A multiple wavelength, portable, solar spectroradiometer designed to measure atmospheric optical depth at several discrete wavelengths between 1 mum and 4 mum has been developed. The instrument employs a manually operated filter wheel to select wavelength bands with minimal gaseous absorption, and a thermoelectrically cooled PbSe photoconductor is used as the detector. Mechanically chopped solar radiation is converted by the photoconductor into a modulated electrical signal which is then processed by a miniaturized lock-in amplifier to produce a DC voltage proportional to solar irradiance. Example optical depth measurements are presented and discussed.