Photographical CIELAB Color Coordinates Measurements Compared to a Clinical Spectrophotometer

Objectives: Digital photography is a simple and widespread method to document clinical cases and its use to measure CIELAB color coordinates has been suggested in recent publications. The aim of this study is to evaluate the null hypothesis that digital photography of shade guide tabs, using polarized filters or not, measures CIELAB color coordinates no differently than a clinical spectrophotometer.
Methods: 1M2, 2M2, 3M2, 4M2 and 5M2 shade tabs of the Vita 3D-Master shade guide had its color coordinates measured using a Vita Easyshade spectrophotometer (EAS) and were photographed under a standardized protocol (Nikon D7000, 105mm Sigma lens, two Nikon SB-200 flashes at 45^o) using (POL) or not (no-POL) polarizing filters to eliminate specular reflection (n=3 for each shade tab and measurement method combination). All pictures were taken with a reference gray card (Whibal, with known CIELAB coordinates) to allow for exposure/color balance. Exposure/color balancing procedures and CIELAB coordinates measurements were performed using Adobe Photoshop. Data were evaluated using ANOVA and Tukey(α=0.05).
Results: All methods were capable to measure the expected reduction of L* and increase of a* and b* coordinates of the five shade tabs (from 1M2 to 5M2). The measured CIELAB coordinates were significantly different between color measurement methods for same shade tabs. Measured △E between color shade tabs was affected by the color measurement method.
Conclusions: Clinical pictures provide valuable visual information and, when performed under a standardized protocol, may be used to measure color coordinates. However, CIELAB coordinates obtained by digital photography cannot be directly compared to those obtained by a clinical spectrophotometer. Since literature also reports disagreements between spectrophotometers themselves, more studies are needed to increase reliability of different clinical color measurement methods.