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  1. #1

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    Microdensitometery for understanding Stand Development

    Pyrocat-HD is my primary film developer and I use it with either minimal agitation or semi-stand agitation. I can see positive differences in my LF contact prints and would like a better understanding of what is going on.

    The approach to understanding that I favor is a Statistical DOE (Design Of Experiments). In order to accomplish this I need to make repeatable, quantifiable (and applicable) measurements.

    My old Perkin-Elmer microdensitometer is not up to the task.

    Can anyone identify a microdensitomery instrument (or alternative) which has the right spectral response coupled with the requisite spatial resolution to quantify changes in microdensity?

    Any recommendations?
    Tom Hoskinson
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  2. #2
    Photo Engineer's Avatar
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    Tom, please let me clarify one point for you.

    Several of my techie friends have told me that a scanner can be used as both a densitometer and a microdensitometer. I have not personally reduced it to practice, but merely have their description of how it is done.

    Basically, as I understand it a set of micro and macro exposures are made and scanned at very high resolution. Then they are all displayed at the same magnficatation on-screen in Photo Shop (forgive the digital here all), and the densities are plotted as a function of distance across the exposure, and also as a function of Log Exposure, so you have a micro denstiometer trace of the line, and the characteristic curve that a particular size line will yield. By further manipulation you can then display the macro and micro images at the same magnficiation and view and compare the results.

    One of the problems is that you must have a practical microsensitometer to get the original images, and to do that you need appropriate charts for making the exposures. So far, I have the charts, but that is about it.

    Forgive any errors above and any misunderstanding, but I am strictly an analog person and do very little digital at all. I am still learning how to use a scanner properly.

    I think your effort here is quite commendable. I will try to do all I can to get some kind of support going at this end from some my former associates here. They may have some suggestions that might help.

    PE

  3. #3
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    Excuse me for butting in. Silicon phototransistors are obtainable with very small apertures and very high sensitivity and linearity. High quality silicon diodes have an exponential voltage-current transfer which, in the feedback loop of a high quality operational amplifier with the phototransistor connected as a current source to the inverting input of the op amp, provide an output that is accurately the logarithm of the input times a constant.

    The aperture of the photocell is ot necessarily the limiting factor, as we are planning to look at very small parts of negatives which an be greatly enlarged, imaged on a very small aperture. A further consideration is that for first attempts, extremely accurate measurements of absolute density may not be required. You will be looking for very small changes in density over very small distances.

    Just some thoughts for anyone who wants to try it.

    I have done the scanner thing, but in order to get a wide range of densities you need a lot of bits per sample. My equipment would only allow 256 gray levels, the increments of which get larger as the level gets smaller in log terms.
    Gadget Gainer

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    Tom et al. - I don't have time now but I'll get my thoughts on this to you all tomorrow.

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    Thanks to PE and Pat for their suggestions.

    Looking forward to your input, Kirk.

    I think one of the keys to this is sufficiently high spatial resolution. It seems that Zeiss has a scanner/optical microscope based microdensitometer with resolution in the 7 micron range. I would like to do better than that if I can.

    A colleague of mine has interfaced a sensor with an optical microscope to scan Xray film and TEM film. I will take a look at that as well.
    Tom Hoskinson
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  6. #6

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    OK - Here's my initial thoughts on the subject. (I love instrument and analysis design, by the way.)

    I see two basic approaches. One can either:

    1) make something, or
    2) adapt existing equipement.

    With that in mind, let's look at Option 1. Going with Patrick's excellent suggestion of using silicon phototransistor would be the first step. Some method would be needed to convert the voltage signal from the op amp into something usable, i.e. a volt meter at the least.

    1.a) Adding on to the phototransistor idea, some sort of an optic path would be needed to isolate and focus on a small section of a negative. And a light source as well. And finally, some sort of mechanism like a micrometer to move the negative.

    For this approach, probably basing the design off of traditional microdensitometers would be best. That means a microscope-like type of optics would be needed.

    This may be a lot of work. Data collection from the volt meter and then conversion to density readings could be painful for the number of readings that would be needed.

    Perhaps easier, would be:

    1.b) Keep the negative in a fixed location, and move the sensor. Some sort of mechanism could be used to move the sensor under a projected image of the neg. Using an enlarger (which most of us already have) to focus and project the image onto the baseboard, a baseboard densitometer like the phototransistor/volt meter combo or perhaps easier, something like on of the old Minolta PM densitometers could be used. The PM (or other baseboard meter) would be direct reading in density, so that would simplify the conversion of voltage readings in to density.

    Things to keep in mind with this approach:

    1.b.1) Move the enlarger up as high as it goes to increase the image size, as well as using a shrt focal length lens to increase the amount of magnification at the baseboard. (We don't need the lens to cover the entire neg, just something that will project a sharp and contrasty portion of the negative.)

    1.b.2) Mask the opening of the sensor to get a smaller aperature to further increase the effective magnification. Ideally, the size of the aperature should be somewhere around the step size of the linear distance that the sensor will be moved.

    1.b.3) Use a set threaded rods (i.e. long screws) to make a "sled" for the sensor to sit on, and then a "dial" could be added so that fine turns of the rod could be measured. A 1 mm pitch rod being turned 15 degrees per step would be 1/24th of a mm increments. Multiple by the baseboard magnification to get the actual step sizes on the negative.

    1.b.4) Use of the color filters of the Minolta PM (or other) baseboard densitometer unit could be used to isolate color/stain effects.

    For Approach 2 - using existing equipment, off hand, I can think of:

    2.a) find a microdensitometer of some commercial equipment that is similar in function. If actual microdensitometers were easy to come by and cheap, we would not be having this thread...

    But I think Tom has a good approach - use a microscope with a digital cameral and take images of the film. I spent about 2 hours trying this one afternoon at a friend's house who is a microscopist. He had a nice Zeiss scope and a digital camera that is designed for use with the scope. I had issues with getting enough magnification while still keeping the image sharp. The contrast was not what I was hoping for as well. And then I had problems getting the image bright enough to shoot with the camera. (I burned a few holes in the test neg a could of times before I figured out how to not do that!)

    But I think this idea has a lot of merit. But it is not a cheap solution for those of us that do not have access to expensive scopes and digital cameras. So on to the next idea:

    2.b) Film Scanners - I think this apporach should be a good one, and since many of us already have scanners, then that is one big hurdle down as far as the equipment side of this problem goes.

    2.b.1) Resolution - I have a Nikon Coolscan V that can do 4000 dpi. That's about 157 points per mm. Looking in James and Mees' microdensity plots, that looks like it should be sufficient, if not just what is needed. Even though that scanner only does 35 mm format, cutting negs up to feed it should not be a issue.

    2.b.2) Data Collection - another big plus for the scanner, simply scan the film. Once the scan is in the computer, then we can use software to get the density readings.

    Take a look at this page: http://www.efg2.com/Lab/ImageProcessing/TestTargets/ (this will be useful for our next big project on how to test for resolution!) and then notice the graphs at the bottom of that page - they used the software that can be found here: http://www.efg2.com/Lab/ImageProcess...xelProfile.htm

    I've played with the Pixel profile software and it is pretty simple and easy. Bring a picture into it, drawn a line across the image, and then it makes a spreadsheet page full of data from the pixels under the line.

    The issues I can forsee with this are not enough resolution in the scanner's density readings - scanners are designed for macrodensity and probably not as much for microdensity. I have a feeling that 8-bit resolution will not be enough. So perhaps 12 or 14 or 16 bit image files would be usable. (I don't know if the Pixel-Profile software can handle these files, then then we can do stuff in photoshop to extract a set of pixel data I'm sure.

    And in general, Ron and Patrick are right that calibration issues will be important. Especially after reading Dr. Henry's account of his attemps at microdensitometery in "Controls in Black and White Photography".

    And targets as well. Perhaps Ron could elaborate on the target he has. I was thinking that a USAF 1951 contact target for resolution would go a long ways here, but the chrome on glass ones that would be best are pretty spendy. I figured photos of the large USAF 1951 target that Edmunds Scientific sells would be good for starting - it has high and low contrast patches, as well as R, G, and B ones.

    Hope this gets the ball moving along on this idea! See, I told you I like this sort of stuff!

    Kirk - www.keyesphoto.com

  7. #7
    mikewhi's Avatar
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    Quote Originally Posted by Tom Hoskinson

    Can anyone identify a microdensitomery instrument (or alternative) which has the right spectral response coupled with the requisite spatial resolution to quantify changes in microdensity?

    Any recommendations?
    Hey, I recall that in the movie ALIENS, the robot-thingy made a handheld device to locate the aliens by identifying micro chances in air density. Apparently when aliens crawl along they screw up air density.

    Do you think that could be adapted to suit your needs?

    -Mike

  8. #8
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    When you are probably not required to read the whole density range of the negative at one setting, The scanner approach may (should) allow biasing the level so that the desired range of densities is in the higher resolution part of the scanners brightness range. E.G., if you are looking at an image of a knife edge, you are more interested in the shape of the top and bottom of the image than in the slope of the drop from top to bottom, and can read the two parts separately. For analyzing such edges, shape is more important than absolute amplitude, I would think, especially if there is an extended enought area on each side of the edge that you can read it with an ordinary densitometer.

    The easel densitometer that I have built and use has both level and sensitivity knobs. I can set the sensitivity to read Zones once I have calibrated it for paper and VC filters, and I can set it to read density of a very small spot, less than 1/2 inch across. The meter is a digital panel meter that can probably be bought for about $15 and has a quite high sampling rate. The phototransistor has a very high frequency response, so I can move the probe around the projected image without waiting for the reading to settle down. Most of the circuitry is in the voltage regulated power supply, and that's not much. It only needs + and - 10 volts. When using it to make measurements I have found that the enlarger lamp is the biggest source of error due to line voltage fluctuations. I do my measurements after most of my neighbors have gone to bed.
    Gadget Gainer

  9. #9
    gainer's Avatar
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    "less than 1/2 inch across" should have read "less than 1/32 inch across"
    Gadget Gainer

  10. #10

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    Thanks, Kirk and Pat.

    I have a Nikon Super Coolscan 8000 ED scanner that will handle negatives up to 6x7 cm and has 4000 dpi resolution. I'm going to try it, but I have my doubts that it is going to do the job in terms of contrast resolution.

    My second approach is the microscope/camera approach. I have image processing software that will handle 16 Bit B&W.

    My Macbeth color densitometer has a spot size that's in the size range Pat describes and I've had no joy reading microdensity with it.

    What I'd like to be able to do is produce a density line scan across a nanometer scale region of the negative and I will need a submicron spot size to do that (plus a really good stage).
    Tom Hoskinson
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