Previous authors (J Opt Soc Am A 2000; 17: 1952-1961; J Opt Soc Am A 2001; 18: 253-264) have proposed a pixel-by-pixel image descriptor that is invariant to certain changes in illumination. We have studied the possibility of applying such an invariant descriptor to scenery illuminated by natural light, by choosing sensors that allow the invariant to behave satisfactorily under daylight. We obtained different triads of monochromatic sensors by using an exhaustive-search method and compared the results with those obtained with other triads proposed by different authors. We extended our study to Gaussian sensors centered in the wavelengths considered for the monochromatic sensors and to commercial CCD camera sensors. Satisfactory results are achieved for Gaussian sensors with maximum sensitivities at different locations. (C) 2006 Wiley Periodicals, Inc.