text: Paiis can be used as quality control and marketing tools in commerce and as a quick and easy way to assess and/or map the activities of new photocatalytic materials in research. In addition, it has also been demonstrated that such inks can be used on highly coloured and black surfaces, provided the oxidised and/or reduced form of the redox dye is luminescent, and that they can be effectively used to demonstrate the activity of visible light photocatalysts.  In light of the need for in situ testing of commercial photocatalyst materials, paii labels have been developed that can be applied simply in the field on any surface to be tested, in both a non-reusable and reusable form. One noteworthy application of paiis is where a uniform film has been applied to a photocatalytic surface, and the variation in the rate of colour change across the surface has been monitored and used to generate a surface map of the photoactivity. By this method the uniformity of the surface activity may be investigated, and any "hotspots" of photoactivity identified. By varying the composition of a semiconductor photocatalyst surface across the surface itself, a paii photoactivity surface map may be used to determine the optimal composition which yields the greatest photocatalytic response.
  The rapid colour change of paiis makes them suitable for such applications as:
  * Quality control (in laboratory, factory and on site);
  * Marketing;
  * Counterfeit identification;
  * Research material assessment.