Achromatic and athermal lens design by redistributing the element powers on an athermal glass map

Abstract

We propose a new graphical method for selecting a pair of optical and housing materials to simultaneously achromatize and athermalize a multilens system. To obtain the material combination using an athermal glass map, the material suitable for housing is graphically selected, and then the powers of elements constituting an equivalent single lens are redistributed. Although a material combination does not exist, we can continuously change the power of each element. Thus, we can reasonably identify a pair of optical and housing materials that simultaneously satisfies achromatic and athermal conditions. By applying this method to design a Tessar lens, the chromatic and thermal defocuses are reduced to less than the depth of focus, over the specified waveband and temperature ranges.

Athermal and Achromatic Design for a Night Vision Camera Using Tolerable Housing Boundary on an Expanded Athermal Glass Map

“…From the Sweat model [9], the Abbe number of the diffractive element, V DOE , is given by (4) where λs and λl are the shortest and the longest wavelengths in the spectral bandwidth. For LWIR waveband from 8 µm to 12 µm, VDOE is about -2.5.…”

“…By differentiating the total power  T of Eq. (8) with respect to the temperature T, the thermal aberration is given as [4]: (9) Here, we assume that the first summation is negligible compared to the second summation, because the change of lens power with the temperature has greater influence on the total power fluctuation than that of ray height in a refractive optical system. Thus, in order to have an athermal system with total power  T , Eq.…”

“…Thus, in order to have an athermal system with total power  T , Eq. (9) can be rewritten as [4]: (10) where α h is the CTE of a housing material, and γ i is the thermal power of the i'th lens element.…”

“…The equivalent lens is defined as the lens that represents the optical properties of an original optical system, by use of the equivalent power (  ), equivalent chromatic power (  ′ ), and equivalent thermal power (  ). For the equivalent lens   consisting of N elements, these three quantities are given by [4,5]: (12) (13) (14) Assuming that a DOE has the optical power of Eq. (3) and no thickness, as Fig.…”

“…In the visible and infrared waveband, many design methods to reduce these defocuses have been reported [2][3][4][5]. Although these methods are good, they often lead to unworkable solutions: because of the limited availability of optical and housing materials in a long wave infrared (LWIR) optics, a suitable material combination for an athermal and achromatic design is not frequently obtained using the previously proposed methods.…”