</HEAD> <BODY> <IMG SRC="../nep.jpg" WIDTH=800> <p> Far-IR detector sensitivity requirements. Noise equivalent powers are computed from photon shot noise arising from the thermal emission from the SPICA telescope adn the astrophysical sky. Values for the BLISS Fourier transform spectrometer (FTS, blue curve) and grating spectrometer (red curve) options are plotted. For the FTS, we assume the detector have lambda<sup>2</sup> throughput in a &lambda/&Delta&lambda = 2 bandpass with 25% end-to-end optical efficiency. For the grating, we assume lamba^2 throughput in a &lambda/&delta&lambda = 1000 bandpass with 25% end-to-end optical efficiency and one polarization. Note that the stray emission from 4 K (dotted line assuming A&Omega=3 mm<sup>2</sup> at &lambda/&delta&lambda = 2 and emissivity = 10%) must be carefully shielded. The large leap in capability from Herschel (green) to SPICA / BLISS corresponds to significantly lower photon NEPs. </BODY> </HTML>