Infrared Objective System Lens

1. Field of the Invention

This invention relates to an infrared objective system lens for the wavelength range of 3.5 .mu.m-5 .mu.m.

2. Description of the Prior Art

Known infrared objectives of the above kind (e.g. DE-A-3008343) have either large chromatic aberrations or if not, their cost is rather high because of the required material. An achromatic infrared objective of low price and tolerable correction of monochromatic aberrations is desirable in particular for use with detectors of low resolution.

The principal object of this invention is to provide a low cost achromatic infrared objective system of medium grade correction.

The present invention provides an infrared objective system lens for the wavelength range of 3.5 .mu.m-5 .mu.m, consisting, as viewed from the object side, of a first positive lens made of silicon, a second lens made of calcium fluoride and a third positive lens made of silicon.

The invention will be described more in detail with reference to illustrative embodiments shown in the accompanying drawings.

FIG. 1 illustrates a cross section of a first embodiment of the invention;

FIG. 2 shows the transverse aberrations of an objective system as shown in FIG. 1 and TABLE 1 in the center of the field in dependence upon relative aperture for infrared wavelengths 3.5 .mu.m, 4 .mu.m and 5 .mu.m;

FIG. 3 shows the transverse aberrations of an objective system as shown in FIG. 1 and TABLE 1 for the meridional bundle of rays in the image zone in dependence upon relative aperture for infrared wavelengths 3.5 .mu.m, 4 .mu.m and 5 .mu.m;

FIG. 4 shows the transverse aberrations of an objective system as shown in FIG. 1 and TABLE 1 for the sagittal bundle of rays in the image zone in dependence upon relative aperture for infrared wavelengths 3.5 .mu.m, 4 .mu.m and 5 .mu.m;

FIG. 5 shows the transverse aberrations of an objective system as shown in FIG. 1 and TABLE 1 for the meridional bundle of rays in the peripheral parts of the image in dependence upon relative aperture for infrared wavelengths 3.5 .mu.m, 4 .mu.m and 5 .mu.m; and

FIG. 6 represents the transverse aberrations of an objective system as shown in FIG. 1 and TABLE 1 for the sagittal bundle of rays in the peripheral parts of the image in dependence upon relative aperture for infrared wavelengths 3.5 .mu.m, 4 .mu.m and 5 .mu.m.

In the cross section of the first embodiments shown in FIG. 1 the materials of the single lens elements are indicated. The front lens 1 and the meniscus lens 2 on the image side are made of silicon (Si). The third lens element 3 in between consists of calcium fluoride (CaF.sub.2). This objective lens is intended for use in a thermal imaging device having a scanning mirror 4 and a radiation detector 5 with a sapphire cover 6 in front of it. This sapphire cover 6 has been considered in optimizing the data of the first embodiment given in the following TABLE 1, where t and s are the axial thicknesses of and separations between the respective lens elements, R are the radii of curvature of the successive surfaces, f is the focal length of the lens system, F is the aperture or speed, s.sub.s is the air separation between the lens 2 and the cover 6 and S.sub.D is the air separation between cover 6 and detector 5.

Image size=2.7, half field angle=1.6.degree., radius of curvature of the image field=17.

In the following TABLE 2 are given values of the modulation transfer function (MTF) in percent in dependence upon spatial frequency in cycles per mm for the lens system according to TABLE 1 with a radius of curvature of the image field R=17, at aperture 0 (center of the image field), aperture 0.707 (image zone) and aperture 1.0 (periphery of the image field). For the calculation of the polychromatic MTF the different wavelengths are given the following weights: wavelength 3.5 .mu.m weight 1, wavelength 4.0 .mu.m weight 3, wavelength 4.5 .mu.m weight 3 and wavelength 5 .mu.m weight 3. MTF for image zone and for periphery of field are mean values for sagittal and meridional bundles.

In the following TABLE 3 are given the data of a second embodiment of an objective lens according to the present invention. R are the radii of curvature of the successive surfaces, t and s are the axial thicknesses of and separations between the respective lens elements, f is the focal length of the lens system, F is the aperture or speed and s.sub.D is the air separation between lens 2 and detector 5.

Half field angle=1.6.degree..

The state of correction of the embodiment of TABLE 3 is similar to that of TABLE 1, although the radius of curvature of the image field is smaller in the case of TABLE 1.

It is believed that the advantages and performance furnished by the infrared objective system lens of the invention will be apparent from the foregoing detailed description of the preferred embodiments of the invention. It will be apparent that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the accompanying claims.