According to one example, there is provided a printing system. The printing system comprises a support having a plurality of spaced apertures and a color sensor moveable to measure light from each aperture. The printing system further comprises a controller to control the color sensor to measure characteristics of light emitted through each aperture, and to determine, for each aperture, light calibration data. The printing system is particularly adapted for use with substrates that are intended to be viewed when backlit, that is in conjunction with backlighting.