Abstract: An athermal compound lens includes a plano-concave lens and a plano-convex lens. The plano-concave lens has a first focal length, a first refractive index n1, and planar object-side surface opposite a concave image-side surface. The plano-convex lens is axially aligned with the plano-concave lens and has (i) a second focal length, (ii) a second refractive index n2, (iii) a planar image-side surface, and (iv) a convex object-side surface between the planar image-side surface and the concave image-side surface. In a free-space wavelength range and temperature range: (a) the first focal length divided by the second focal length is less than ?0.68, and (b) first and second refractive indices n1 and n2 have respective temperature dependences ? ? ? n 1 ? ? ? T ? ? and ? ? ? ? ? n 2 ? ? ? T that satisfy ( ? ? ? n 1 ? ? ? T ) / ( ? ? ? n 2 ? ? ? T ) ? 2.

Abstract: An athermal compound lens includes a plano-concave lens and a plano-convex lens. The plano-concave lens has a first focal length, a first refractive index n1, and planar object-side surface opposite a concave image-side surface. The plano-convex lens is axially aligned with the plano-concave lens and has (i) a second focal length, (ii) a second refractive index n2, (iii) a planar image-side surface, and (iv) a convex object-side surface between the planar image-side surface and the concave image-side surface. In a free-space wavelength range and temperature range: (a) the first focal length divided by the second focal length is less than ?0.68, and (b) first and second refractive indices n1 and n2 have respective temperature dependences ? ? ? n 1 ? ? ? T ? ? and ? ? ? ? ? n 2 ? ? ? T that satisfy ( ? ? ? n 1 ? ? ? T ) / ( ? ? ? n 2 ? ? ? T ) ? 2.