Computer processing of the infrared (IR) images speeds up considerably the evaluation of the measurement results. Processing of digitized IR images may be carried out by a variety of approaches and methods. On the whole, it may be said that it is expedient to tailor the strategy and the method of evaluation to the phenomenon under scrutiny. This, in some cases, calls for both heat engineering experience and technical intuition. Image filtering is a special and not commonly used method of IR image analysis. The present author has no information about publications on the mathematical filtering of IR images. The first paper on that topic by the present author was published in 1997. The mathematical IR image filtering in general is suitable for detecting some thermal faults which are difficult to identify otherwise. Image filtering is based on the adequate transformation of the temperature field, i.e. the modification of the temperature values in the pixels of the image.