Optical brightening agents (OBAs) or fluorescent whitening agents (FWAs) are often used additives in paper and board products as they improve both whiteness and brightness of the products. When printed, OBAs can even contribute to colour tone reproduction. Fluorescent emissions of OBAs depend on the UV content of the illuminant (light source). Adequate adjustment (control or adjustment) of UV content of measurement apparatus (e.g. spectrophotometer) is essential for accurate colour measurement and printing colour reproduction. We proposed a method to adjust the UV content against assigned spectra rather than, as adopted in current ISO standards, against single assigned values. Demonstrations of applying this method to the CIE standard illuminants used in papermaking and graphic industries, D65, C and D50 have been given. Thanks to the well-established traceability of reference standards (IRs), the UV contents of a spectrophotometer corresponding to the standard CIE illuminants have been achieved.