A contactless method for determining the spatial variation of the free carrier density in CdS is presented. The method involves the correlation of IR transmission with the free carrier density via the IR absorption coefficient. Preliminary measurements indicate a free electron density sensitivity of less than 5×1014cm−3and a spatial resolution which is limited only by the spot size of a CO2(&lgr;=10.6 &mgr;m) laser; this should be of the order of 100 &mgr;m. Experimental results indicate that the predominate scattering is due to impurity interaction at room temperature; this is indicative of the high degree of compensation in CdS.