SPIRIT is a spatial and spectral interferometer with an operating wavelength range 25 μm - 400 μm. As a double-Fourier interferometer, SPIRIT features sub-arcsecond spatial resolution and R≡λ/Δλ=3000 spectral resolution over a 1 arcmin field of view. Its three primary scientific objectives are to: (1) Learn how planetary systems form from protostellar disks, and how they acquire their chemical organization; (2) Characterize the family of extrasolar planetary systems by imaging the structure in debris disks to understand how and where planets form, and why some planets are ice giants and others are rocky; and (3) Learn how high-redshift galaxies formed and merged to form the present-day population of galaxies. The detector subsystem provides a set of far-infrared detector arrays in the SPIRIT instrument. These arrays are used for science purposes by detecting the faint interferometric signal. The resulting technology requirement is for a set of eight arrays operating at wavelengths of 25 μm - 400 μm, divided into two arrays (one for each interferometer output port) per octave of wavelength. At the short wavelength end, the arrays are 14×14 pixels, shrinking to 2×2 at the longest band. The per-pixel sensitivity requirement of 10-19 W/√Hz, coupled with speed of τeffective ~150 μs, make these relatively small arrays challenging. The operating temperature necessary to provide this sensitivity is around 50 mK. Over the majority of the SPIRIT wavelength range and sensitivity requirement, there are no commercial vendors of such detector arrays, and thus they will require a separate NASA-supported development.