The Photodetector Array Camera and Spectrometer on Herschel provides imaging line spectroscopy and imaging photometry in the 55–210 μm wavelength band. In photometry mode, two filled silicon bolometer arrays with 16×32 and 32×64 pixels, respectively, are used to simultaneously image two bands, 60–85 μm or 85–130 μm and 130–210 μm, over a field of view of ~1.75'×3.5', with Nyquist beam sampling in each band. In spectroscopy mode, two Ge:Ga photoconductor arrays (stressed and unstressed) with 16×25 pixels, each, are used to image a field of ~50''×50'', resolved into 5×5 pixels, with a spectral resolution of ~175 km s-1 and an instantaneous spectral coverage of ~1500 km s-1. In both modes the performance is expected to be not far from background-noise limited, with sensitivities (5σ in 1 h) of ~4 mJy or 3–20×10-18 W/m2, respectively.
We summarise the design of the instrument and its subunits, describe the observing modes in combination with the telescope pointing modes, report results from instrument level performance tests of the Flight Model, and present our current prediction of the in-orbit performance of the instrument.