A ZnSe positive single lens with aspheric rear surface is designed for $CO_2$-laser output (λ = 10.6㎛). The lens is of focal length 5cm, F/1.0, field of view $\pm2^\circ$.
In the present design work, firstly, the spherical surface design with minimum spherical aberration is obtained by using the bending process. Then the rear surface is aspherized by the iterative algorithm SLEM (simultaneous linear equations method) until the spherical aberration becomes so small that the geometric optical image of the axial object point is smaller than the Airy disk.
The effect of various choices of the adjustable SLEM parameters is investigated until the iteration becomes stationary. The result shows clearly and as usual that the correction of spherical aberration associated with higher incident height requires further detailed attention than that of spherical aberration associated with higher incident height requires further detailed attention than that of spherical aberration associated with lower incident height. The SLEM is insensitive to the differential increments of aspheric coefficients so that the increments may be chosen with less care in the approximation of the first order derivatives of the spherical aberration with respect to the aspheric coefficients. The aspherization of the front surface is also tried, and the result shows that the rear surface aspherization gives better lens performance.
To find the path of the principal ray associated with given field angle, an iterative method is developed, and it has proved to be effective in the calculation of tangential image field at large field angle.
