The present specification discloses and claims a particle-size spectrometer in which a stream comprising a fluid carrier medium containing particles under test, is conducted through a slender beam of light, the stray light appearing on the particles being conducted to a photomultiplier, and the concentration and/or size distribution of the particles is determined with reference to the output signals of the photomultiplier. The spectrometer has a chamber through which the stream and said beam of light, passes and intersects to form a centre of stray light. The chamber has an inner surface which surrounds the centre of stray light at least along an arc which extends up to the edge of the beam of light both in front of and behind the centre of stray light, the inner surface being arranged to receive and conduct the stray light to the photomultiplier which is connected to the chamber.
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