Imaging medium with bubble-suppressant layer

An imaging medium capable of being imaged to form a transparency comprises a substantially transparent support having a thickness of at least about 20 mu m, a color-forming layer superposed on the support and comprising a color-forming composition adapted to undergo a change of color upon increase in the temperature of the color-forming layer above a color-forming temperature for a color-forming time; and a bubble-suppressant layer superposed on the color-forming layer and having a thickness of at least about 10 mu m. Upon imagewise increase in the temperature of the color-forming layer above the color-forming temperature for the color-forming time, in heated regions the color-forming layer undergoes its change of color but remains essentially free from bubbles, thereby providing an image in which the colored regions are not blackened when viewed in transmission.