Abstract: The apparatus employs two or more beams of radiation of different spectral composition which are transmitted through a sample and received by a detector which provides signals for computing the required parameter by ratio measurements. The beams are directed to a sample zone where a part is transmitted through, and a part is reflected by a sample. The invention provides diffusing means to receive the transmitted and reflected parts and pass the same onto a radiation responsive detector. The diffusing means may be a ground glass plate or a member made of fused alumina. A concave mirror may be used to reflect the reflected part back through the sample zone which is close to or at the center of curvature of the mirror. A beamsplitter may also be used so that the beams are directed substantially normal to a plane in the sample zone.