1. A method of monitoring particles in a stack, the method comprising: generating on a first side of the stack a beam of light directed through the particles; and obtaining an image of light scattered from the particles by using an imager including a collecting lens through which the light scattered from the particles passes; CHARACTERISED IN THAT unwanted scattered light also passes through the collecting lens, and in that the method includes the step of blocking from the image the unwanted scattered light that has passed through the collecting lens.