Assessment of the Sources for Disagreement Between Two Spectrophotometers


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John Seymour


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Agreement between different models of spectrophotometers is increasingly being recognized as an issue in the industry. Brand owners are requesting color tolerances that are in some cases too tight to be met when using different make and model spectrophotometers along the supply chain. This often means that printers are being required to purchase a different model of spectrophotometer for each print buyer that they supply. It is a particular problem when printers seek to improve accuracy (and efficiency) by incorporating spectrophotometers into the press. Such inline spectrophotometers are necessarily of a different design than hand-held devices.

This paper is part of ongoing work. The ultimate goal of this work is to develop a way to reliably standardize one instrument to another. The assumption is that understanding the physical nature of the differences between instruments will lead the way to a standardization process that provides the best performance without being prone to large errors.

In this paper, I look first at how well five spectrophotometers of different make and model agree on what "black" is, and then on how well they agree on what "white" is. Several differences have been determined, including rejection of specular light, calibration of absolute white, aperture size, and goniophotometry. With the exception of calibration of white light, the traditional models for standardizing one spectrophotometer to another do not account for these differences.

Thus, it is necessary to proceed with caution when attempting to standardize one instrument to another. Standardization may actually worsen the agreement between two instruments. At the very least, it is necessary to make sure that the physical properties of the standardization set (the set of samples used to correct one instrument's readings to match another) are similar to the physical properties of the samples to be measured. In particular, translucency and gloss are important.

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