Optical and electrical techniques are used to characterise deep- levels in silicon that could have applications in extrinsic silicon thermal imagers. The concentration of indium in indium-doped silicon substrates is estimated from Hall-effect and capacitance-voltage measurements together with chemical analysis and infra-red absorption. Junction space-charge measurements on diode structures are used to give information on the thermal emission rate, photoionisation cross-section, and enthalpy values of impurities and radiation-induced defects in silicon. The responsivity, noise and detectivity of indium-doped silicon detectors are measured and found to be in very good agreement with calculated values of these quantities. A model for the responsivity is developed which explains the general features of the experimental responsivity results in terms of capture and emission processes from acceptor levels. Finally, the use of the punch-through technique to produce a compact monolithic extrinsic silicon thermal imager is described and results of its low temperature operation are given.