A converter and method of converting thermal images into secondary images, e.g. visible images. The infrared radiation emitted by an object is focused by an optical system to form a thermal image on the free surface of a thermal radiation absorbing layer to one side of a liquid-liquid interface which has low surface tension and is a mixture of two liquid partially miscible at operating temperature and pressure. To convert the thermal image to a visible a parallel beam of light or a beam of light from a point source in the visible range is directed to the other side of the interface and, depending on the selected indices of the interface liquids and whether a prism is used or not; is either transmitted or reflected. The emerging transmitted or reflected beam is then received on a screen or in a detection system where variations of the optical properties of the emerging beam are observed.
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