Originally Posted by Kirk Keyes
In the sentence that you cite I was talking about the speed point of a print, not negative film speed. There is an ANSO method for determining the speed point for silver papers, and it is the exposure necessary to produce a print density of 0.6 over B+F.
Alternative printers use different speed points for their prints. Dick Arentz, for examples, calculates speed point at 90% of Dmax, taking into account the long sweeping toe of the Pt./Pd. process. I think that is a very reasonable choice for Pt./Pd. and also for the closely related kallitype process (not VDB but kallitype based on ferric oxalate). On the other hand with a process like carbon that has an almost absolutely straight line it makes more sense to me to establish the speed point at between 95% to 100% of Dmax.
However, to be precise about film speeds, they are not determined by using the first step that shows any density. There is also an ANSI standard for making this determination and it is based on the exposure value needed to produce a log density of 0.1 over B+F when development is such that an SBR of 1.3 will give a negative DR of 0.8. I pretty much quote from Davis above but other sources use very similar language.
Sandy writes, "Alternative printers use different speed points for their prints. Dick Arentz, for examples, calculates speed point at 90% of Dmax, taking into account the long sweeping toe of the Pt./Pd. process. I think that is a very reasonable choice for Pt./Pd. and also for the closely related kallitype process (not VDB but kallitype based on ferric oxalate). On the other hand with a process like carbon that has an almost absolutely straight line it makes more sense to me to establish the speed point at between 95% to 100% of Dmax."
Thanks - that what I was wondering about.
"However, to be precise about film speeds, they are not determined by using the first step that shows any density."
Yeah, I know. From the tables in the article it kind of looks like you were just visually comparing step densities. I don't think we would have made it this far into this thread if I was not familiar with this.
Thanks for the answer above.
Last edited by Kirk Keyes; 08-18-2004 at 07:45 PM. Click to view previous post history.
Reason: added more info
Another quick correction -
On page 8 I said:
"The Xrite 810 is designed to do Status M transmission RGB filters. These are designed for use with internegatives that will be printed on color neg paper. This is not the right kind of densitometer for measuring the materials under discussion here. Ideally, you should find a densitometer that uses Status A filters, e.g. the Xrite 811, 820 or 310."
This is not correct - densitometers with Status M filters are fine for this type of testing. Status A filters are recommended for measuring materials designed for direct viewing or projection, such as transparencies. Status M filters are recommended for measuring densities in film that are intended for printing, such as color negative and internegative films, as well as reversal films when used as originals for printing. The use of Status M filters in our situation is completely appropriate here.
Surely, if I know whether it is flat across that band, or if these are widths where the response is down 3 db or 6 db or whatever. I'm nitpicking, of course, but I'm also curious.
It was fun part of the time. I graduated from WVU with a BS in aeronautical engineering and went to work for NACA in 1952. Did analysis of maneuver loads flight test data, analysis of wind tunnel data, fatigue tests of B-47, and without moving from my desk became employed by NASA. I designed star charts for the Mercury astronauts to use as backup reentry guidance, worked on aerial combat simulations, measurement of eye movements for the purpose of improving instrument panel layout, worked on mathematical models of human operators. My last position description referred to me as " an internationally known expert in non-linear mathematical models of the human operator." Somewhere along the line, I took a couple of graduate level courses in Psychology at William and Mary. Now I'm resting.
Patrick wrote: "I'm nitpicking, of course, but I'm also curious."
I'm curious too, as I hope you can tell. I was looking through my 1980 vintage Kodak Wratten Filter handbook today, and I noticed that they had no mention of the Status A and M filters. I had hoped that they were possibly part of that series. If you know of someplace to find absorption spectra for these, I'd love to see them.
You home-made your densitometer right? I can't remember if you said what sensor - Si photodiode? What did you do as far as filtration on it?
"It was fun part of the time..."
As I said, cool!
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I had a long reply set to send and got disconnected from my server. I'm going to write it in notepad as I should have done in the first place.
It was fun while we were doing our own work on a tight budget. When it became mostly monitoring contracts, it was no fun.
I hate when that happens!
Hey, isn't it the middle of the night where you are? I'm going to bed now here on the west coast so I'll see your reply in the morning.
Here it is, whenever you are.
I am using a silicon phototransistor in current mode. The sensitive
area is no bigger than a pinhead. It has sensitivity from UV to IR, but
the response is not flat, being higher in the red.
The meter uses the exponential relationship between current through a
semiconductor diode and the voltage drop across it. I use two op amps,
each with a diode as feedback. In this mode, the current into the
summing junction of one op amp comes from the photosensor and that into
the other op amp comes from a variable resistor. The output of the
first op amp is proportional to the log of the photocurrent. The
resistor is used to set level. The outputs of the two op amps are
summed in a simple resistor network into a potentiometer, the arm of
which goes to a digital panel meter. This pot sets the constant of
proportionality between the log of the photocurrent and the meter
reading. The two pots act independently of one another in that I can
set the level without affecting the scale, or the scale without
affecting the level.
This arrangement allows me to set the scale to read zones for any grade
of paper and the level to make the zone readings come true, at least
for two points on the paper's scale. There is one scale setting that
will read density. So far it has been of great use for comparative
tests and in printing. The use of two log channels provides at least a
first order compensation of temperature effects. The residual is taken
care of by the fact that I don't want to be in my darkroom when it is
too hot or too cold.
The probe is a simple affair on about two feet of twin lead. In
measuring projected density there can be a problem with flare in the
projection lens. I put a mask in the negative carrier just large enough
to pass one step of the step wedge, and slide the wedge into the slit
to make each reading. I block off extraneous light from other sources.
When I use it for printing, The same flare that the photosensor may see
will also br seen by the printing paper. In this sense, it has an
advantage over densitometers that are outboard of the enlarger on
negatives that are to be enlarged. My meter could be adapted to work
over a light table to measure for negatives to be contact printed, and
it could be equipped with an attachment to read reflection density.
Excuse the length, but I thought some of you might be interested in the
I do not currently have any set of filters for special purposes. If I
had my druthers, I would have a fixed, flat, wide band response with a
set of filters to modify the response as needed. If anyone knows of a
phototransistor with very low dark current that fits the basic
requirement, I would like to know about it.
It is interesting to note that when I worked for NASA, much of our
simulation was done with a large analog computer.You could fit maybe 6
of the op amps into an average bathtub. Of course, they operated with
voltage swing of + or - 100 volts, but they required constant
attention. The rectifiers and amplifying elements in these things were
vacuum tubes,and if the air conditioner ever failed, we couldn't use
the thing. It took about 9000 watts to run it. There were 100
amplifiers, some of which were dedicated integrators. They were
programed by wiring plug boards to connect various amplifiers,
integrators, resistors, capacitors, pots, and gadgets dreamed up by
Gadget Gainer. The other guys said they could always tell which board
was mine because it had something hanging on it that didn't come with
Enough for now.
And I do not know how it came to be double-spaced. It was not so in Notepad. Maybe I shouldn't do these things at 3 AM.
Patrick wrote, "I do not currently have any set of filters for special purposes. If I had my druthers, I would have a fixed, flat, wide band response with a set of filters to modify the response as needed. If anyone knows of a phototransistor with very low dark current that fits the basic requirement, I would like to know about it."
Good luck finding that! Let me know too if you do.
You did not mention your light source - I'll assume you are using a tungsten lamp of some sort. So a tungsten lamp, combined with your IR-loving photodiode is probably severely biased towards the IR. For making "visual" readings, you could add a temperature correction filter such as a 80A to take some of the excess red from the lamp out, and then perhaps a second 80A combined with a IR cutoff filter to take some of the red/IR response out of the photodiode. This would go a long way towards adjusting your system.
For making measurements of specific colors -R, G, B, or UV you could add a Wratten #92 for Red, a #93 of Green, a #94 for Blue. The 92/93/94 combo is used for densitometric measurements of color films and papers. The precision of these filters according to the guide is +/- 5%. The guide does recommend using a #301A Infrared rejection filter in combination with these filters as they all pass IR.
For more accurate measurements, there are Status A, D (?), and M filters, which according to the guide, "are carefully selected and calibrated to conform to close spectral tolerances. Their use allows more direct comparison of measured densities with other similarly equipped electronic densitometers." They must also be used with the #301A IR cut-off filter.
Kodak does not sell Status filter despite making them - they must be purchased from densitometer manufacturers (at least in 1980...).
Narrow band color filters could also be used - the #25 for red, #61 for green, and the # 47B for blue.
Partrick, for UV you could add the 34A I mentioned to Sandy yesterday IF you could find a way to filter out the blue that it passes. I would suggest looking for a dichroic filter for this though.
I suspect that the filters in Sandy's Gretag are probably not gel filters, but are dichroic filters because of the narrowness of the bandpass. This is actually better than a gel-based filter as the dichroic filters will essentially last for ever, do not fade on exposure to light, and can have much sharper shoulder's than the gels usually do.
Patrick - you mentioned "comparitive" measurements - what do you do for calibration?