A Densitometer for Reading Pyro Negatives

by Bob Herbst

(The following article was written in December 2002)

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) density 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.