With regard to the more stringent legislation in environmental protection strong efforts have been made to reduce fine dust emissions. As a result filter devices equipped with dry operating cleanable textile filter media have become one of the most favoured separation techniques for fine dust. For improvement of cleanable filter media it is necessary to understand the influence of their structure on their filtration behaviour and to develop parameters for characterising their clogging and penetration behaviour. The aim of the research work presented in this paper was to improve the already developed reflected-light method [W. Koschutnig, G. Mauschitz, W. Hoflinger, Charakterisierung der Oberflachenbehandlung und des Verstopfungsverhaltens von abreinigbaren Staubfiltermedien mittels Bildanalyse, CIT 76, 2004, 10, 5 pp.] to get a structure parameter of cleanable non-woven dust filter media, which can give information about the particle penetration. To reach this goal transmitted-light is used instead of reflected-light. By that way images with higher contrast will be achieved which enable to define a mean hydraulic diameter for the pores near the outer surface of cleanable non-woven dust filter media. Experimental measurements were carried out which prove that the mean hydraulic pore diameter can be a measure for the particle penetration of a filter medium. The pore volume equivalent, which can be used as a measure of the dust storage capacity of the filter medium can also be detected by the transmitted-light method. (c) 2007 Elsevier B.V. All rights reserved.