The PhotoCheck test set is designed for testing the wavelength accuracy of all Merck photometers and spectrophotometers. It can be used as a secondary standard for testing instruments according to DIN/ISO 9000 and ISO 14000. The stable colour solutions can be used to test the following spectrophotometer filters and wavelength settings: 445 and 446 nm, 520 and 525 and 690 nm. The accuracy of the wavelength setting and the linearity of the extinction measurement is checked with four solutions per wavelength. The traceability of these test solutions to international standards is guaranteed by examining them in a reference photometer that is checked against the primary standard (NIST standard). These values are properly documented and supplied as part of the set.