Abstract: A thermal transfer printer includes three light emitting diodes (7, 8, 9) emitting red, green and blue light respectively, and respective detectors (10, 11, 12) mounted on the opposite side of a dyesheet (1) passing through the printer. The detectors (10, 11, 12) detect the light absorption ratios of three colour print panels (Y, M and C) of the dyesheet, and these detected ratios are compared with acceptable ranges of light absorption ratios. If the detected light absorption ratio for any colour falls outside the corresponding range, use or further use of the dyesheet in the printer is prevented, for example by disabling an essential function of the printer or ejecting the dyesheet from the printer.