Abstract: A compact, rugged sensor for use in the measurement of particle sizes in a fluid stream is disclosed. The sensor permits the use of a light emitting diode as a light source by mounting a high grade optical system with its focal point at the point light source provided by the light emitting diode to collimate the generated light. This collimated light passes through an aperture formed in one dimension by the width of the fluid flow stream and formed in a second perpendicular dimension by a narrow transparent slit in an otherwise opaque shield adjacent the fluid flow stream. The collimated light source, together with the use of the flow stream as a light path boundary, substantially increases the accuracy of the sensor. All of this is accomplished in conjunction with a solid state light emitting diode source which permits the sensor to be used in shock and vibration environments which would have prohibited particle measurement using prior art sensors.