Why is athermalized design needed for the infrared optical system?

Temperature variation will result in changes in all parameters of the infrared system, which will influence its image plane position and image quality. Therefore, athermalized design is required. Generally, temperature affects the infrared system in the following three ways:

1.     The refractive indexes of the infrared optical components will change when the temperature changes.

Under normal circumstances, the refractive indexes of the infrared optical components will change when temperature changes, which will change the focal length of the lens or the optical system.  The temperature coefficients of infrared optical materials are much larger than those of ordinary optical glass. For instance, the temperature coefficient value of K9 glass is only 2.8x10-6C-1, while that of the single crystal germanium dn/dt (a commonly used material for making infrared lenses) is 396x10-6C-1, about 141 times larger than the former. Therefore, the influence of temperature on the refractive index is quite apparent in the infrared system.

2.     The radius of curvature and the center thickness of the infrared optical components will change when the temperature changes.

This change is caused by the fact that the material of the components expands on heating and contract on cooling, which is related to the optical material’s linear thermal expansion coefficient (a0). When the temperature changes, its radius of curvature and center thickness will turn into:

D' =D+dD=D+D* a0*dT

R' =R+dR=R+R* a0*dT

Note: R and R' are respectively its radius of curvature before and after temperature change; D and D' are respectively its center thickness before and after the temperature change. dT refers to the temperature variation.

3.     The thermal effect of the lens tube material

When the temperature changes, the dimensions of the assembly material will change, which will cause a change in the air gap between the optical components. Ultimately, it will influence the image quality. This change is associated with the assembly material’s linear expansion coefficient.

Among the above three main factors, the change of the refractive index of the optical material has the greatest influence on the image plane position and image quality; the influence of the radius of curvature is the second greatest, while changes in the thickness of optical components and the space between them have the least influence.