The use of single crystal MgF.sub.2 material as a window for missile systems has at least the advantages of having superior transmission for both the UV and IR wavelength regions, being stable in sunlight, having a low refractive index, thereby eliminating the necessity for anti-reflection coatings, being unaffected by water, therefore eliminating the need to provide wetness protection, having the mechanical strength to match the applicable usage, being able to be fabricated without excessive complication, thereby reducing manufacturing costs while assuring a high quality product, generating low scatter and being readily available from multiple domestic sources.