to fade through being heated to a given degree affords an indication of the depth of the blockage by carrying out a similar measurement of the same blockage using heat energy for example electromagnetic radiation applied from the other end of the filter three dimensional information about the blockage may be obtained heating times will also depend on factors such as the nature and power of the electromagnetic radiation the amount of radiation that is coupled with the face of the filter and the composition of the filter material and dimensions of the channels blockage depth may be measured directly using other means for example by measuring the depth to which a metal rod can be inserted into a channel such measurements may be used to calibrate the method by relating the measured imaging times to the known blockage depth as an alternative source of electromagnetic radiation to cause heating of the material of the filter we propose to use a heated metal plate notably a blackened copper plate heated to a temperature of a few hundred degrees celsius by embedded electrical elements an advantage of this arrangement is that it can provide very uniform heating of the filter surface by longer wavelength radiation over a much broader spectrum much of the radiation emitted by the black body will be in the 7 to 14 u03bcm range matching the sensitivity of the thermal imaging camera this is expected significantly to improve the detection of damage within filters as the camera will be.