A Densitometer for Reading Pyro Negatives
by Bob Herbst
The last year has brought many changes to my photography and teaching of platinum printing workshops. Kodak finally made good on its attempts to discontinue SO-132 direct duplicating film, which I have used in teaching
workshops for years. NA2 (sodium chloroplatinate) emerged as one of the most significant developments in platinum/palladium printing in decades. I began working in 12x20 format necessitating becoming familiar with a
whole new set of films. And early in 2002, I learned of a commercially available densitometer that can accurately read pyro negatives for platinum printing and other UV processes.
A friend and former workshop student had
recently purchased an X-Rite Model 361T densitometer on ebay. The user’s manual indicated that this particular model had an “ortho” channel and a “UV” channel. He brought the unit over
to my house and I used it to read the same negatives of the Stoufer Step Wedge I had used for my article on “The Effects of Pyro in Platinum Printing”. By comparing the densitometer readings to the derived
densities from the platinum prints and D-76 negative data, to my surprise, the “UV” channel appeared to be accurately reading the negative density of the pyro negative including stain! I plotted the negative
densities from the UV channel on the graphs of negative density for RGB and B&W channels from traditional black and white and color densitometers. That data and chart are below. Red values are omitted.
I compared the green channel readings from the X-Rite 820TR densitometer with the “ortho” (green) readings from
361T unit and the readings were virtually identical – within 0.02 in all cases. This normalizes the data between the
two densitmeters meaning I can draw direct comparisons between the density readings. The readings in the chart
above already have film base + fog (fb+f) desnity subtracted. (The stain added a half stop to (fb+f).) When you compare the UV channel readings from the 361T with the color channels from the 820TR and those from a B&W
densitometer, the stain is adding 2/3 stop more density than the blue channel readings, 1-1/2 stops more density
than the green (ortho) channel readings, and a full two stops more than readings from a traditional black and white densitometer.
The next test was to use it in a real application so my friend was gracious enough to let me use it in my two platinum
workshops later in the year. We used the UV channel for measuring both pyro and non-pyro negatives and used
the readings to establish density ranges of the negatives, the starting contrast mixture, and initial exposure times. It
worked for all negatives just as traditional densitimeters work for non-pyro negatives. Shortly thereafter, I purchased a Model 361T unit on ebay as well.
This unit became invaluable as I began my journey into 12x20 format. There are a very limited number of films
available for this format and I was working with a completely different set of lenses so I really didn’t know where to
start on rating the film speed and development times for the two films with which I was experimenting. By using the
361T densitometer, I was able to very quickly figure out how to adjust film speed and development to yield a good
negative for platinum before ever making a single print. I used an iterative process of exposure, development and
proofing the negatives on AZO. I read the negatives and made adjustments accordingly with each change of film
speed or development time. The densitometer also helps determine starting print times because I can now read the
negative densities directly and establish the printing time by comparing to the print times of the equivalent step in a Stouffer step wedge.
X-Rite Model 361T densitometers were designed for the graphic arts industry where UV processes have been a standard for decades. The emergence of digital technology in this industry is causing many graphic arts houses to
sell off this equipment because it is no longer needed for many applications. Model 361T units still show up on
ebay. I have seen them sell for between $200 and $500. They are also still available new from X-Rite for $2250 if
you have the extra change to spend. A couple of cautions are necessary when buying these units on ebay. The unit
should come with the calibration step wedge and manuals, and it should display “Test Pass” in the LCD display
when powered up. If the lamp has been replaced, there is an alignment procedure that must be performed for the
unit to work properly. These units can be anywhere from a few years to 20 years old so you may also want to ask
the age. The lamps do wear out and have to be replaced. If no calibration wedge comes with the unit, they can be purchased on X-Rite’s web site.
The Model 361T densitometer has been out there all along just waiting to be discovered for a completely different
application. Platinum printers like me who use pyro for developing negatives now have a viable tool for use in platinum printing and other UV processes.
Bob Herbst has been photographing and printing in platinum and silver gelatin for more than 32 years. He has gained
international recognition as a master platinum printer, and his work is represented in collections in the U.S. and Europe.
Bob is a contributing writer for View Camera magazine and he prints platinum editions on a commission basis. Bob
teaches workshops on platinum/palladium printing and has taught undergraduate and graduate classes at the Cleveland
Institute of Art, the University of Akron and at Hiram College in Ohio. He is an officer and a member of the Board of
Trustees of the Friends of Photography of the Cleveland Museum of Art. A selection of his work and information on his
workshops can be viewed at www.bobherbst.com. He can be reached via email at firstname.lastname@example.org , by phone at (330) 562-4854, or by mail at 619 Deepwood Drive, Aurora, Ohio, 44202.
This article was originally published in View Camera magazine, March/April 2003.