We describe the design and performance of a liquid helium-cooled As:Si blocked-impurity-band photodetector system intended for spectrophotometry in the thermal infrared (2 to 30 mum) spectral region. The system has been characterized for spectral sensitivity, noise, thermal stability, and spatial uniformity, and optimized for use with a Fourier-transform infrared spectrophotometer source for absolute goniometric reflectance measurements. Its performance is evaluated and compared to more common detector systems used in this spectral region, including room-temperature pyroelectric and liquid-N(2)-cooled photoconductive devices.