Results of study optical performance of single pixel transition edge sensor (TES) bolometer presented at wavelength band 30-60 μm are presented. FIR radiation is coupled into a multimode horn with entrance aperture of 450 μm, length 4.5 mm and exit aperture of 45 μm , which feeds a metal integrating cavity containing the detector. The radiation band is defined by a pair of lowpass and highpass mesh filters in front of the horn. Here we present measurements of optical noise equivalent power (NEP), optical efficiency, dynamic range and time constant. The results show that measured TES detectors are close to meeting the requirement of the “Band S” of SAFARI FTS imaging instrument on the SPICA mission.