  The next generation of far infrared space observatories will require
extremely sensitive detectors that can be realized only by combining extremely
low intrinsic noise with high optical efficiency. We have measured the
broad-band optical response of ultra-sensitive TES bolometers (NEP$\approx2\rm\
aW/\sqrt Hz$) in the 30--60-$\mu\rm m$ band where radiation is coupled to the
detectors with a few-moded conical feedhorn and a hemispherical backshort. We
show that these detectors have an optical efficiency of 60% (the ratio of the
power detected by the TES bolometer to the total power propagating through the
feedhorn). We find that the measured optical efficiency can be understood in
terms of the modes propagating through the feedhorn with the aid of a spatial
mode-filtering technique.
