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# Thread: Microdensitometery for understanding Stand Development

1. Just for future reference, if we can get to 1/20th on a mm aperatures then graininess measurements may be possible...

From http://www.kodak.com/global/en/profe.../e58/e58.jhtml

THE RMS-GRANULARITY METHOD
The characteristics that cause graininess in a photographic image can be measured using a micro-densitometer to scan density fluctuations in a uniformly exposed and processed piece of film. In practice, a single microdensitometer reading aperture of 48 micrometres in diameter is used. The 48 micrometre aperture corresponds to about a 12X magnification. You could vary the size of the aperture to represent changes in film magnification, but it's not necessary. For most films, the relative differences in granularity will also apply to other magnifications.

The rms deviations of the microdensitometer scan are then calculated using statistical formulas for standard deviation. This is the source of the term "rms granularity" which is generally used to express granularity measurement. You record the standard deviation of density as a decimal number, which is multiplied by 1,000 to make comparisons easier. The resulting number is usually between 5 and 50.

2. For those interested in using a film scanner, here's a nice paper on using a Nikon Coolscan 8000 as an automated densitometer.

http://repositories.cdlib.org/cgi/vi...5&context=lbnl

3. Originally Posted by Kirk Keyes
No hurry, Patrick. I understand the computer crash bit myself. Sorry to hear about the viral encepalitis - I crushed a few of the disks in my neck last month that put my arm into a bunch of pain for some time. Thing's are getting much better but I'm not moving as fast as I would like either...
Hope you guys don't mind a little OT humor. A friend of mine, may he rest with the angels, had to have some discs fused. On his way in to the OR he asked the doctor if he would be able to play the violin when it was over. The doc said "Of course." He replied, "That's strange. I never could before."

When I woke up after the attack of encephalitis, a nurse asked "Mr. Gainer, have you lived all your life in West Virginia?" I said "Not yet."

4. Originally Posted by Kirk Keyes
For those interested in using a film scanner, here's a nice paper on using a Nikon Coolscan 8000 as an automated densitometer.

http://repositories.cdlib.org/cgi/vi...5&context=lbnl
An interesting paper Kirk, Thanks. The custom designed robotic film loader is a neat concept - but not a requirement (I should think) for what we are trying to do.

5. Originally Posted by Kirk Keyes
For those interested in using a film scanner, here's a nice paper on using a Nikon Coolscan 8000 as an automated densitometer.

http://repositories.cdlib.org/cgi/vi...5&context=lbnl
Kirk,
I am attaching a copy of a page from a Philbrick manual I used many years ago when working for NASA. We used analog computers to drive our simulators and for other simulations of dynamic systems when digital computers were not fast enough to run them in real time. I earned my Gadget nickname mostly because my program board often had something plugged into it that didn't come with the analog computer. Anyway, this is where I got the idea for my easel densitometer.

Circuit 2.22 (a) is a practical circuit for a log amp. Replace the resistor R with a phototransistor connected directly to the positive power supply and the output will be a constant times the log of the photo current over quite a wide range. Use the same circuit with the resistor atached to the arm of a pot connected between + and - power supplies to get a reference voltage that you may sum into another op amp (not a log amp) or a simple resistor summing net and then into a panel meter.

Using two log amps allows the level setting and the photosensing to be independently adjusted. Once you have the gain set to measure log, changing the level will not affect that gain. Also, you get a first order temperature compensation. I have never seen the need for any more complex temperature correction, mainly because if the darkroom changed temperature that much I wouldn't be in it.

The 741 op amp is not good enough. It has too much input current. You need a JFET or CMOS op amp. TL082 will do, but they may have something a lot better now. They are still cheap.

The second attachment shows essentially what I have been using. I didn't put in the circuitry for the powers supply. IC voltage regulators are cheap and easy to use. If you want to get fancy, you can use 10 turn precision pots for P1 and P2. Its good to have one for P2 so that you can easily return to a setting. I use the circuit to set printing exposure by Zones, which requires a different setting of P2 for each grade of paper or printing filter. P1 is then dependent on paper speed.

6. Thanks for the circuit, Pat. I think I even have some TL082s laying around. Time to dig around in the basement.

Kirk

7. OK - I've got some light to voltage photodiodes on the way. Should be here next week some time. I got some with blue, green, and red integral filters, and a few without to play with.

The lens on the Taos diodes are 0.75 mm diamter, or 750 um. To get to the sensor size to the equivalent to the 48 um that Kodak mentions for the granularity measurements that I referenced above, then 750/48=15.6 times magnification. I think that can be achieved with a 50 mm lens and a baseboard height of about 850 mm. My Saunders 4500 can do that with no problem.

8. Originally Posted by Tom Hoskinson
The custom designed robotic film loader is a neat concept - but not a requirement (I should think) for what we are trying to do.
No, but I thought they did a good job of discussing the settings for the scanner for precision use.

Kirk

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