A high accuracy dispersive spectrophotometer for transmittance and reflectance measurements has been designed to provide calibration and standards in the 2 - 25 micrometers wavelength region. The system consists of the traditional modules of a double monochromator based spectrophotometer, i.e., source unit, predisperser, monochromator, detector, and sample, but organizes them in a novel way. The sample is positioned between the predisperser and the grating monochromator to achieve the highest feasible signal-to-noise ratio with a room temperature system. A convertible beam path allows measurements either in a collimated or in a converging beam. The minimum optical bandwidth of the system is 0.2%, consequently high accuracy measurements of state-of-the-art interference filters can be carried out. The system is under construction. The optical lay-out, the error sources, and uncertainties of the system are discussed.