps in mind. The effect I've noted is that VC paper prints to a higher grade by default with the white LEDs I used (4500K is cooler/more blue). A print in my condenser enlarger with no filter requires a #1 or a #1/2 VC filter in the LED enlarger to get the same contrast grade.

    Coincidentally, the 4500K LEDs I choose are a near perfect match grade for grade for the Aristo V54 fluorescent lamp I have. Even though one is white to the eye and the other a blue-green, the filters print the same contrast ranges on each grade despite the distinctly different lamps. I thought it was rather interesting. This was with Ilford RC paper.
    Last edited by konakoa; 04-16-2011 at 09:26 PM. Click to view previous post history. Reason: clarification

  10. #20

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    I wish there were spectral response data available for available variable contrast papers. Unfortunately, that data does not seem to be available anywhere.

    Alan

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