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, wherein unwanted scattered light also passes through the collecting lens, and blocking from the image the unwanted scattered light that has passed through the collecting lens; wherein the beam of light is directed obliquely through the particles, so that different areas of the imager receive light scattered from particles at different ranges across the stack; and wherein the light scattered from the particles and passing through the collecting lens has been scattered in a direction that forms an angle of less than 30 degrees from the direction of the light beam as it passes through the particles.