Abstract: A sheet of thermally reflective material (1) has a surface texture comprising a plurality of reflecting elements (2), wherein each element has a first facet (4) which is substantially reflective at thermal infrared wavelengths and wherein the respective first facets (4) form an angle ? with the plane of the sheet (A-B) (0°<?<90°). Preferably, the first facets (4) are aligned such that, in use, thermal radiation is reflected from a common direction. By orienting the sheet of thermally reflective material to reflect cold regions of the sky, a marking material exhibiting a cold spot in a thermal imager can be provided.