  We describe the optical characterisation of two silicon cold-electron
bolometers each consisting of a small ($32 \times 14~\mathrm{\mu m}$) island of
degenerately doped silicon with superconducting aluminium contacts. Radiation
is coupled into the silicon absorber with a twin-slot antenna designed to
couple to 160-GHz radiation through a silicon lens.The first device has a
highly doped silicon absorber, the second has a highly doped strained-silicon
absorber.Using a novel method of cross-correlating the outputs from two
parallel amplifiers, we measure noise-equivalent powers of $3.0 \times
10^{-16}$ and $6.6 \times 10^{-17}~\mathrm{W\,Hz^{-1/2}}$ for the control and
strained device, respectively, when observing radiation from a 77-K source. In
the case of the strained device, the noise-equivalent power is limited by the
photon noise.
