Abstract: A method and apparatus for detecting the mean atomic number of a mass of material, for example freight in a vehicle comprises: subjecting the mass to X-rays and determining the mean number N.sub.A passing therethrough; subjecting the mass to higher energy X-rays and determining the new mean number N.sub.B ; and determining the mean atomic number of the mass from look-up tables against the computed ratio N.sub.A /N.sub.B. The mass, such as a railway wagon, may be scanned by two linear accelerators arranged perpendicular to each other, detector arrays being disposed respectively opposite the accelerators, so that a three dimensional image can be built up of the mass.

Abstract: Apparatus as described by which very low light levels emitted typically by very small regions such as individual cells in cultured monolayers, can be more readily detected by a detector such as a CCD detector or image intensifier. The light capturing aid is a fibre optic face plate located between the light emitting material and the detector, but not in contact with the light emitting material. Where the latter is contained on a surface such as a petri dish, the face plate is positioned between the dish and the detector with at least an air gap between the face plate and the detector.