Abstract: Method and apparatus for investigating a sample particularly a pharmaceutical tablet. An emitter and/or the sample are initially positioned so that the emitter is at a predetermined distance and normal angle to a first point on a surface of the sample. The emitter then irradiates the sample with radiation having a plurality of frequencies in the range from 25 GHz to 100 THz at a plurality of points on the surface of the sample. Relative motion is possible between the emitter and the sample so that the surface of the sample can be tracked to maintain the predetermined distance and normal angle at each of the plurality of points, and allow radiation transmitted and/or reflected from the sample at the plurality of points to be detected. This has particular application to imaging the structure or composition of a coating on a pharmaceutical tablet.

Abstract: An apparatus for varying the path length of a beam of radiation, the apparatus comprising: an element (51) rotatably mounted about an axis, said element comprising two reflective surfaces in fixed relation to one another such that radiation may be reflected between said reflective surfaces and out of the element (51); and driving means (55) for rotatably oscillating said element about said axis.

Abstract: A circularly polarizing antenna feed comprises a feed waveguide having a short circuit reflecting plate at one end and a radiating horn at the other, a wave exciter for launching linearly polarized plane waves axially along the feed waveguide in opposite directions, and a grid of parallel reflector strips disposed in a plane which is perpendicular to the waveguide axis and which is located between the wave exciter and the reflecting termination at a distance of approximately .lambda.g/8 from the exciter and approximately .lambda.g/4 from the termination. The reflecting strips of the grid are inclined at an angle of 45.degree. to the direction of polarization of the waves propagated by the exciter, the grid and the reflecting termination together forming a twist reflector which effectively reflects and rotates through 90.degree. the waves incident upon the grid. These reflected waves together with the forwardly propagated waves from the exciter represent circularly polarized waves.