Thursday, May 17, 2007

Low Commitment Spectrophotometer Care

On March 5, I posted an entry on the aging of technical publications and how their derivates need to be monitored. Today I need to write a similar update regarding the repeatability of spectral measurements.

Almost a decade ago I had the lucky chance to work with Kathleen Berrigan and David Wolf on detecting very small drifts on a manufacturing line. Than line was instantiated on various continents and operating at different ambient temperatures. Part of the result was published in the technical report HPL-1999-2, Low Commitment Spectrophotometer Care. This report had quite a few downloads and it is time to publish an important update.

The report discusses mostly a very structured procedure to follow, and the technical aspect requiring the most attention is thermochromism. The statistical part was a very simple repeatability estimate based on calculating ΔE values.

A lot of water has flown under the bridges in the past decade and today’s faster PCs allow for much more sophisticated statistical methods than ΔE. In fact, in the meantime the ASTM has created specification E2214-02, a Standard Practice for Specifying and Verifying the Performance of Color-Measuring Instruments.

The recommended multivariate methods allow for a much more accurate assessment of repeatability. Maybe more importantly, however, multivariate methods allow you a much faster diagnosis when a problem does occur.

A more comprehensive procedure entails a steeper learning curve, and this might have hold off some adopters. Fortunately, the current issue of Color Research and Application, Volume 32, Number 3, dated June 2007, has an easy-to-read paper by David Wyble and Danny Rich entitled Evaluation of methods for verifying the performance of color-measuring instruments. Part I: Repeatability. Their paper describes in an clear way the methods, then they present repeatability results from a long-term study of twelve commercial spectrophotometers.

If you are using what I described in HPL-1999-2, I recommend you study this paper and update your practice. There is also a Part II, but that is on inter-instrument reproducibility, which was not addressed in my technical report.