In the colorimetric or spectral characterization of imaging devices such as digital cameras and scanners, the optoelectronic conversion functions (OECFs) are traditionally obtained from standard gray samples. However, these gray samples are sometimes unavailable when conducting color characterization. We propose an efficient method for recovering OECFs by using nongray samples, based on the finite-dimensional modeling of spectral reflectance and the second-order polynomial fitting of OECFs. Experimental results indicate that the accuracy of the estimated OECFs are close to those obtained from gray samples, with the correlation coefficients R 2 larger than 0.995. The proposed method should be useful in colorimetric and spectral characterization of imaging devices by using custom-made color samples in textile or other industries, when standard gray samples are not available and not easily made. (C) 2008 Wiley Periodicals, Inc.