What are the types of mechanical compensation in the athermal design of an infrared lens?

1)  Mechanical (electromechanical) active temperature compensation

From the basic theory of optics, we know that when the axial position of a lens (or lens group) in the optical system is changed, the focal plane position of the system will change accordingly. Active temperature compensation uses this principle to reproduce the system. The focus and adjustment method can be manual or electric. Motorized IR lens and manual IR lens can be understood as a kind of active mechanical/manual compensation athermal lens.

To improve the sensitivity of adjustment and maintain the stability of the optical axis, this temperature compensation method generally requires a precise mechanical transmission mechanism. At the same time, the required stroke of the temperature compensation lens should be investigated, and the most sensitive lens affected by the focusing surface should be selected as the temperature compensation element.

The basic mechanical transmission structure of this compensation method is basically the same as the focusing mechanism of the optical system. The method is simple in principle and easy to implement, but it increases the weight of the optical instrument, and at the same time easily brings about aiming errors.

2)  Mechanical (electromechanical) passive temperature compensation

The principle of mechanical (electromechanical) passive temperature compensation is basically the same as that of active temperature compensation, except that the way of changing the lens group has changed. It uses certain materials or mechanisms with special functions to realize automatic adjustment.