Low thermal stress birefringence imaging lens

A lens system having reduced susceptibility to thermally-induced stress birefringence for imaging an object plane onto an image plane comprising;

• a plurality of lens elements arranged to image the object plane onto the image plane,
• wherein the lens element that experiences a highest optical power density from transiting light when the lens system is used to image the object plane onto the image plane is fabricated using a glass having a negligible susceptibility to thermal stress birefringence as characterized by a thermal stress birefringence metric;
• wherein the other lens elements are fabricated using glasses having a negligible or a moderate susceptibility to thermal stress birefringence as characterized by the thermal stress birefringence metric, at least one of the other lens elements being fabricated using a glass having a moderate susceptibility to thermal stress birefringence;
• wherein the thermal stress birefringence metric has the form:
• M1=ρκαE/(K·(1−μ).
• where ρ is a coefficient of thermal expansion of the glass, κ is a stress optical coefficient of the glass, α is a light absorption coefficient of the glass, E is a modulus of elasticity of the glass, K is a thermal conductivity of the glass, and μ is a Poisson's ratio of the glass;
• and
• wherein the glasses having the negligible susceptibility to thermal stress birefringence satisfy the condition that M1≦0.1×10−6 W−1 and the glasses having the moderate susceptibility to thermal stress birefringence satisfy the condition that 0.1×10−6 W−1≦M1<1.6×10−6 W−1.