Principal component analysis (PCA) is widely used to reconstruct the spectral reflectance of surface colors. However, the estimated spectral accuracy is low when using only one set of three principal components for three-channel color-acquisition devices. In this study, the spectral space was first divided into 11 subgroups, and the principal components were calculated for individual subgroups. Then the principal components were further extended from three to nine through the residual spectral error of the reflectance in each subgroup. For each target sample, the extended principal components of the corresponding subgroup samples were used in the common PCA method to reconstruct the spectral reflectance. The results show that this proposed method is quite accurate and outperforms other related methods.

A new spectral reflectance estimation method based on CIE XYZ values under multi-illuminants was proposed to obtain multi-spectral images accurately by using digital still cameras. CIE XYZ values under multiilluminants were initially predicted from raw RGB responses by using a polynomial model with local training samples. Then, spectral reflectance was constructed from the predicted CIE XYZ values via the pseudo-inverse method. Experimental results indicated that the new spectral reflectance estimation method significantly outperformed the traditional colorimetric characterization method without requiring extra training samples or greatly increasing computational complexities.

Metamer mismatching (the phenomenon that two objects matching in color under one illuminant may not match under a different illuminant) potentially has important consequences for color perception. Logvinenko et al. [PLoS ONE10, e0135029 (2015)] show that in theory the extent of metamer mismatching can be very significant. This paper examines metamer mismatching in practice by computing the volumes of the empirical metamer mismatch bodies and comparing them to the volumes of the theoretical mismatch bodies. A set of more than 25 million unique reflectance spectra is assembled using datasets from several sources. For a given color signal (e.g., CIE XYZ) recorded under a given first illuminant, its empirical metamer mismatch body for a change to a second illuminant is computed as follows: the reflectances having the same color signal when lit by the first illuminant (i.e., reflect metameric light) are computationally relit by the second illuminant, and the convex hull of the resulting color signals then defines the empirical metamer mismatch body. The volume of these bodies is shown to vary systematically with Munsell value and chroma. The empirical mismatch bodies are compared to the theoretical mismatch bodies computed using the algorithm of Logvinenko et al. [IEEE Trans. Image Process.23, 34 (2014)]. There are three key findings: (1) the empirical bodies are found to be substantially smaller than the theoretical ones; (2) the sizes of both the empirical and theoretical bodies show a systematic variation with Munsell value and chroma; and (3) applied to the problem of color-signal prediction, the centroid of the empirical metamer mismatch body is shown to be a better predictor of what a given color signal might become under a specified illuminant than state-of-the-art methods.

To improve the spectral image color reproduction accuracy, two novel interim connection spaces (ICSs) were proposed. The dominant structure of spectral power distributions was extracted by principal component analysis for the widely used illuminants and light sources, and then further transformed to three synthetic illuminants. The CIEXYZ tristimulus under two or three synthetic illuminants was employed to construct two novel ICSs. The two ICSs were compared with LabPQR and the ICS with two sets of tristimulus under two real light sources according to the spectral and colorimetric representing accuracy of Munsell and Natural Color System (NCS) chips. The results indicated that the two ICSs proposed in this study outperformed the other two ICSs as a whole.

To improve the spectral image representation and reproduction accuracy, an interim connection space (ICS) was proposed. Three offset functions were defined to make up the wave regions uncovered by the CIEXYZ color matching functions. The XYZLMS ICS was determined based on the CIEXYZ and defined offset functions. The spectral and colorimetric representation accuracy of the proposed ICS was compared to LabPQR and LabRGB with various spectral datasets as testing samples. The results indicated that the proposed ICS outperformed the other two ICS as a whole.