The ink is applied to the photocatalyst coating, usually using either a felt-tipped pen, air-brush, rubber stamp, paint brush, or a drawdown bar, and then exposed it to sunlight or an alternative, appropriate light source.  The ink identifies the presence of the photocatalyst coating by changing colour upon irradiation of the latter at a rate (usually < 10 min) which provides a measure of the film's activity.  
For example, it has been established that the rate of change in colour of an paii on commercial self-cleaning glass is directly related to the rate at which the glass is also able to photo-oxidatively mineralise, via reaction (1) the wax-like, natural fatty acid, stearic acid, found in finger prints. The rate of the rapid colour change associated with photocatalyst activity indicator inks has also been directly correlated with the photocatalytic oxidation of methylene blue  and NO.  It has also been shown that digital colour analysis of photographs monitoring the colour change of a paii can be used to extract apparent absorbance data which correlates well with UV-vis absorption data for the same sample, without the need for expensive spectrophotometric instrumentation.  
By making the dyes in the ink increasing difficult to reduce chemically, for example by using: basic blue 66, resazurin, and acid violet 7, respectively, it is possible to make paiis which are effective on photocatalyst coatings which exhibit, respectively: low (most self-cleaning tiles), moderate (self-cleaning glass) or high (self-cleaning paints) activities. paiis based on the dyes 2,6-dichloroindophenol (DCIP) and methylene blue have also been reported.