The present disclosure provides a method and apparatus for measuring ash deposit levels in a particulate filter comprising at least one cell. One or more indicator components is inserted into the at least one cell so as to lie upon any deposits contained therein. Radiation is emitted from a radiation source into the cell, and the position of the one or more indicator components is detected on the basis of the emitted radiation. The deposit level within the cell is then calculated on the basis of the position of the one or more indicator components.