Apparatus for monitoring particles flowing in a stack, the apparatus comprising: a light source for emitting light along an optical path for scattering from the particles, wherein the intensity of the light from the light source can be varied; a rotatable monitoring assembly mounted in the optical path; a detector for detecting the scattered light; wherein the rotatable monitoring assembly contains at least two apertures (25, 27) and is arranged to be rotated into a plurality of different configurations, including: a contamination-check configuration, wherein the light is reflected through the rotatable monitoring assembly onto the detector by reflectors, without entering the stack; an operation configuration, wherein the light passes along the optical path through a first aperture in the rotatable monitoring assembly and into the stack unimpeded and light scattered from particles in the stack passes through a second aperture in the rotatable monitoring assembly to the detector; a zero-check configuration, wherein the rotatable monitoring assembly blocks the light from reaching the stack and background light passes through the second aperture in the rotatable monitoring assembly and is detected by the detector; a span-check configuration, wherein light of varying intensity from the light source passes along the optical path, through the first aperture in the rotatable monitoring assembly, and into the stack and light scattered from particles in the stack passes through the second aperture in the rotatable monitoring assembly and is detected at the detector; and a safety-shutter configuration, wherein the rotatable monitoring assembly blocks particles from reaching optical components in the apparatus by forming a barrier. a light source for emitting light along an optical path for scattering from the particles, wherein the intensity of the light from the light source can be varied; a rotatable monitoring assembly mounted in the optical path; a detector for detecting the scattered light; wherein the rotatable monitoring assembly contains at least two apertures (25, 27) and is arranged to be rotated into a plurality of different configurations, including: a contamination-check configuration, wherein the light is reflected through the rotatable monitoring assembly onto the detector by reflectors, without entering the stack; an operation configuration, wherein the light passes along the optical path through a first aperture in the rotatable monitoring assembly and into the stack unimpeded and light scattered from particles in the stack passes through a second aperture in the rotatable monitoring assembly to the detector; a zero-check configuration, wherein the rotatable monitoring assembly blocks the light from reaching the stack and background light passes through the second aperture in the rotatable monitoring assembly and is detected by the detector; a span-check configuration, wherein light of varying intensity from the light source passes along the optical path, through the first aperture in the rotatable monitoring assembly, and into the stack and light scattered from particles in the stack passes through the second aperture in the rotatable monitoring assembly and is detected at the detector; and a safety-shutter configuration, wherein the rotatable monitoring assembly blocks particles from reaching optical components in the apparatus by forming a barrier. a contamination-check configuration, wherein the light is reflected through the rotatable monitoring assembly onto the detector by reflectors, without entering the stack; an operation configuration, wherein the light passes along the optical path through a first aperture in the rotatable monitoring assembly and into the stack unimpeded and light scattered from particles in the stack passes through a second aperture in the rotatable monitoring assembly to the detector; a zero-check configuration, wherein the rotatable monitoring assembly blocks the light from reaching the stack and background light passes through the second aperture in the rotatable monitoring assembly and is detected by the detector; a span-check configuration, wherein light of varying intensity from the light source passes along the optical path, through the first aperture in the rotatable monitoring assembly, and into the stack and light scattered from particles in the stack passes through the second aperture in the rotatable monitoring assembly and is detected at the detector; and a safety-shutter configuration, wherein the rotatable monitoring assembly blocks particles from reaching optical components in the apparatus by forming a barrier.