Spectrophotometric Color Matching: A Straightforward Alternative to Kubelka-Munk Analysis of Reflectance Data for Readout of Water Quality Monitoring Disks

We are developing a spacecraft water quality monitoring system based on measuring the change in diffuse reflectance of indicator disks following exposure to a water sample. The most widely used approach for deriving quantitative information from a diffuse reflectance spectrum involves an initial Kubelka-Munk transformation of the reflectance data. Provided the assumptions underlying the transformation are satisfied, the magnitude of the resulting function varies linearly with analyte concentration. We measure diffuse reflectance spectra using a commercial-off-the-shelf handheld spectrophotometer designed for color matching applications. Spectra are currently downloaded and transformed to obtain the value of the Kubelka-Munk function at a detection wavelength diagnostic of the analyte. This value is then used to quantify the analyte via a standard response curve. Data workup would be greatly simplified if the color matching capabilities of the spectrophotometer could be used to read out analyte concentrations directly by matching the color coordinates obtained from the indicator disks to those of premeasured standards. Results will be presented comparing the utility of color matching to that of the Kubelka-Munk transformation for the quantitative analysis of spacecraft water constituents using diffuse reflectance spectrophotometry.