GB2141556A

GB2141556A - Glass and plastics objective lens for low light level viewing apparatus

Info

Publication number: GB2141556A
Application number: GB08413094A
Authority: GB
Inventors: Philip John Rogers
Original assignee: Pilkington PE Ltd
Current assignee: Qioptiq Ltd
Priority date: 1983-06-14
Filing date: 1984-05-22
Publication date: 1984-12-19
Also published as: GB8413094D0

Abstract

A lightweight and compact objective lens for a night sight has several plastics lens elements (2, 3, 4) bounded by glass lens elements (1, 5) which provide optical correction. A positive glass focussing element (6) and a vignetting stop (9) may be located between the glass bounded plastics arrangement and the photocathode (8) of an image intensifier (7) onto which light is in use focussed by the objective lens. <IMAGE>

Description

SPECIFICATION Improvements in or relating to low light level viewing apparatus This invention concerns improvements in or relating to low light level viewing apparatus.

Low light level viewing apparatus, such as is used for night vision sights, commonly comprises an objective lens which focusses incident light on to a photocathode of an image intensifier tube. The image of the scene or object under view thus formed on the photocathode is intensified by the action of the tube to produce a corresponding image of increased brightness on a phosphor at the other end of the tube. The phosphor image is viewed by an observer through an eye-piece lens or other magnifier.

In use such apparatus is commonly hand held, or mounted on a weapon which is hand held, or in some cases the apparatus is head-mounted. There is thus generally a requirement for the apparatus to be light in weight and compact so as to facilitate portability. However, there is also of course a requirement for satisfactory optical performance. Plastics optical elements have the advantage over glass elements of lightness in weight but can suffer from inadequate optical properties and are also subject to damage by abrasion.

According to the present invention there is provided an objective lens for use in low light level viewing apparatus comprising a plurality of lens elements of plastics material which provide at least the major part of the optical power of the overall lens, and a pair of relatively thin outer glass lens elements which effect optical correction disposed one in front of and one behind the lens elements of plastics material.

Preferably the glass lens elements are of negative power and provide chromatic correction and the lens elements of plastics material are all of positive power. The plastics material may be an optical plastics material having low internal scatter and in particular acrylic (polymethylmethacrylate).

For simplicity of focussing there may be provided a single positive glass lens element behind the above-mentioned arrangement of plastics elements bounded by glass elements, i.e. in use between the back above-mentioned outer glass lens element and the image intensifier. The back outer glass lens element may act also as a colour correction filter, and the lens may include a vignetting stop.

The refracting faces of the glass lens elements are preferably of spherical curvature. The refracting faces of the lens elements of plastics material can be aspherised with relative ease, and may therefore be of aspheric curvature in particular to increase the aperture. It will be understood, however, that the plastics elements could have refracting faces all of spherical curvature, and/or the glass elements could have one or more faces of aspheric curvature, if desired.

The present invention further provides low light level viewing apparatus, and in particular a night vision sight, comprising an objective lens as set forth above and an image intensifier having a photocathode onto which light is in use focussed by the objective lens.

In order that the invention may be better understood, a preferred embodiment thereof will now be described, by way of example, with reference to the accompanying drawing, the single figure of which is a schematic representation of an objective lens with an image intensifier tube in a night vision sight.

The objective lens comprises from front (which in use faces towards the scene or object being viewed) to back individual singlet lens elements 1 to 6. The front lens element 1 is of optical glass and is a relatively thin negatively powered meniscus element with a convex front face R1 and a concave rear face R2. The following three lens elements 2, 3 and 4 are all of an optical plastics material and are all of positive power so that they provide the bulk of the optical power of the overall objective lens. Specifically the first element 2 of the three is bi-convex with convex front and rear faces R3 and R4 respectively, and the other two elements 3 and 4 are meniscus with respective convex front faces R5 and R7 and concave rear faces R6 and R8.Behind the positive plastics lens elements 2, 3 and 4 is another optical glass lens element 5 which is a relatively thin negatively powered meniscus element with a convex front face R9 and a concave rear face R 10. The two glass lens elements 1 and 5, being negative in power, provide the chromatic correction in the system so that the plastics elements 2, 3 and 4 can all be positive and thus made from an optical plastics material having low internal scatter, namely acrylic (polymethylmethacrylate). The glass lens elements 1 and 5 disposed one in front of and one behind the plastics elements 2, 3 and 4 constitute a pair of outer elements which protect the plastics elements 2, 3 and 4 against damage by abrasion.

The stop S of the objective lens system is at the back face of the back outer glass lens element 5, and behind that is a focussing member in the form of a small single glass lens element 6 of positive power. Specifically, the element 6 is bi-convex with convex front and rear faces R11 and R12. The focussing element 6 is disposed between the glass-bounded plastics construction formed by lens elements 1 to 5 and the image intensifier tube 7 and its position along the optical axis can be adjusted in known manner to effect correct focussing of light from the scene or object under view on to the photocathode 8 at the front end of the image intensifier tube. This element 6 thus provides a simple means for adjusting the focus of the objective lens to form a clear image on the photocathode 8 of a scene or object at a particular distance.The drawing also indicates a vignetting stop surface 9 disposed between the focussing element 6 and the photocathode 8. The image formed on the photocathode 8 is intensified by the action of the tube 7 to produce a corresponding image of increased brightness on a phosphor at the back end of the tube (not shown) which can be viewed by an observer through an eye-piece lens or other magnifier in well known manner.

It will be seen that the system described above and shown in the drawing resembles a tripletderivative of fairly short length relative to its focal length. The solid lens elements are gas, and conveniently air, spaced. In the preferred embodiment the refracting faces R1, R2, R9, R10, R11 and R12 of the glass lens elements 1, 5 and 6 are all of spherical curvature, but it will be understood that one or more aspheric faces could be used if required and the additional manufacturing expense can be accepted. The refracting faces R3 to R8 of the plastics elements 2 to 4 can be aspherised with relative ease and one or more of these faces can be made aspheric to enable a higher aperture (e.g. F/1.5) to be achieved. However, if such higher aperture is not required, these faces could all have spherical curvature.

It will be appreciated that an objective lens as described above can be of light weight and short in length whilst achieving satisfactory optical performance, and with the plastic lens elements protected by the outer glass lens elements against abrasive damage. It can therefore contribute to a light weight and compact viewing apparatus.

A particular example of objective lens in accordance with the embodiment shown in the drawing has numerical data as follows, the dimensional units being millimetres but the values are relative and can be scaled accordingly. The lens elements 1, 5 and 6 are of optical glass with refractive indices Nd and constringences V as follows: Element Nd V 1 1.70585 30.30 5 1.80911 25.28 6 1.64769 33.85 The plastics lens elements 2, 3 and 4 are of acrylic (Perspex) material with refractive index Nd of 1.49176 and constringence V of 57.4. The solid lens elements are air spaced.

Radius of Axial Thickness/ Element Surface Curvature Separation R +621.02 1 2.566 R2 + 105.31 1.745 R3 Aspheric 2 12.512 R4 Aspheric 0.246 R5 Aspheric 3 16.166 R6 Aspheric 0.246 R7 Aspheric 4 , 16.166 R8 Aspheric 9.412 R9 + 273.00 5 2.053 R10 + 27.990 20.128 R11 +113.56 6 4.434 R12 -79.594 Equivalent Focal Length: 100 Aperture: F/i.5 Field of View: 10 degrees A vignetting stop, which limits the beam to the width of the full aperture axial beam, is provided by a planar vignetting surface 9 located 2.607 mm axially behind the surface R12 and 35.900 mm axially in front of the input fibre-optic face place of the tube 7.

The aspheric surfaces of the plastics lens elements 2, 3 and 4 have curvatures defined as follows where X, Y and Z are mutually orthogonal co-ordinates and S2 = y2 + Z2 Surface R3:

Surface R4:

Surfaces R5 and R7:

Surfaces R6 and R8: X = + 1.5646 X 10-3S2 + 1.4785 X 10-7S4-2.7980 X 10-' S6 + 1.3926 x 10-'4S8 It should be noted that in the above example the first air gap (i.e. between surfaces R2 and R3), which is nominally 1.745 mm, can be adjusted to take out the effects of consistent errors in the aspheric curves or consistent (i.e. batch to batch) variation of the acrylic material refractive index.

Further, the rear glass lens element 6 can be of one of several glass types having nominally or approximately the same refractive index (Nd) but different constringences (V values) to take out the effect of the partial dispersion of the arylic material differing from published data (Attman and Lytle, Optical design techniques for polymer optics, SPIE Vol. 237, page 380). Such possible glass types are: Glass Type Nd V SF2 1.64769 33.85 BaSF10 1.65016 39.15 BaF5l 1.65224 44.93 SSkN5 1.65844 50.88 SkNi8 18 1.63854 55.42 LaKN7 1.65160 58.52 The small performance changes resulting from change of glass can be compensated by adjusting the first air gap as mentioned above.

The back outer glass lens element 5 is a hard radiation stabilised (cerium doped) glass (type SF6 G5) which has a strong absorbtion below 500 nm wavelength. The element 5 thus acts both as a lens element and as a minus blue filter. It will be appreciated, however, that a colour correction filter, if required, could be provided as a separate component and the lens element 5 could then be made of a glass without special absorbtion characteristics.

It will be appreciated, in any case, that the above example, and the embodiment shown and described herein, are given by way of illustration and example and that other examples and embodiments of lens in accordance with the invention may be designed by those skilled in the art.

Claims

1. An objective lens for use in low light level viewing apparatus comprising a plurality of lens elements of plastics material which provide at least the major part of the optical power of the overall lens, and a pair of relatively thin outer glass lens elements which effect optical correction disposed one in front of and one behind the lens elements of plastics material.

2. A lens according to Claim 1 in which the glass lens elements are of negative power and provide chromatic correction and the lens elements of plastics material are all of positive power.

3. A lens according to Claim 1 or Claim 2 in which said plastics material is an optical plastics material having low internal scatter.

4. A lens according to Claim 3 in which said plastics material is acrylic.

5. A lens according to any preceding claim further comprising a single positive glass lens element behind the arrangement of plastics elements bounded by glass elements.

6. A lens according to any preceding claim in which the back outer glass lens element is arranged to act also as a colour correction filter.

7. A lens according to any preceding claim including a vignetting stop.

8. A lens according to any preceding claim in which the refracting faces of the glass lens elements are of spherical curvature.

9. A lens according to any preceding claim in which the refracting faces of the lens elements of plastics material are of aspheric curvature.

10. An objective lens for use in low light level viewing apparatus substantially as described herein with reference to the accompanying drawing.

11. A lens according to Claim 10 substantially in accordance with the particular example set forth herein.

1 2. Low light level viewing apparatus comprising an objective lens according to any preceding claim and an image intensifier having a photocathode onto which light is in use focussed by the objective lens.

1 3. A night vision sight according to Claim

1 2.