The present invention is directed to a method of locating leaks in a substrate (30) having a first and a second surface wherein the substrate (30) is adapted for preventing the flow of a fluid, or components contained in the fluid, through the substrate (30) from the first surface to the second surface, and a system (10) useful in the method, wherein the method comprises: a) isolating the first surface from the second surface; b) creating a pressure differential between the first surface and the second surface wherein the pressure on the first surface is higher than the pressure on the second surface; c) contacting the second surface or the exit (32) of the device (11) containing the substrate (30) with a baffle (23), wherein the baffle (23) has a plurality of interconnected parts which form a pattern and the baffle (23) is of a sufficient size to cover the second surface of the substrate (30) or the fluid exit point (32) of the device the substrate (30) is disposed in and the parts of the baffle (23) create openings that particles (33) can pass through; d) exposing the surface of the baffle (23) to light from a source of diffuse light (24); e) contacting the first side of the substrate (30) with a carrying fluid containing particles (33) of a particle size that the substrate (30) is a designed to retain in the first surface of the substrate (30); f) monitoring the space above the surface of the baffles (23) for the light scattered by particles (33) that have passed through the substrate (30).