plate 110 acting as a mirror is 7 9 1 5 5 3 times weaker than thermal lens of the lens 120 this means that for this example the absorption of the coating of the thermal lens compensation plate 110 should be 5 3 times the absorption of the lens 120 to achieve a compensating thermal lens effect other conditions being the same with the absorption thus determined the thermal lens for the thermal lens compensation plate 110 should be the same in magnitude as but opposite in sign from the thermal lens for the lens 120 so that the former compensates for the later and reduces the temperature dependency of the overall focusing system in some circumstances it may also be desirable to compensate for the change in absorption of the lens 120 over its service lifetime a lens 120 made of znse has a typical initial absorption of about 0 15 percent this absorption typically doubles over the lifetime of such a final focus lens to about 0 3 percent it is preferable to choose the coating absorption on the thermal lens compensation plate 110 to match the absorption of the thermal lens 130 at the end of the lifetime of the thermal lens 130 this means that the coating on the thermal lens compensation plate 110 will have absorption of at least 0 3 5 3 1 6 molybdenum with an optical absorption of about 2 percent would provide an acceptably close match as a coating material if absorption of the coating on the thermal lens compensation plate 110 is based on absorption of the lens 120 at the end of its lifetime then it is preferable to.