GB2178557A - Visual optical systems - Google Patents

Abstract

In a visual optical system (40) having an image relay optical assembly for relaying an image (44) via a substantially parallel light space (47) to an image (45), the assembly includes a first lens group E spaced by parallel light section (47) from a second lens group F. Lens groups E and F are each doublets and each has a positively powered lens element E2, F1 and a negatively powered lens element E1, F2 arranged such that lens element E1 is proximal image (44) and lens element F2 is proximal image (45). The positively powered lens elements E2, F1 are made of a first glass and the negatively powered lens elements E1, F2 are made of a second glass, the V-value of the first glass being greater than the V-value of the second glass by at least 10 units and the refractive index of the first glass being greater than 1.54 and having a value similar to or greater than the refractive index value of the second glass which is itself numerically less than 1.74. The relative partial dispersion of the first glass is substantially equal to the relative partial dispersion of the second glass. <IMAGE>

Description

SPECIFICATION Visual optical systems This invention relates to visual optical systems.

In certain forms of visual optical systems there is a requirement to relay an image between first and second image surfaces by means of an image-relay optical assembly incorporating a parallel-light section. By way of example such assemblies are utilised in microscopes where the parallel-light section is used to inject a graticule by means of a partial mirror located in the parallel-light section. Such assemblies are also used in viewing systems where image rotation is encountered and the parallel-light section is used to accommodate an image de-rotation prism.

Such assemblies are also used in long optical systems where the parallel-light section is used to confine the diameter of the system so that it is only a fraction, e.g. 5%, of the length.

Hitherto in the design of visual optical systems having an image-relay optical assembly incorporating a parallel-light section the nature of the components has been determined on the basis that detection of the image would be by the human eye so that the resolution and aberration performance of the assembly has been tailored accordingly. However, in more modern systems there is a need to provide for non-human-eye detection since either an electro-optic or a photographic detector may be used and this requires improved resolution and aberration performance of the assembly.

It is an object of the present invention to provide a visual optical system having an imagerelay optical assembly incorporating a parallel-light section and which has a resolution and aberration performance suited to detection of the image by a non-human-eye detector.

According to the present invention there is provided a visual optical system having an imagerelay optical assembly for relaying an image between first and second image surfaces, said assembly comprising a first lens group which is spaced from a second lens group by a parallellight section, each of said lens groups comprising a positively-powered lens element and a negatively powered lens element, the lens elements which are proximal the respective image surfaces having like power, each of said positively-powered lens elements being made of a first glass, and each of said negatively-powered lens elements being made of a second glass, wherein the V-value of the first glass is greater than the V-value of the second glass by at least ten units, the refractive index of the first glass is greater than 1.54 and in value is similar to or greater than the value of the refractive index of the second glass, the refractive index of the second glass is less than 1.74, and the relative partial dispersion of the first glass is substantially equal to the relative partial dispersion of the second glass.

It will be understood that the optical system is designed for use in the visible wavelength region which has a waveband between about 0.48 microns and 0.66 microns and that the Vvalue is determined by the equation nd-1 V = nf-nc and the relative partial dispersion (P) is determined by the equation nd-nc P = nc-nf where n is the refractive index of the pertaining glass at the known spectral wavelengths which are subscripted to n, where f=0.486 microns d=0.588 microns (21 - (f+c)) c=0.656 microns By virtue of the present invention the optical system is mechanically and optically simple in that only two glasses are used and because of the selected V-values primary colour aberrations both monochromatic and polychromatic can be substantially corrected with relatively simply shaped refractive surface curvatures.Because of the selected P-values secondary longitudinal chromatic aberration is substantially eliminated. Because of the selected values of refractive index (n) of the two glasses the system does not require use of fluoro-crown glasses. Furthermore, the combination of selected values provides a very low Petzval sum (i.e. a good aberration performance) and the performance of the system within the parallel-light section is substantially polychromatically diffraction limited. Accordingly the system of the present invention is suited to a non-human-eye detector, for example, a TV detector, a CCD detector or a photographic film detector.

Preferably said image-relay optical assembly is one of a pair of like assemblies optically connected in cascade in said system whereby an image is relayed between said first and second image surfaces and between said second image surface and a third image surface, and a field lens made of a third glass, is located adjacent at least one of said image surfaces to provide for reduction of vignetting.

Conveniently said field lens is formed by split lens elements located on either side of said at least one image surface to reduce image contamination by the presence of the field lens.

Conveniently the negatively-powered lens elements of each of said assemblies are proximal the pertaining image surface. Alternatively the negatively-powered lens elements of one assembly may be proximal the pertaining image surfaces and the positively-powered lens elements of the other assembly proximal the pertaining image surfaces.

In the latter case spherochromatism (i.e. variation of spherical aberration with wavelength) is substantially eliminated from the system.

It is preferred that the air space between the lens elements forming each lens group is substantially zero on the optical axis but, if present, does not esceed about 20% of the diameter of the lens elements so that the lens elements form a doublet.

By way of example the first glass may be that manufactured and sold by Schott under their designation LgSK2; the second glass may be that manufactured and sold by Schott under their designation KzFSN4; and the third glass may be that manufactured and sold by Schott under their designation BK7.

For LgSK2 glass P=0.3004, V=61.04 and no=1.58599 For KzFSN4 glass P=0.3003, V=44.30 and no=1.61340 An embodiment of the present invention will now be described by way of example with reference to the accompanying schematic drawing which illustrates a long optical system 40 axially split at lines X-X and Y-Y in the interests of clarity.

System 40 is formed by a series of lens elements arranged in lens groups A,E,F,I,J,K with an additional lens section 50 which comprises an accumulation 50' of similar lens groups as will be explained. The lens groups A,E,F,I,J,K are in the form of doublets and triplets each made from only two materials a doublet or triplet being defined as a closely air spaced (compared to diameter) or contacted series of elements. Additionally system 40 comprises field lenses C,G,H lying close to or at an image, and protective windows B,D.The optical system is of the refractor type, wherein all lens elements lie upon an optical axis 43 and are so positioned that radiation entering the optical system 40 from object space 41 via a pupil 0 forms a final real image 46 in image space 42, the axial field pencil having no vignetting and the field pencils being progressively more vignetted up to full field. The maximum vignette being about 18% by area of the axial field pencil and being caused by physical stops (not shown) within the optical system 40.

The overall length of said optical system is about 14.7 metres with the maximum aperture diameter about 0.139 metres. The substantial length of the optical system is achieved by means of repeated and similar optical assemblies so that in this example a first image 44 is formed and transferred via a substantially parallel light space 47 to a second image 45 which is similarly transferred via a second substantially parallel light space 48 and so on until a final parallel light space 49 and final real image 46 are formed.

The doublets and triplets are each made of only two optical materials, the positive element being a 'crown' glass and the negative element being a 'flint' glass, and the other lens elements are made of any suitable glass, all materials are transmissive in the visible waveband and can readily accept optical coatings which improve transmission, reduce unwanted reflections and are protective to the glass substrate. Both the glass types and the general configurations of optics has been chosen so that the resolution of said optical system is near-diffraction limited over about 3/4 of the field of view at the first and last images and at all the substantially parallel light spaces. Conveniently only three glass types have been used throughout the optical system, LgSK2 and KzFSN4 being used in doublets and the triplet and BK7 (Schott glass designations) used in all other lens elements.

Lens element C is so positioned that it can afford a graticule on one of its surfaces and both surfaces are protected from contaminations from foreign bodies by elements B and D; an added precaution is increased centre thickness of element C which is not required for strength or rigidity but does make the non-graticule surface less contamination sensitive, the losses of transmission and resolution being insignificant compared to the aforesaid benefit. Field lenses G and H have been placed at an appropriate distance from the image 45 to make them also less sensitive to contamination and the resultant loss of performance.

The optical system substantially corrects all aberrations except field curvature and a small residual of spherochromatism. However in order to minimise the latter aberration the lens doublet J has its order of elements, i.e. positive and negative power in reverse from the normal as shown in I. In lens group I the near parallel or parallel radiation first hits the positive element 12 then hits the negative element 1, whereas lens group J has the reverse order. A further reduction in spherochromatism has been achieved by using a triplet K instead of a doublet and positive element K, has been air-spaced at an optimum distance from K2 to compensate for spherochromatism.

The refractive surfaces 1,2,3,4.... 36 are each preferably substantially 'spherical' (or 'flat') within the meaning of the art.

The optical system is designed for use in the visible wavelength region (i.e. 0.48-0.66 micrometres), and consequently has been colour corrected over this waveband, calculated according to the following equation: nd-nc P = nc-nf where P is the relative partial dispersion over the waveband 0.486-0.656 micrometres and n is the refractive index of the material at the wavelengths subscripted to n i.e. d=0.588 micrometres, C=0.656 micometres and f=0.486 micrometres; then LgSK2 and Pa+0.3004 and KzFSN4 has P=0.3003 which as can be seen are almost equal.Calculated according to the following equation: nd-1 V= nf-nc where V is V-value or inverse primary dispersion over the wavebank 0.486-0.656 micrometres and n is the refractive index of the material at the wavelengths subscripted to n and previously described, then LgSK2 has V=61.04 and KzFSN4 has V=44.30, the difference between them being 16.74. The refractive index at a wavelength of 0.586 micrometres, i.e. nd, is 1.58599 for LgSK2 and 1.61340 for KzFSN4 from which it will be seen that the two refractive indices are similar in value.

One example of the optical system 40 is detailed in Table I wherein the radius of curvature of each refractive surface is given together with the aperture diameter of each surface and of the pupil 4), the position of which is used as a datum from which the separation of successive refractive surfaces is defined, together with the nature of the material relevant to such separation interval.Thus, for example, surface 3 has a radius of curvature of -79.83 millimetres, the negative sign indicating that the centre of curvature is to the left hand side of surface 3; it is separated by an air space of 1.00 millimetre from the preceding surface, No. 2 in the direction of the pupil 4); it has an aperture diameter of 66.9 millimetres; and is separated from the succeeding surface, No. 4 by a distance 8.00 millimetres in glass known under the Schott glass designation KzFSN4. The aperture diameters given are for about 18% vignette by area of the full field pencil compared to the axial field pencil.Specific values of image quality for this optical system are given in Tables II, I1I, IV, V and VI the former and latter providing data relevant to the first and last real images, respectively, and the others commencing with Ill providing data relevant to the first, second and final near parallel light spaces, respectively, the quantity of field curvature being apparent from the difference due to focus shift of the optimum R.M.S. spot positions.

The optical system which has been described provides high performance over at least 2 of the full field with a maximum vignette of less than 20% at full field.

The system detailed in Tables l-VI inclusive can be scaled and optimised to provide various overall lengths, overall diameters, fields of view, entrance pupil diameters, quantity of vignetting and effective focal lengths. It is also possible to optimise this optical system and/or an additional attached optical system to provide different conjugates for the output radiation and variations in the field curvature. It is also possible to attach at either end and/or insert additional optics to provide discrete dual and multiple fields of view and it is possible to insert a zoom lens in a similar manner. Because of the large number of high performance spaces it is also possible to extract, insert and reinsert radiation.Although the example given describes an optical configuration having three real images it is possible to have a minimum of two real images (and one internal near parallel light space) and to have more than three real images. Although the refractive surfaces are preferably 'spherical' or 'flat' they can be non-spherical and non-flat such as aspheric and toric if so desired.

It is to be noted that all details given in Tables I to VI inclusive are for 20"C and that the optical system described has been optimised for the waveband 0.48-0.66 microns-i.e. about 0.2 microns broad. Non-human-eye detectors have the capability of detecting 'visible' radiation slightly outside this waveband and the system could alternatively be optimised for a different 0.2 micron band such as 0.55-0.75 microns by the use of slightly different glasses. In this connection it will be appreciated that the specific Schott designated glasses referred to previously are members of respective families, or glass types, viz. LgSK, KzFSN, BK so that differently designated members of these glass types may be used to optimise the system over slightly different wavebands.

Table I Radius of $ Aperture * Lens Surface Separation Curvature Material Diameter Entrance Pupil &num; # 0 Flat Air 36.0 A1 1 370.97 271.56 67.9 2 20.00 -80.81 LgSK2 67.6 A2 3 1.00 -79.83 66.9 4 8.00 -745.43 KzFSN4 66.8 5 251.42 Flat 48.7 B 6 5.00 Flat BK7 48.4 C 7 100.00 657.89 BK7 41.2 8 20.00 Flat 40.0 9 100.00 Flat 44.7 D BK7 10 5.00 Flat 44.8 E1 11 832.31 2377.84 90.8 KZFSN4 12 10.00 199.94 KzFSN4 91.3 E2 13 1.50 200.73 91.8 14 15.00 -633.70 LgSK2 92.1 F1 15 606.32 894.70 91.7 16 15.00 -479.70 LgSK2 91.4 F2 17 1.50 -473.57 4 91.2 18 10.00 3174.60 KzFSN4 90.9 19 1920.00 1147.83 100.7 G BK7 20 10.00 Flat 100.5 21 960.00 Flat 101.7 H BK7 22 10.00 -1435.38 101.9 I1 23 2303.00 Flat 108.1 KZESN4 24 10.00 863.50 @@@@@@ 108.2 12 25 2.00 854.11 LgSK2 108.4 LgSK2 26 15.00 -1621.53 108.6 27 2020.03 1267.99 138.8 J1 LgSK2 28 19.00 -584.25 138.4 J2 29 11.84 -560.23 KzFSN4 136.1 30 12.00 Flat 135.8 K1 31 169.40 668.31 129.1 32 19.00 -382.33 LgSK2 128.1 K2 33 9.63 -320.55 124.9 34 12.00 325.28 Z 123.3 K3 35 2.00 293.81 123.7 36 19.00 -831.08 LgSK 122.9 All data in millimetres and at 200C. 0 &num; Maximum field angle at entrance pupil = 6 .

* As required by full field beam with 82% unvignetted (by area) of axial beam $ Schott glass designation.

TABLE II

Approximate R.M.S. Spot Sizes at first intermediate image (in micrometres) Monochromatic at Focus Shift &num; *Polychromatic Focus Shift &num; Field 0.588 micrometres (in micrometres) over 0.476-0.656 (in micrometres) micrometres Axial 1.9 +109 7.8 +77 13.9 + 92 16.4 +72 15.4 + 9 17.9 + 1 Full 18.1 -199 19.7 -194 * Given as an equally weighted three wavelength accumulated measurement, the wavelengths being 0.486, 0.588 and 0.656 micrometres.

&num; Positive focus shift denotes movement towards the final image space and negative shift denotes movement towards object space. All shiftes are measured from surface 6, Table I.

TABLE III

Approximate R.M.S. Spot Sizes at first afocal space (in microradians) Monochromatic at 1/Focus Shift &num; *Polychromatic 1/Focus Shift &num; Field 0.588 micrometres (in micrometres) (1/metres) over 0.486-0.656 (1/metres) micrometres Axial 2.0 -.0030 8.1 -.0028 8.6 -.0016 12.2 -.0015 11.1 +.0008 13.5 +.0007 Full 23.4 +.0049 20.2 +.0044 * Given as an equally weighted three wavelength accumulated measurement, the wavelengths being 0.486, 0.588 and 0.656 micrometres.

&num; Positive focus shift denotes movement towards the final image space and negative focus shift denotes movement towards object space. All shifts are measured from surface 14, Table I.

TABLE IV

Approximate R.M.S. Spot Sizes at first afocal space (in microradians) Monochromatic at 1/Focus Shift &num; *Polychromatic 1/Focus Shift &num; Field 0.588 micrometres (in micrometres) over 0.486-0.656 (1/metres) micrometres Axial 3.2 -.0032 7.3 -.0032 4.4 -.0015 7.5 -.0016 9.2 +.0012 9.6 +.0009 Full 24.0 +.0054 20.9 +.0048 * Given as an equally weighted three wavelength accumulated measurement, the wavelengths being 0.486, 0.588 and 0.656 micrometres.

&num; Positive focus shift denotes movement towards the final image space and negative focus shift denotes movement towards object space. All shifts are measured from an imaginary flat surface located # +0.353 metres from surface 26, Table I.

TABLE V

Approximate R.M.S. Spot Sizes at first afocal space (in microradians) Monochromatic at 1/Focus Shift &num; *Polychromatic 1/Focus Shift &num; Field 0.588 micrometres (1/metres) over 0.486-0.656 (1/Metres) micrometres Axial 1.7 -.0037 10.5 -.0034 6.1 -.0018 9.5 -.0018 16.0 +.0009 14.7 +.0008 Full 31.6 +.0056 27.3 +.0051 * Given as an equally weighted three wavelength accumulated measurement, the wavelengths being 0.486; 0.588 and 0.656 micrometres.

&num; Positive focus shift denotes movement towards the final image space and negative focus shift denotes movement towards object space. All shifts are measured from surface 30, Table I.

TABLE VI Approximate R.M.S. Spot Sizes at final image space (in micrometres)

Monochromatic at Focus Shift &num; *Polychromatic Focus Shift &num; Field 0.588 micrometres (in micrometres) over 0.486-0.656 (in micrometres) micrometres Axial 7.4 +5.6 9.0 +1.0 8.0 -199.9 8.7 -190.4 13.0 -459.4 11.9 -434.5 Full 27.0 -856.0 23.5 -808.2 * Given as an equally weighted three wavelength accumulated measurement, the wavelengths being 0.486, 0.588 and 0.656 micrometres.

&num; Positive focus shift denotes movement away from the optical system and negative focus shift denotes movement towards optical system. All shifts are measured from an imaginary flat surface located # 0.720 metres from surface 36, Table I.

Claims (7)

1. A visual opticl system having an image-relay optical assembly for relaying an image between first and second image surfaces, said assembly comprising a first lens group which is spaced from a second lens group by a parallel-light section, each of said lens groups comprising a positively-powered lens element and a negatively powered lens element, the lens elements which are proximal the respective image surfaces having like power, each of said positivelypowered lens elements being made of a first glass, and each of said negatively-powered lens elements being made of a second glass, wherein the V-value of the first glass is greater than the V-value of the second glass by at least ten units, the refractive index of the first glass is greater than 1.54 and in value is similar to or greater than the value of the refractive index of the second glass, the refractive index of the second glass is less than 1.74, and the relative partial dispersion of the first glass is substantially equal to the relative partial dispersion of the second glass.

2. A system as claimed in claim 1, wherein said image-relay optical assembly is one of a pair of like assemblies optically connected in cascade in said system whereby an image is relayed between said first and second image surfaces and between said second image surface and a third image surface, and a field lens made of a third glass, is located adjacent at least one of said image surfaces to provide for reduction of vignetting.

3. A system as claimed in claim 2, wherein said field lens is formed by split lens elements located on either side of said at least one image surface to reduce image contamination by the presence of the field lens.

4. A system as claimed in any preceding claim, wherein the negatively-powered lens elements of each of said assemblies are proximal the pertaining image surface.

5. A system as claimed in any one of claims 1-3, wherein the negatively-powered lens elements of one assembly are proximal the pertaining image surfaces and the positively-powered lens elements of the other assembly are proximal the pertaining image surfaces.

6. A system as claimed in any preceding claim, wherein the lens elemens forming each lens group form a doublet.

7. A system as claimed in claim 1 and substantially as hereinbefore described with reference to Table I.