A Full Circle: the DT NGT2 returns to George Eastman Museum

Elizabeth Chiang, museum photographer, shares how the museum beta tested a new color reference target. What is that? It ensures that when we take photographs of objects, regardless of technology, that we’re accurately capturing the colors in digital!

One of the many components of cultural heritage preservation is the documentation of objects. Here at George Eastman Museum, we have two separate copy stands that document flat objects. These workstations use entirely different cameras and different sets of lights. So how do we achieve uniformity between different work stations? To start, object-level targets or color checkers, included in the capture frame, are a good way to reference known color and neutral values.

How do we manage the differences in gear? How can we be certain that this camera’s sensor, in conjunction with this model of light, is actually reproducing a color patch that is true to it’s measured values? This is where overall system calibration tools come into play. Color reference targets (with many many more patches of color than an object-level target) help measure the performance of a capture device and lighting combination, and in turn, are used to build ICC profiles, which are sets of data that characterize a color input or output device.

Robust cultural heritage digitization hinges on accurate color and tonal reproduction, with measurable results being favorable to simple visual confirmation. Whether it be back in the days of microfilm, through the era of scanning, or to today’s fast capture systems — this is a niche of photography where scientific advances in color accuracy make a discernible impact.

In 2015, George Eastman Museum was approached by David Wyble of Avian Rochester LLC with a request to make spectrophotometric measurements of samples in the collection. Wyble is a scientist who was developing a new reference target at the request of the Library of Congress. Spectrophotometry measures the relative intensity of light that either passes through or reflects off of a sample. We call these quantities transmittance and reflectance. After measuring how a material behaves for each wavelength in the visible range, we can calculate definitive values for the color of that material.

Our museum was among several cultural institutions, including the Rochester Museum and Science Center, the Cary Graphic Arts Collection at the Rochester Institute of Technology, the Penfield Public Library, and the Library of Congress, whose collections were measured. Samples were taken from books, magazines, newspapers, periodicals, journals, and photographs — some of the more commonly found flat objects in museums. These measurements were eventually translated into color patches E7 to J7 and E8 to J8 on the DT NGT2 target.

Fast forward to November 2019 and the Photography Studio at Eastman Museum became a beta tester for the final product. Other beta testers included the National Gallery of Art and the National Geographic Society.

Color reference targets are vital in the cultural heritage industry because one of the primary goals of cultural heritage digitization is to accurately portray the physical object. Images of collection objects are used in many outward-facing capacities — such as social media, publications, and websites — but they also serve as markers of time that record the (inevitable) physical changes of an object.

As a camera calibration target, the NGT2 has several useful features: it has a wide gamut of color patches (equal or better than other commonly available targets); it is uniformly glossy, and it is infinitely cleanable. The longevity of the NGT2 is an extraordinary feature. A smudge can change the measured value of a color patch, which causes lost time and energy needed to re-measure and remake profiles. Imagine doing this throughout the years and the number of new targets that would have to be purchased over and over when one gets damaged.

I spoke to David in February 2020 and he stressed the importance of having uniform gloss across all the patches. It made logical sense to create a target that would give uniform measurements no matter the angle of light because if the program used to measure a target was always asking the same question — what is the measurement of this color patch at a 45-degree lighting angle? — then in order to get solid results, the input and output questions should be the same.

The studio was delighted to be able to beta test the NGT2. Here’s what we did:

1. Photographed the reference target under standard copy stand lighting conditions (45-degree cross lighting)
2. Used the produced file to create a custom icc profile (using BasICColor5)
3. Applied the profile to subsequent captures that include an object-level target (we used a Golden Thread Object Level Target)
4. Exported the files and verified the color accuracy of the capture (we used OpenDICE)

There are two medium format cameras in the studio — the PhaseOne IQ180 and the P45. While the newer IQ180 produced great results (as expected), the P45 digital back predated the adoption of the linear scientific curve, which provides absolutely linear tonal values. This was resolved by the technical team at Digital Transitions Cultural Heritage who reverse calculated a linear scientific curve for the P45. Some might question whether it’s worth the hassle, however, many studios in the nonprofit sector are using older gear and it becomes a practical measure to extend the useful life of such gear.

Looking forward to the next steps, with the help of our Conservation Department, we will measure the specific values of our Golden Thread object-level targets and use those values for verification. By using measurements specific to our day-to-day object-level target, we can get even more accurate, real-world, results. One of the many goals of the studio in this coming year is to fine-tune our color accuracy and to introduce a more robust quality assurance and quality control workflow. Creating custom color profiles is the first step to reaching these goals.

The George Eastman Museum studio is currently undertaking many exciting digitization projects, including the NEH grant-funded cataloging and digitization of the Alden Scott Boyer collection, and the comprehensive digitization of Nikolas Muray’s work in anticipation of an upcoming exhibition. For more information, please visit www.eastman.org