Potassium dichromate dissolved in perchloric acid is a highly suitable method to check for photometric accuracy (absorbance) in the UV region. Potassium dichromate gives a spectral scan containing characteristic peak maxima at 257 nm and 350 nm and minima at 235 nm and 313 nm. The potassium dichromate solution is sealed in an airtight sealed quartz precision cell with 10 mm light path. The absorbance values of the filter are determined at the wavelengths 235 nm, 257 nm, 313 nm, and 350 nm and are specified in the certificate. It should be noted that the measurements are carried out with the filter 667-UV14 (0.01 N perchloric acid) in the reference beam path. Measured values of the reference filter are separately displayed in the certificate.