A new lead sulfide integrating detector assembly has been developed for applications in NIR spectroscopy in the 1 - 3 micron spectral region. The assembly contains a 128 element, thermoelectrically cooled, multiplexed detector array sensor, a driver control unit, and a software interface for signal digitization and processing. The system has selectable single scan integration times of 1 ms - 25 ms, and a signal-to-noise dynamic range of greater than 1,000 at 10 ms integration time. Dark referencing is used to control dark signal drift and to maintain pixel-to-pixel offset uniformity to within 1%. The PbS detector assembly has been interfaced with an optical fiber, a dispersion grating, and a laptop computer, to produce a compact 1 - 3 micron spectrophotometer with a per pixel resolution of 12 nanometer.