We have measured the optical response of detectors for SAFARI, the far-infrared imaging spectrometer for the SPICA satellite. SAFARI's three bolometer arrays, coupled with a Fourier transform spectrometer, will provide images of a 2′×2′ field of view with spectral information in three bands covering the wavelength range 34 - 210 μm. Each bolometer consists of a transition edge sensor (TES) with a transition temperature close to 100 mK and a resistive absorber on a thermally-isolated silicon nitride membrane. Each detector sits behind a concentrating horn and in front of a hemispherical backshort. SAFARI requires extremely sensitive detectors (NEP∼2×1019 W/√Hz) to take advantage of SPICA's cooled optics. To meet the challenge of testing such sensitive detectors we have constructed an ultra-low background test facility with a cryogen-free high-capacity dilution refrigerator, paying careful attention to stray-light exclusion. The SAFARI Detector System Test Facility provides a high degree of electrical, magnetic, and mechanical isolation for the detectors, allowing us to use a pulse-tube cooler to pre-cool the dilution refrigerator. We have verified the test facility's performance in terms of background power loading and carried out cross-calibrations with other test systems. The SAFARI Detector Test Facility is now being used for routine dark and optical measurements of ultra-sensitive TES bolometers. We report on optical measurements of prototype SAFARI detectors.