High-Accuracy Colorimetric Characterization of Photographic Films

A desktop scanner was colorimetrically characterized to average CIELAB error of less than unity for Kodak Ektachrome transparencies and Ektacolor paper, and Fuji Photo Film Fujichrome transparencies and Fujicolor paper. Independent verification on spectrally similar materials yielded average ∆E*ab error of less than 2.1. The technique first modeled the image formation of each medium using either Beer-Bouger or Kubelka-Munk theories. Scanner digital values were then empirically related to dye concentrations. From these estimated dye concentrations, either spectral transmittance or spectral reflectance factor was calculated from an a priori spectral analysis of each medium. The spectral estimates can be used to calculate tristimulus values for any illuminant and observer of interest. Results and Discussion The ∆E*ab histogram for the Ektachrome IT8.7/1 modeling target (as a representative material) is shown in fig. 1 where the predicted CIELAB values are compared with the measured CIELAB values. The histogram of an independent target with 216 patches (6x6x6 digital factorial design) is shown in fig. 2. The table lists the results for the four films analyzed. The 95th percentile was slightly greater than two times the mean of each data set. This technique was very effective for positive films and slightly less effective for photographic papers. In the case of Ektacolor Plus paper, the empirical model relating linearized scanner values (Dr, Dg, Db) to dye concentration had greater residual error than the transparent materials; this was due to the wide-band scanner spectral sensitivities and quantization errors at high concentrations. For the Fujicolor paper, the independent data had large systematic errors. In a post hoc analysis, it was found that the independent target had different dye characteristics than the IT8.7/2 target; as a result, the spectral reconstruction was in error.