Conventional diffraction gratings are designed so that most of the light striking the grating is diffracted toward the "first-order beam" direction, but in practice, some of the light may be diffracted towards the "second-order beam" direction, so that the light striking a particular sensor pixel may be a combination of both first-and second-order components. This causes errors in the output of diffraction-grating based UV-visible spectrometers. A new method has been developed to correct such errors by calibrating the optical system with a UV-excluded standard (such as an orange tile) and a UV-included standard (such as a white tile). These two calibrations can predict the relationship between first-order-only signals and first-and-second-order combined signals at sensor pixels (nominal wavelengths) where there is secondorder light, and thus help correct the second-order diffraction error. This compensation method applied on the reflectance measurement of a set of color ceramic tiles showed significant improvement on the accuracy of the result. (C) 2016 Wiley Periodicals, Inc.