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  1. #131

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    Quote Originally Posted by Kirk Keyes
    Sandy - Here you go:

    http://www.bhphotovideo.com/bnh/cont...ku=8207&is=REG

    That's cheap enough I might get one to play with!

    Kirk
    According to information at the B&H site the 403 filter completely blocks wavelengths longer than 360nm.

    I am curious why you believe this is good filter for the kind of testing I discussed given the fact that the peak reading in UV mode of the Gretag D-200-II is 373 nm, with a bandwith of 45 nm?

    Sandy

  2. #132
    gainer's Avatar
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    I think I have cooled down a little.

    Let me reiterate something I think I said a long time ago, but maybe not in this forum.

    If a densitometer has a truly logarithmic response to input current, i.e. if its output follows the equation:

    Eout = k log(Iin)

    and we make the constant k = 1 by adjusting the gain, then:

    Eout = log(Iin)

    Now you see there are no requirements placed on the source of the input current. It could come from a photosensor or it could come from some other source. The meter's output will still be the logarithm of the input current. In our case, we would like to have the input current be a linear function of the illumination on the sensor. Phototransistors are very linear over a long range when used in the current mode, without a load resistor.

    Suppose then that my phototransistor has a transfer funstion like:

    I = K*L

    where L is the illumination of the cell.

    When this current is used as input for the meter, the output reading is:

    Eout = log(K*L)

    or Eout = log(K) + log(L)

    But log(K) is another constant that may be set to zero by the level adjustment on my meter.

    Consequently, whether I use a step density wedge or a precise arrangement of resistors and a regulated power source, the slope of the output vs log(input current) equation will be the same. The only change will be in the intercept of that line. Hard as this may be to swallow, it is true.

    Nowhere in this derivation is there any requirement placed on the color of the illumination. If I measure in a part of the spectrum to which the photosensor is either more or less sensitive, I will find a shift in the calibration curve, but not a change in slope.

    The questions are, how and what do we measure? The questions are answered one way if for some reason we want to know the spectrum passed by a pyro developed negative or step wedge, and another way if we want to know how to estimate paper grade and exposure. In the first case, the H&D curve (I still prefer to honor the men who got us started) would have to be three dimensional, with the spectrum plotted at each point on the log relative illumination axis. Then we would know the spectral content of a pyro-developed piece of film. In the second case, we would want to know the spectral response of our printing material as well as the content of each part of the negative we want to print. I doubt that any of us are willing to undertake all the experiments and concatenation of transfer functions that would be necessary. It will not be done simply by taking densitometer readings. If you are making contact prints by a printing-out process, I'll bet you will still lift up a corner now and again to see if it's done.
    Gadget Gainer

  3. #133

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    Sandy writes, "According to information at the B&H site the 403 filter completely blocks wavelengths longer than 360nm."

    "I am curious why you believe this is good filter for the kind of testing I discussed given the fact that the peak reading in UV mode of the Gretag D-200-II is 373 nm, with a bandwith of 45 nm?"

    Sandy - did you look at this link I gave you on the previous page?

    http://msp.rmit.edu.au/Article_01/06.html

    Again, I say "They say that Tiffen makes a similar filter and B&W as well - B&W 403. They show a Schott UG1 that looks perfect for this test as well."

    Appearantly you did not. From the graphs they show on that web page, it looks to me that the B+W 403 may actually be a better match to your Gretag UV filter than the Wratten 18A.

    I like B&H a lot, but I'm not sure they are a better source of scientific information than a web site devoted to Medical and Scientific Photography. Perhaps B&H has a typo on their page. I hope that they at least got the price right...

    Look at the other link I gave you and then please let me know what you think about that filter.

    Kirk

  4. #134

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    Quote Originally Posted by Kirk Keyes
    Sandy - did you look at this link I gave you on the previous page?

    http://msp.rmit.edu.au/Article_01/06.html


    Kirk
    No, I missed the link on first reading of your message. The charts do indeed suggest that the 403 would be a good inexpensive substitute for the 18A.

    Sandy

  5. #135

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    Patrick - I'm sorry if my comments upset you. That was not my intent.

    Patrick wrote, "Are you also telling me that knowing the exact densities of a negative in any and all parts of the spectrum will somehow cause the paper to respond obediently?"

    No. But it may lead us to making some measurements that may allow us to predict how the paper responds.

    "We can use them to test theories or to make prints. Prints are seldom used to test theories, and theories do not always make good prints. Now I'm telling you something you already know."

    There is an entire science called sensitometry that does this. And we are attemping to scratch the surface of it ourselves here.

    "Let me reiterate something I think I said a long time ago, but maybe not in this forum. If a densitometer has a truly logarithmic response to input current, i.e. if its output follows the equation: Eout = k log(Iin) [...]"

    I believe you did say this in this thread. And I even understand the math and electronics behind it.

    "Nowhere in this derivation is there any requirement placed on the color of the illumination. If I measure in a part of the spectrum to which the photosensor is either more or less sensitive, I will find a shift in the calibration curve, but not a change in slope."

    I'm not saying that your densitometer's slope will change with wavelength - otherwise it would be impossible to build things like spectrophotometers.

    I'm saying the density of the materials you are trying to measure may change with wavelength. And sometimes significantly. That's why it could be important to control both the souce of illumination as well as the spectral response of your densitometer.

    If your densitometer is responding to light from blue to near IR, without some sort of control of the light source and the sensor response, how will it be able to give us an accurate measure (say +/- 0.02D) of the density on a piece of stained film? How will it tell us about the UV absorption? I can't, as it is being influenced by all the other wavelengths that are striking the sensor - it is integrating the densities of all the wavelengths into one number. That is what my response to Donald's question was about.

    And what about making measurements on a something that is obviously not neutral - say a piece of color transparency film. Without controlling the light source and the sensor, any measurements made would not be useful for much.

    Patrick - I understand you are concerned about what wavelengths all these measurements are made at - it's a valid concern. That's why at this point, we should be using standardized materials for our density readings, things like Status filters or filters with specified properties, like the ones in Sandy's Gretag densitometer. At least that we we have some way to try and make comparable numbers. And there are probably better suited filters that could be used for stained negs, but until that has been determined, we must use what we have.

    "In the first case, the H&D curve (I still prefer to honor the men who got us started) would have to be three dimensional, with the spectrum plotted at each point on the log relative illumination axis."

    Yes, I've come to that conclusion as well. I really would like to see a graph of that...

    And I've got no problem with honoring Messrs. H and D. - please do, we owe them a lot.

    "Then we would know the spectral content of a pyro-developed piece of film. In the second case, we would want to know the spectral response of our printing material as well as the content of each part of the negative we want to print."

    I believe that point is what got this entire thread started.

    "If you are making contact prints by a printing-out process, I'll bet you will still lift up a corner now and again to see if it's done."

    Yeah, that's why I don't like printing out papers...

    Kirk

  6. #136
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    Okay. Now my point is that I have a meter that reads out the log of its input current. This kind of meter is well suited to use with silicon sensors in the current mode, both for linearity of photocurrent with illumination and for response speed. I used photoresistors at first, but the risetime of a change in reading was too long. Furthermore. they also had problems with the spectral response.

    The available phototransistors have changed since I built my contraption, I'm sure. I have heard of silicon blue cells that have increased response in blue or decreased in red, I don't know which. as I said, I would like to have a flat response, because then I know what my filters are actually doing. We had a light meter at NASA that none of us could afford that had a flat response. The sensing element seemed to be about the size of a nickel and would have been of little use as a densitometer for our purposes anyway.

    You may have missed the point that I have both slope and intercept adjustable. There are scales on the pots so I can return to any previous setting. I have one setting of slope for measurement of density, and several others for different contrasts of printing paper. The intercept setting varies with paper speed when I am set to read Zones, and with the maximum light level when I am reading density. In my next version, I am going to use 10 turn precision pots which can be set and reset to 1/100 of a turn for the adjustments. Another point I want to stress is that the setting of the slope of the meter response for reading density can just as well be done with a pot, some resistors and a regulated power supply. In fact, that part of the calibration will be built into my next version.

    The secret to using this meter to set exposure for projection prints from stained negatives is to make a test print based on zone 3 where the stain is least. Now I look at the print and see where one of the higher zones lies. I then read that part of the negative and set my slope and intercept if needed to make those two parts of the negative read the two zones that I observed. Now I am pretty well set for deciding which grade of paper to use. Heaven help me when I go to VC!
    Gadget Gainer

  7. #137

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    Patrick wrote, "The available phototransistors have changed since I built my contraption, I'm sure. I have heard of silicon blue cells that have increased response in blue or decreased in red, I don't know which."

    I don't think it's that a Silicon Blue diode has an increased response to blue so much as they have added a filter to reduce the red response. Here's a blurb from a Perkin Elmer data sheet: Blue "series devices have a built-in infrared rejection filter for those applications where a detector is needed that approximates the human eye. Typical transmission of wavelengths greater than 750 nm is less than 3% when measured with an incandescent source operating at 2850 K."

    Check out the Absolute Spectral Resonse graph with the filtered Absolute Spectral Response graph on this data sheet.
    http://optoelectronics.perkinelmer.c...%20process.pdf
    The filtered diode does look pretty close to the response curve that one finds for human vision. A sensor like this would be good in a densitometer that was trying to approximate the resonse of the eye.

    "...as I said, I would like to have a flat response, because then I know what my filters are actually doing."

    That's true. However, if we are using filters to examine a (relatively) small range of wavelengths, then the response curve of the sensor will become negligible as it is the filter that will be in control of the wavelengths for our density measurements.

    "The secret to using this meter to set exposure for projection prints from stained negatives is to make a test print based on zone 3 where the stain is least. Now I look at the print and see where one of the higher zones lies. I then read that part of the negative and set my slope and intercept if needed to make those two parts of the negative read the two zones that I observed. Now I am pretty well set for deciding which grade of paper to use. Heaven help me when I go to VC!"

    I'm amused by that last sentence, because VC papers are where things get hairy, and more interesting.

    For graded papers, I find that stained negs print just like non-stained negs. I probably would not bother using a staining developer unless I was using VC papers. (And that's only because I dont need the extra density or a dual purpose neg like Sandy has designed the pyrocat to do.) Although I do think stained negs are lower in grain - so I guess I would stick with them afterall...

    Kirk

  8. #138

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    Kirk Keyes said:
    For graded papers, I find that stained negs print just like non-stained negs. I probably would not bother using a staining developer unless I was using VC papers. (And that's only because I dont need the extra density or a dual purpose neg like Sandy has designed the pyrocat to do.) Although I do think stained negs are lower in grain - so I guess I would stick with them afterall...


    My results are exactly opposite of your suppositions in this regard. The proportional stain exhibited by certain staining developers is counter productive to highlight tonal separation on VC materials. Most notable in this regard is the stain color of the PMK pyro formulation.

    In fact the proportional stain is most effective with graded materials, I can not understand your statement about stained negatives printing the same as non stained negatives on graded materials. Have you done any testing of the reflection density in the zone VI and higher tonal regions on prints made from negatives developed in both types of developers? If you haven't I think that you may be surprised at the results of these density readings. At least my testing has indicated this.

    Regarding your statement about less grain with staining developers, I have found that ABC pyro exhibits noticeably more grain then some non staining developers.

  9. #139
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    Kirk,

    If I do not know what the response of my sensor is without a filter, then I do not know what it is with a filter, regardless of how much I know about the filter. True, I may know which wavelengths are not passed, but of the ones that are passed what gets through is a combination of the passband of the filter and the sensitivity band of the sensor.
    Gadget Gainer

  10. #140

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    Patrick wrote "If I do not know what the response of my sensor is without a filter, then I do not know what it is with a filter, regardless of how much I know about the filter."

    If you have calibration standards, you do know the response - or at least you can measure/calculate it.



 

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