A measuring apparatus including a ring-shaped base (11) surrounding a telltale plate (P1a) contaminated by particles. Uniformly distributed LEDs (D1, Dx) emitting on a wavelength of 630 nm are attached to the plate. In a preferred embodiment, twelve LEDs (D1, Dx) illuminate the top surface of the telltale plate (P1a) at a low angle of incidence. The lower ring-shaped structure (11) supports a cylindrical optical enclosure (10) underneath an upper ring-shaped structure (12) which in turn supports a CCD detector (122) for measuring the light diffused by the particles, and an optical filter (124) centred on the 630 nm wavelength. The filter (124) and the detector (122) may be removed and replaced with a microscope.