The detection of thermal radiation has received much attention in the past, but little has been done on the production of a visible picture of a thermal pattern. We can distinguish three independent processes in the operation of a thermal image-converter. First, the focused thermal radiation falling locally on the sensitive layer disturbs the thermal balance between this region and its surroundings, and a new radiation balance is set up; as a result the local temperature changes. Secondly, the temperature pattern consequent on the new equilibrium results in a variation over the layer area of some physical property. Finally the changes in this property are communicated to the eye. The efficiency of each of these processes is analysed, and it is shown that a converter is not most sensitive when the layer has an overall high absorption. The ideal layer has high absorption over the angle subtended by the optics, and zero absorption elsewhere. Calculations are made of the properties of a composite metal-dielectric film, and it is shown that this combination approximates to the ideal layer. A simple converter, the absorption-edge image tube, is used to test these ideas. Measurements are made of the shape of the absorption edge of amorphous selenium, and the shift of the edge with temperature. Other materials are also examined. The sensitivity of a tube using selenium is calculated, and then measured by photographic photometry. Good agreement is obtained between theory and experiment. The design principles which have resulted from this analysis are applied to obtain the highest sensitivity, and it is shown that bodies less than 10 °C above ambient temperature can be imaged with good resolution.