Why is athermalized design needed for the infrared optical system?

Why is athermalized design needed for the infrared optical system?

  1. Temperature change brings about changes in the parameters in the infrared system, which will influence its image plane position and image quality. Therefore, athermalized design is needed. Generally, temperature change 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 cause a difference in the focal length of the lens or of 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 evident in an 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. It is related to the optical material’s linear thermal expansion coefficient (a0). When the temperature changes, its radius of curvature and center thickness will become:

    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 temperature change. dT refers to the temperature variation.

    3.     The thermal effect of the lens cone material

    When the temperature changes, the dimensions of the assembly material will change, which causes a change in the air spaces between the optical components. Ultimately, it will influence the image quality. This change is related to 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.