The International Standards Organization's ISO R105 standards consist of a series of eight blue-wool cloths containing dyes selected so that each one of the series requires twice as much exposure to light as the preceding one in order to fade to the same degree. The standards are designed to characterize the lightfastness of dyed cloth, but have been used to characterize the lightfastness of colored materials other than textiles and to provide a measurement of the total radiation falling on a display over time. ISO standards were intended to be used without instruments, although at the Center on the Materials of the Artist and Conservator (Carnegie Mellon University, Pittsburgh, PA.), spectrophotometers are used to make degree-of-fading measurements more precise and objective. The authors discuss that and other modifications of the traditional use of blue-wool standards.