The availability of small detector arrays that operate in either the lower or the upper thermal infrared atmospheric windows has created the need for relatively simple low-mass optics. Crucial to the success of such an optic is the correct choice of optical materials. Infrared optical materials can be ranked in terms of their relative `lightness', but material choice also depends on other factors such as the correction of primary chromatic aberration, secondary spectrum and thermal effects. This paper provides details of the relative mass of materials when used in single element or achromatic doublet form, the limit set by secondary spectrum on the diameters of specific low-mass combinations, and the possible elimination of thermal defocus by the choice of lightweight combinations of materials. The use of surface relief holograms on candidate materials is included as part of the investigation.