Ramble on a new darkroom. And LEDs for contact printing.

Discussion in 'B&W: Film, Paper, Chemistry' started by CBG, Jun 10, 2009.

  1. CBG

    CBG Member

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    Miscellaneous thoughts as I start up a new darkroom, my first in thirty plus years.

    I am setting up to do contact printing but do not wish to use an enlarger as a light source. I'll be working with 8x10 negs primarily, and 5x7s otherwise, and don't want to use an enlarger as an overly complex lamp. It occurred to me that I could use blue and green lights as exposing light sources and avail myself of the benefits of split grade printing. So I obtained one each of a blue and a green LED unit pre wired as a screw in bulb. Each screw in unit has some 24 individual LEDs in it.

    Despite many years lapsed, things feel utterly familiar, except for the contact printing. In the past I always used an enlarger as the light source to do contact prints, and 4x5 film or smaller. Now, with 8x10 negs and a very different light source, the mechanics of making a print change.

    After hearing so much about how little light LEDs supposedly put out, I was surprised that my LEDs are so bright that I am having to cut way down on their intensity to get workable exposure times. To obtain soft even illumination of the printing frame, I bounced the LEDs off my rather low ceiling, and have exposure times about 2 seconds. Way too short for any meaningful control I bought the blue and green units on this: www.superbrightleds.com/specs/E27-W24.htm page. Each unit runs off a separate Time-O-Lite timer to keep things simple. Now I'm in process of putting a dark gray painted area on the ceiling where I am bouncing the LED light so I can get some thing more like the 20 to 40 second area.

    Even with the way too short times I can see that the contrast range should be very workable. The test prints range from verrrry soft with the green LED to very hard with the blue LED. If I find I want to extend the contrast range further, I'll try painting the bounce zones on the ceiling blue purple and yellow green to max out the respective hard and soft contrast emulsion layers.

    I also bought a red LED unit to test as a safelight. I have not yet had time to evaluate that. It seems very bright and very red so I'm hopeful.

    So far, I dislike the contact printing frame I just bought from PF. I got the 11x14 frame so I can do 8x10 negs within a 11x14 paper size to get really wide borders. Basically it seems well constructed, but I don't like the snapping metal clamps that hold the back on. The clamps are noisy, unkind to borderline arthritic hands, and slow since there are six clamps to do and undo for every test.. Does anyone else know of any other brands on the market with more humane clamping action???

    I want to explore the use of print pre-flashing for contrast control after I get the basic contact printing setup working.

    Guess I'll need to post photos in the darkroom portraits thread soon. It's going to be an interesting voyage.
     
  2. Jerevan

    Jerevan Subscriber

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    From the description of your set-up, I assume you do silver printing on variable contrast paper. In that case, I think it is easier to use a heavy plate of glass, putting the negative right on to the paper. If you use some sort of mat or foam under the paper you will get good contact between the neg and the paper. Otherwise your ideas seem great! Post more of your experiences with the LEDs as you go along.
     
  3. glbeas

    glbeas Member

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    Have you tried the leds with a dimmer yet? Extended exposure times will allow for burning and dodging of the contact print as well as local contrast control.
     
  4. Lee L

    Lee L Member

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    The electronic circuitry built into these units for converting 120VAC to the 2-5 VDC with limited current for driving the LEDS allows only limited control with a dimmer, and it may be hard to adjust accurately. In addition, the vendor posts this warning in the product description:

    It might be better to try these http://superbrightleds.com/specs/e27-w8.htm as an alternative. (BTW, the blue and green dominant wavelengths are switched on this data page.) These lamps have 8 LEDs rather than 24, and appear to be the same output and same color per LED.

    Because the output of the lamps is well matched to the sensitivity of the VC layers of the paper, they are more efficient than you might assume.

    Lee
     
    Last edited by a moderator: Jun 11, 2009
  5. CBG

    CBG Member

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    Unfortunately, these LEDs have a dire warning about dimmers and any other control circuits so dimming is probably not an option for them. Too bad since dimming had been a back up strategy...
     
  6. CBG

    CBG Member

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    Yup. I hadn't read down before I made my last post. You found that text too.

    I'll check out that other set of bulbs - it never struck me that I might be getting bulbs too strong. In the meantime I have darkened the area I use for bouncing the light off of. Haven't had time to test.
     
  7. Struan Gray

    Struan Gray Member

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    There are LED light panels sold for - don't laugh - discos and DJs which would make a nice light source for contact printing. They dim, can have variable colour, and will even pulse on and off in time to your darkroom music. There are UV ones which are (just) deep enough to do some alt process work.

    http://prolight.co.uk/item/ledj63/
    http://prolight.co.uk/item/ledj66/

    You can of course make your own using simple electronics breadboarding kits and packs of LEDs and limit resistors from eBay. This is a nice project (hint: it doesn't have to be UV):

    http://www.instructables.com/id/UV-LED-Exposure-Box/
     
  8. Lee L

    Lee L Member

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    superbrightleds.com also has 12VDC light bars and various other 12VDC configurations that can be easily and properly dimmed with a potentiometer (standard 'volume' control) and a 12VDC power source. I do this with 12VDC LED safelights and a variable DC power supply from partsexpress.com like this one: http://www.parts-express.com/pe/showdetl.cfm?Partnumber=120-536 which provides switchable 3 - 4.5 - 6 - 7.5 - 9 - 12 VDC

    You could run that power supply at 12VDC and put a potentiometer in line for continuously variable output from a 12VDC LED array.

    Lee
     
  9. George Collier

    George Collier Member

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    These LED light source options are kind of interesting, especially for a non electro-techie like me.
    Would it make sense to have a forum dedicated to this technology for the future, so that all threads would be in one place?
     
  10. jgjbowen

    jgjbowen Member

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    I have found a vacuum easel much easier to work with than a printing frame.
     
  11. Joe VanCleave

    Joe VanCleave Member

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    Conventional resistive light dimmers, built for incandescent bulbs, don't work on LEDs because the LED has a minimal bias voltage it needs to maintain in order to work, since it's a semiconductor device. Flourescent lamps have a similar problem with not being able to be dimmed using a resistive-type dimmer switch, since they too require a minimum voltage in order to ionize the gas into plasma.

    A better idea worth trying is a DC motor control module, the type used to vary the speed of a motor by chopping the duty cycle. For use with an LED, the dimmer would merely pulse the LED off and on, the duty cycle of which will determine average exposure.

    ~Joe
     
  12. BetterSense

    BetterSense Member

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    What you describe is pulse-width modulation. It's the most power-efficient way to do it, but I accomplish my LED safelight (and LED flashlight) dimming with a common 7805 voltage regulator IC, available at radioshack for a couple dollars. It can easily be wired up into a linear current source, with a simple pot to adjust the brightness.
     
  13. domaz

    domaz Member

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    I hope you are dimming the current and not the voltage. Varying the voltage and letting the current float is usually a bad idea and can cause your LEDs to lose life.
     
  14. BetterSense

    BetterSense Member

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    No, you can easily wire up a 7805 as a current source. I don't think I've ever used one as a voltage regulator, actually.
     
  15. CBG

    CBG Member

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    I need to learn more about how one calculates the correct circuitry for LEDs. Unfortunately I am electronics illiterate; the explanations I have read on the web make no more sense than Sanskrit to me so far. I need to keep reading till I find a way into the subject. There is so much basic theory and terminology I am missing. I have no idea, for example, what a bias voltage is, or what chopping the duty cycle by using a DC motor control module implies.
     
  16. CBG

    CBG Member

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    That's a great idea!
     
  17. BetterSense

    BetterSense Member

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    When I said 7805, I meant to say LM317.

    [​IMG]
     
  18. paul_c5x4

    paul_c5x4 Subscriber

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    Even although your sketch shows an LM117 :surprised:
    That aside, the circuit will work for any 3pin fixed voltage regulator including the 78xx & 79xx series.
     
  19. Steve Smith

    Steve Smith Subscriber

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  20. Struan Gray

    Struan Gray Member

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    I think that is why it is good that ready-made panels are now available. The ones I linked to include dimming, colour adjustment and a nice stable case, all for not much more than the cost of parts (in small quantities).

    Making your own can be fun - and with a basic electronics education not terribly hard - but getting a large array of LEDs to illuminate evenly and reliably takes a little care.
     
  21. Steve Smith

    Steve Smith Subscriber

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    Here is the circuit I use for controlling small DC motors. It will work for LEDs but will need a series current limit resistor fitted.


    Steve.
     

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  22. domaz

    domaz Member

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    Switching current regulators are the most efficient and best thing to use for dimming. Buckpucks are good and not too expensive- some will even take AC power right out of the box and support using a Pot for dimming. Can't get much easier than that.