JPH0279809A - Lens system - Google Patents

Abstract

PURPOSE: To make it possible to form an image on a common image plane over specified two light wave bands and to form a lens system from just tow kinds of materials by forming one optical path by one or more lens elements composed of zinc selenide and one or more lens elements composed of zinc sulfide or cesium bromide. CONSTITUTION: The lens systems has the first lens 1 composed of the zinc selenide and the second lens 2 composed of the zinc sulfide. A diaphragm 3 is disposed in front of these lenses 1, 2 so that the transmitted rays gather at the focus of the image plane 4. The lens system is so constituted as to exhibit a conjugation ratio, i.e., ratio of an object and the image plane of 2:1. The lens system which forms the image of 0.5 times the size of the object and brings about the imaging performance of the proximity diffraction limitation on the optical axis over a transmission wave band of 3 to 5μm and 8 to 12μm is obtd. according to this constitution As a result, the image good in both of the prescribed wave bands is formed by just two kinds of the different materials.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a lens system, and in particular to a lens system for use with infrared light.

PRIOR ART AND PROBLEM TO BE SOLVED BY THE INVENTION As is well known, all optically transparent materials have a refractive index n(λ) and a corresponding reciprocal of the dispersion power associated with a wavelength λ and an asserted wavelength spacing Δλ, respectively. That is, it has a V-value. If the index of refraction varies with wavelength, a lens exhibits chromatic aberration, ie, the degree of refraction depends on the wavelength of the refracted rays.

Generally, when it is necessary to converge two different wavelengths to a focal point on a common optical axis, two optical materials with different ■-values are required. Similarly, focusing three different wavelengths typically requires three materials with different V values.
The same applies below.

Optical systems used in the far infrared band (approximately 8-12 μm) typically use germanium as the only optical material. Although only one wavelength can be brought to a common focus in a system using only germanium elements, the relatively high V value of germanium, approximately 1000, generally makes chromatic aberration negligible for most applications. It is recognized that it is sufficiently small. However, some more recent applications and uses with more complex designs require, for example, correcting for chromatic aberration to converge two wavelengths to a common focus. In these cases, zinc selenide (ZnSe) or zinc sulfide (Zn
Other materials such as S) have been combined with germanium. Such systems are commonly used for 8μm and 11μm
It is used to focus two wavelengths in the far infrared light band, such as m, to a common focus.

Recent advances in electro-optical systems have enabled optical systems to focus both 3-5 μm and 8-12 μm light rays to a common focus within the limits set by the "Rayleigh quarter-wave law." I needed that. Current methods to achieve this generally involve ZnS, Zn
At least three different materials are used in the optical system, consisting of 5e and Ge. Previously, it was not thought possible to make a lens system meeting these requirements from just two materials.

[Means to solve the problem]

According to the invention, the lens comprises one or more lens elements made of zinc selenide and one or more lens elements made of zinc sulfide or cesium bromide. elements form one optical path,
A lens system is provided, characterized in that it is configured to image in a common image plane over both the 3-5 μm and 8-121 μm wavebands.

In a first preferred embodiment, the lens system includes one element comprised of zinc selenide and a second element comprised of zinc sulfide. The system may include a first lens constructed of zinc selenide and having a biconvex structure of equal curvature separated from a second lens by a relatively narrow air gap, the second lens having a concave meniscus structure. It is composed of zinc sulfide.

It can be seen that such an optical system is capable of producing good images in at least both of the above predetermined wavebands. Since only two different materials are used, this system is relatively easy to implement.

In a second preferred embodiment of the invention, the system comprises one zinc selenide element and one cesium bromide element.

The system preferably comprises a biconvex cesium bromide anterior lens of positive optical power separated by a relatively narrow air gap from a second zinc selenide lens of negative optical power.

Preferably, the second lens has a meniscus cross section.

This configuration, like the first configuration, can create a good image over the above-described waveband.

〔Example〕

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings.

Referring to FIG. 1, a lens system according to a first embodiment of the present invention includes a first lens 1 made of zinc selenide and a second lens 2 made of zinc sulfide. An aperture stop 3 is provided in front of these lenses.
The transmitted light rays are brought to a focal point in the image plane 4.

In this particular embodiment, the lune 1 is configured to be biconvex with equal curvature, as shown in FIG. 2, with radii on both sides of 36.51. In FIG. 2, the "Radius" column refers to the radius of curvature of the particular surface indicated, "Thickness" refers to the separation between said surface and the next surface, and "Aperture" refers to the diameter of said surface. This diameter is twice the aperture, and the "Material" column indicates what optical material is between this surface and the next. The system thus consists of a first lens 1 of biconvex structure of equal curvature with a radius of curvature 36.51 separated by an air spacing of 0°965 from a second lens 2 of zinc sulphide, the second lens having a negative radius of curvature 26. It has a first surface with a radius of .13 and a second surface with a negative radius of curvature of 167.67.

The lens system is thus configured to exhibit a conjugate ratio of 2:1, ie, an object to image plane ratio. Zinc sulfide lenses can also be used with any other material with similar chemical composition,
For example, it may be manufactured from a product that is a pure molded product of zinc sulfide known as C1eatran, a trademark owned by CVD, Woburn, MAO 1801, or from Multispectral, a product of Schott Glass-Welch. . According to the configuration shown in FIGS. 1 and 2, an image 0.5 times the size of the object is created,
3-5 μm and 8-12 μm measured in common image plane
A lens system is obtained that provides near-on-axis, diffraction-limited imaging performance over a transmitted waveband of .

However, many other lens systems can be configured within the scope of the present invention to exhibit different Gaussian characteristics, such as an equivalent focal length, in which case the focal length is 2
4.58 or a multiple thereof, but with Zn5e in the optical element.
It should be understood that it is also possible to use only ZnS and ZnS.

Figures 3 and 4 show the first and second lenses 5.6, respectively.
The second lens 2 is a biconvex lens with a positive optical power made of cesium bromide, and the rear lens 6 has a negative optical power. ,
It has a meniscus cross section and is made of zinc selenide. The actual dimensions of this system are shown in FIG. 4 as well as in FIG.

This second embodiment exhibits a conjugate ratio of infinity to 1 and 12
It shows an equivalent focal length of 4.969. Regarding the first embodiment described above, other configurations having more lenses or adapted for use with different wavelengths of light are also within the scope of this second aspect of the invention. You should know that it is considered. Also,
The example shown in Figure 3 is 3-5 μm measured at the same image plane.
Diffraction-limited performance can be achieved over two wave bands of 8 and 12 μm.

In addition, only Zn5e and C5Bh, or ZnS and Zn
When used alone, thermal changes in the axial position of the optical image are significantly reduced. In this regard, previous systems with germanium in the optical system were prone to thermal changes in optical performance. The systems disclosed herein are not prone to such thermal changes to the extent that germanium is.

Additionally, the optical system according to the invention is not only configured to operate over the 3-5 μm waveband and the 8-12 μm waveband, but also produces good images at visible wavelengths down to perhaps <0.5 μm. Can be done.

Lens systems according to embodiments of the invention can be advantageously used as diffractive elements in thermal imaging devices.

[Brief explanation of the drawing]

1 shows a second lens system according to the invention, FIG. 2 is a diagram showing the dimensions of the structure shown in FIG. 1, FIG. 3 shows a second lens system according to the invention, and FIG. 3 is a diagram showing the dimensions of the system of FIG. 3; FIG. 1---First lens, 2--Second lens, 3--Aperture, 4--Image plane. Front film ratio Figure 1 Equivalent focal length Figure 3 Equivalent focal length

Claims (1)

[Claims] 1. One or more lens elements made of zinc selenide and one or more lens elements made of zinc sulfide or cesium bromide, A lens system characterized in that the lens elements of are configured to form one optical path and to produce an image in a common image plane over both the 3-5 μm and 8-12 μm wave bands. 2. The lens system of claim 1, comprising one zinc selenide lens element and one zinc sulfide lens element. 3. Claim comprising a first zinc selenide lens with biconvex surfaces of equal curvature and a second zinc sulfide lens with negative optical refractive power, both lenses being separated by a relatively narrow air gap. 2. The lens system described in 2. 4. The lens system of claim 1, comprising one zinc selenide lens element and one cesium bromide lens element. 5. Comprising a first cesium bromide lens with a positive optical power and a second zinc selenide lens with a negative optical power, both lenses being separated by a relatively narrow air gap. The lens system according to claim 4. 6. A thermal image forming apparatus comprising the lens system according to any one of claims 1 to 5.