The infrared radiation measurement system in the moisture meter based on the cooled PbS photoconductive detector is designed by the authors and analysed in this paper. It consisted of a preamplifier and a mechanical chopper. The basic noise sources and their influence on this measuring system are studied out. One solution for the maximisation signal to noise ratio based on the theoretical and laboratory practical consideration has also been made. Under the circumstances, the possible accuracy of the mentioned system is founded out by the measurement. The final aim is to develop an optimal optoelectronical measuring system for the infrared moisture meter for practical use but with some specific substances firstly. The overall results can be kept in the numerous variety applications later.

Water has an absorption band of four wavelengths, namely 1.2, 1.45, 1.94, and 2.94 micrometers, in the near-infrared light range. When a sample is irradiated with light of the above wavelengths, this sample will absorb light in accordance with its moisture content. As moisture content of wheat increased, reflectance from the wheat sample decreased, which indicates that water is a strong light absorber than a sandy wheat sample. When light hits a sample, several interactions can occur: specular reflectance, diffuse reflectance, absorption, transmittance, refraction and scattering. In this paper we try to find out influence of several interactions on accurate measurement for moisture wheat.