Abstract: The invention provides an optical apparatus configured to monitor a state of a diffuser illuminated by a light source, at least one light beam comprising at least one bundle of rays of light exiting the diffuser being for illumination of a transmissive or reflective spatial light modulator of a projector, further comprising a sampler that samples the light beam exiting the diffuser and re-directs the light beam sample towards a detection system, wherein the detection system is coupled to an analyzer that generates a signal when an anomaly is detected in the distribution of the light intensity in a cross-section of the sample of the light beam. It also provides a method of monitoring a state of a diffuser illuminated by a coherent light source.