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Thread: Photronix Delta III Calibration

  1. #1

    Photronix Delta III Calibration

    I'm getting back into film photography after a 12-or-so year hiatus, and picked up an older Photronix Delta III transmission densitometer to help me in various tasks. Unfortunately, it did not come with instructions or a calibrated wedge. I need to call up Stouffer to get a new wedge, and I'd like to be prepared in case I need to recalibrate the densitometer. Does anyone happen to still have a scan of the instructions & calibration procedures for this densitometer? I took the bottom off & found three potentiometers, and while I could probably figure out how to use them, I would prefer to know the procedure before I get started. I saw some instructions being passed around in a post from 2010, but I hated to reactivate such an old thread!

    If anyone still uses one of these and can help, I appreciate it!

  2. #2

    Join Date
    Jun 2014

    Re: Photronix Delta III Calibration

    A quick Google exercise turns up this thread:
    It features a schematic (of doubtful image quality, but it's sort of readable):
    Looks like P2 is the HI (sensitivity ?) adjustment and P3 is the LO adjustment. P4 sets the zero point for the ammeter element. P1 seems to be the offset adjustment for the 741 used to set the zero point of the sensor/opamp combination.

  3. #3

    Re: Photronix Delta III Calibration

    Koraks, thanks or the tip! I actually found the same post on Photrio after I asked here, and it did put me down the correct path to getting the densitometer calibrated (it still took a lot longer than I expected). I wound up having to replace all three potentiometers. The original potentiometers were simple single-turn trimmer type and their wipers had all been bonded in place with fingernail polish. Instead of trying to clean the fingernail polish off the wipers & traces, I ordered some new, modern, 12-turn trimmers, and they worked great for this project. I'm planning to take a few pictures and put together a little tutorial in this thread on how I did it - just in case someone else needs to keep one of these densitometers working.

    As a possibly interesting and somewhat related aside, I worked at Ritz Camera off and on from 1999 through 2009 and I always thought it was wasteful policy for our X-Rite densitometer to remained turned on 24 hours a day. It just seemed silly because those little calibrated light bulbs were pretty expensive. Now, having my own densitometer and researching the underlying opamp a little, I think I understand why. The opamp offset voltage requirement varies with temperature, so the unit really should be calibrated and used under stable temperature conditions (i.e. after a suitable warm-up period, which could be a couple hours).

  4. #4

    Join Date
    Jun 2014

    Re: Photronix Delta III Calibration

    Sounds good!

    As to the opamp and its offset voltage: it's not so much the opamp, although the 741 is ancient technology and positively abysmal in terms of today's quality standards. Yes, it will drift with temperature. But that's not the biggest problem; it's most likely the dark current of the photodiode, which is also highly dependent on several factors, temperature being a big one. In that sense, keeping the unit turned on might make sense.

    Btw, if you search a little more on Photrio you'll find a fairly recent thread of someone who's in the process of making a transmission + reflectance densitometer with concrete plans to launch it to the market. Might be good to check that out. Due to the technology choices he's made, the whole issue of drift as discussed above is no longer a significant issue at all.

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