Â© 2014 SPIE. We have characterized the optical response of prototype detectors for SAFARI, the far-infrared imaging spectrometer for the SPICA satellite. SAFARI's three bolometer arrays will image a 2'Ã—2' field of view with spectral information over the wavelength range 34-210 ÃŽ1/4m. SAFARI requires extremely sensitive detectors (goal NEP âˆ¼ 0.2 aW/Ã¢Å¡Hz), with correspondingly low saturation powers (âˆ¼5 fW), to take advantage of SPICA's cooled optics. We have constructed an ultra-low background optical test facility containing an internal cold black-body illuminator and have recently added an internal hot black-body source and a light-pipe for external illumination. We illustrate the performance of the test facility with results including spectral-response measurements. Based on an improved understanding of the optical throughput of the test facility we find an optical efficiency of 60% for prototype SAFARI detectors.