ISO 21066:2018 specifies a method, the Resazurin (Rz) ink test, for the qualitative assessment of the activity of a photocatalytic surface, and its classification as below, within, or above the applicable range of the test. The method then allows for the subsequent semiquantitative evaluation of the activities of photocatalytic surfaces that are within the applicable range of the test. In all cases, artificial ultraviolet (UV) radiation is used.

The test method specified is appropriate for use with all flat, smooth, photocatalytic surfaces, which are not macroporous, examples of which include: commercial photocatalytic glass, paint, tiles and awning materials. The method is not applicable to assessing the visible-light activity of photocatalytic surfaces, nor their ability to effect: air purification, water purification, self-cleaning or disinfection, although some relevant correlations have been reported^[4][5].