 The kinetics of reduction of the dye in a "paii" have been studied in great detail.Figure 2 illustrates the basic principles of operation of a "Paii" when applied to a product that has a thin photocatalyst film coating.The ink is applied to the photocatalyst coating, usually using either a felt-tipped pen, air-brush, rubber stamp or paint brush, and then exposed it to sunlight or an alternative, appropriate (invariably UVA) light source.