Wavelength-dependent spatial correction and spectral calibration of a liquid crystal tunable filter imaging system

Abstract

The centroid wavelength of liquid crystal tunable filters (LCTFs), by definition, changes with incident angle, resulting in an imaging system with spectral sensitivity that varies across the image plane. Prior to this research, this effect was considered negligible. Imaging uniform colored papers with high chroma revealed that the effect is appreciable. A regression-based method of modeling and correcting systematic spectrophotometric errors in conventional reflectance spectrophotometry was used to correct this wavelength-dependent spatial nonuniformity in an LCTF imaging system. For six high chroma sheets of spatially uniform paper, the uniformity was reduced from an average and maximum of 0.95 and 4.00, respectively, to 0.08 and 0.18 CIEDE2000. The same method was used to transfer the scale of spectral reflectance factor from a reference spectrophotometer to the imaging system such that its reported values matched the reference spectrophotometer with a constant bandwidth. For an independent color target, residual errors were 0.008 spectral reflectance factor RMS difference and 0.44 and 1.65 average and maximum CIEDE2000, respectively (0.63 and 2.34 ΔEab*).