Dual-Beam Spectrometer Doesn’t Color Colorimetry

Daniel C. McCarthy, News Editor

Matching the color of an automobile's plastic trim to its painted metal finish is difficult enough when the work is done in the same factory. Consider that these parts often are manufactured and processed in different factories or even on different continents, and it quickly becomes apparent how vital the measurement of color has become in the global economy. Enter David Spooner, a consultant on color measurement who is also an expert on the physics of color measurement errors. Among his tools is the dual-beam AH4230 spectrometer manufactured by American Holographic Inc.

Spooner incorporated the AH4230 into an optical bench colorimeter he designed to investigate the effects of flash xenon sources on measured sample color. "The high intensity of the xenon flashlamp sources used in some color measuring instruments can change the color of the sample while it's being measured," Spooner said. "In a survey of drawdowns of over 100 pigment samples that I did during the past two years, I found that about 10 percent of the samples changed color while being measured."

Causes of this phenomenon range from transient photodarkening, photobleaching and/or movement of spectral absorption edges. How the sample is positioned in each instrument and interactions of the instruments with translucent samples can also cause measurement differences.

To separate light-source-specific disparities from other effects, Spooner designed his optical bench colorimeter so that it would illuminate the sample with either a xenon or incandescent source without moving the sample. In his design, he envisioned a dual-beam spectrometer detection system that would simultaneously measure the sample illumination and reflectance.

Spooner selected the AH4230 because its dual-beam design simplified his instrument by eliminating the need to integrate and calibrate two single-beam spectrometers. In addition, the dual-beam configuration essentially eliminated the effects of temperature and optical path differences on comparative measurements, he said.

American Holographic's device uses one grating and one detector array to take measurements of two spectrometer channels.