An apparatus and method are presented for the analysis of materials. The apparatus includes two or more similar analyzers, with the output of the analyzers combined to provide improved measurements. The apparatus may be, for example, a differential photometric analyzer, such as the AETHALOMETER®. The apparatus includes a processor programmed to accept an instrument constant determined at low filter loadings and use the constant to compensate for non-linear instrument responses. A method is also presented for conditioning filters before use.