Abstract: An interferometer is provided for producing an interference pattern from light such as laser light which travels along first and second optical paths and at the end of each such path there is a respective fixed mirror for directing light to a common optical path. Means are provided to vary the optical path length of light in the or each first or second optical path to produce an interference pattern in the common optical path, which means comprises a pair of parallel-plane optical flats in accordance with one embodiment of the invention, which are each rotated in an oscillating manner to produce a linearly varying optical path length during rotation. According to another aspect of the invention first detector means are provided for detecting a reference part of the interference pattern produced to derive therefrom a reference signal, and second detector means are provided for receiving the interference pattern to thereby derive signals indicative of successive parts of the interference pattern.

Abstract: An apparatus and a method for inspecting cullet for the presence of unwanted materials among the pieces of glass. A light source such as a laser directs light onto the cullet. At least one array of at least two light detectors is positioned to detect reflected light from the cullet. The detectors differentiate between non-uniformly scattered light from the cullet and generally uniformly scattered light from the debris. Signalling means are provided to signal the differentiation thereby to indicate the presence of debris in the cullet under inspection. When cullet is present on a moving cullet belt, light from the light source is intermittently scanned over the cullet in a raster-type scan. A multi-faceted rotatable mirror may be employed intermittently to reflect light onto the cullet.