A thermal cell has an infrared source element for producing an infrared signal and a spatial decorrelator element which spatially decorrelates the generated infrared signal to increase the fill factor of the thermal cell. The infrared source element includes an infrared generating cell, with a resistive element. The spatial decorrelator element is disposed above and spaced apart from the infrared source element and has an aperture formed therein and through which the infrared signal radiates. The aperture extends substantially orthogonal to the primary surface of the source element and between primary surfaces of the decorrelator element, which are in turn substantially parallel with the source element primary surface. The side walls of the aperture reflect the infrared signal. The opening in the primary surface of the decorrelator element which is furthest from the primary surface of the source element has an area which is greater than that of the opening in the primary surface of the decorrelator element which is closest to the primary surface of the source element.