GB2141260A

GB2141260A - Zoom lens assembly

Info

Publication number: GB2141260A
Application number: GB08412291A
Authority: GB
Inventors: Iain Alexander Neil; William Mccreath
Original assignee: Thales Optronics Ltd
Current assignee: Thales Optronics Ltd
Priority date: 1983-06-09
Filing date: 1984-05-14
Publication date: 1984-12-12
Also published as: GB2141260B; GB8412291D0

Abstract

A zoom lens assembly (11), e.g. forming part of an infrared refractor telescope (10) having an eyepiece system (12), is formed by three lenses E, F and G. Lens G is fixedly mounted on support (20) whereas lens F is mounted on carriage (21) which is axially movable along support (20) by electric motor (23), and lens E is mounted on carriage (22) which is axially movable along support (20) by electric motor (25). Motors (23,25) receive respective position signals from computing means (28) which is pre-programmed, according to the predetermined optical parameters of the assembly (11), with data correlating the position of lenses E and F with focal distance, magnification factor and temperature. The required focal distance is manually input to computing means (28) via controller (30) and the required magnification factor is manually input to computing means (28) via controller (32). The prevailing temperature of the assembly (11) is continuously monitored by sensor (34) and directly input to the computing means (28) along line (33). <IMAGE>

Description

SPECIFICATION Zoom lens assembly This invention relates to a zoom lens assembly.

Zoom lens assemblies are already well known and comprise at least one lens which is fixedly mounted on a support which carries two or more movable carriages each incorporating a lens whereby movement of the carriages effects movement of the pertaining lenses axially along the optical axis of the assembly. Usually two of the carriages are mechanically coupled together by means of gearing having an input control for inputing a magnification factor demand. The gearing is predetermined on the premise that focal distance and temperature are constant such that according to the input demand the two carriages are respectively move to predetermined axial positions which provide the required magnification factor.The precise nature of the gearing is dependent upon the optical parameters of the lenses of the assembly, namely refractive indices of the materials used, curvature of the refractive surfaces, lens thicknesses and the practical limits of axial movement of the movable lenses.

The increasing use of zoom lens assemblies in a variety of applications has recently led to a demand for improved performance over the entirety of the zoom range. Primarily this demand has arisen because of the use of such assemblies in environments were temperature is neither constant nor substantially constant as a result of which the optical parameters of the lenses vary. Various proposals have already been made to effect compensation for these varying parameters. For example one proposal is to mount a third lens on a movable carriage and permit manual adjustment of the axial position thereof by an operator until the resolution of the assembly degraded by the temperature variation is restored to a level sufficient for the operator's purposes.

This proposal is mechanically complex in that the assembly comprises three movable carriages and is optically unsophisticated in that optimal resolution cannot be achieved at all settings of focal distance and magnification factor for all temperatures.

It is an object of the present invention to provide a new and improved form of zoom lens assembly the performance of which is substantially independent of temperature variations.

According to the present invention there is provided a zoom lens assembly having predetermined optical parameters comprising at least one lens which is fixedly mounted on a support which carries two movable carriages each incorporating a lens, the lenses of the assembly being aligned on a common optical axis and movement of said carriages being such as to effect movement of the pertaining lenses axially along said axis, wherein the carriages are independently axially positioned according to respective position signals generated by a computing means which is preprogrammed according to said predetermined optical parameters with data correlating lens position with focal distance, magnification factor and temperature, means being provided to input to said computing means demanded focal distance, demanded magnification factor and automatically-measured temperature in the vicinity of the assembly.

It will be appreciated that the assembly may operate in any waveband such as visible or infrared and the optical parameters although predetermined may take any form. Thus each lens may comprise a plurality of lens elements and may be provided with spherical refractive surfaces or aspheric refractive surfaces. The lens materials may be the same from lens to lens or they may differ for such purposes as colour correction.

However, because the optical parameters are predetermined the correlation of lens position with focal distance and magnification factor can be derived as heretofore at any one temperature. Accordingly similar correlation can be derived for all temperatures within a given temperature range in order to provide the data with which the computing means is preprogrammed. This data may be stored in discrete numerical value format or in algorithmic format (analogue) and it will be appreciated that as in heretofore deriving the predetermined nature of the gearing used to simultaneously position two lens carriages, the present invention is based on the premise that the preprogrammed data provides only one position for each carriage for a given set of input demands. This is achieved by selecting these carriage positions which provide optimal resolution of the assembly.Accordingly the present invention provides optimal resolution at each demanded focal distance and magnification factor over the entire temperature range, the temperature in the vicinity of the assembly being automatically measured so that optical resolution is automatically maintained.

Because the assembly of the present invention utilises only two movable carriages the mechanical mechanisms thereof are extremely simple and because only two lenses are movable accurate boresighting alignment is relatively simple. Conveniently the mechanical mechanisms are driven by electric motors each arranged in a servo-loop to which the pertaining position signal generated by the computing means is delivered. By way of example the computing means may incorporate a microprocessor the aforesaid data being preprogrammed in the memory thereof.

An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings and tables in which: Figure 1 schematically illustrates an optical system incorporating a zoom lens assembly according to the present invention, the assembly (in part) being illustrated discretely for three different magnification factors in the interests of clarity; Figure 2 is a graph showing the position variation with magnification factor variation and focal distance variation at constant temperature for each of the two movable lenses of the assembly; Figure 3 is a graph showing the position variations with magnification factor variation and temperature variation at constant focal distance for each of the two movable lenses of the assembly;; Figure 4 is a graph showing optical resolution variation with magnification factor variation and focal distance variation at constant temperature for the system; and Figure 5 is a graph showing optical resolution variation with magnification factor variation and temperature variation at constant focal distance for the system.

The optical system 10 which is illustrated in Figure 1 is in the form of an afocal zoom refractor telescope having a zoom lens assembly 11 and an auxiliary lens assembly 12 each aligned on a common optical axis 13. Accordingly radiation entering the system 10 from object space 14 is formed into an internal real image 15 and is transmitted to image space 16 via a pupil 17.

The auxiliary lens assembly 12 is illustrated by way of example as being formed by four fixedly-mounted lens elements A,B,C and D, the materials dimensions and mutual separations of which are selected to interface the zoom assembly 11 with the radiation detecting arrangement intended to be used in image space 16. It will however be appreciated that various other formats of assembly 12 could be used if so desired.

The zoom lens assembly 11 is schematically illustrated as being formed by three lenses E,F,G of which lenses E and G are single element and lens F is a triplet. Lens G in this embodiment is fixedly mounted on a support 20, lens F is mounted on a first carriage 21 which is carried by the support 20, and lens E is mounted on a second carriage 22 which is also carried by the support 20, carriages 21, 22 being independently movable along support 20 within the practical limits imposed by the presence of the fixed lenses D and G.

Carriage 21 is axially moved by means of a motor 23 forming part of a servo and to which a first position signal is applied on line 24. Similarly carriage 22 is axially moved by means of a motor 25 forming part of a servo and to which a second position signal is applied on line 26, lines 24, 26 being connected to a computing means 28 which is pre-programmed with data as will be explained and which is provided with three input signals.The first input signal is provided on line 29 from a controller 30 which is operator actuable to demand a particular focus or focal distance setting; the second input signal is provided on line 31 from a controller 32 which is operator actuable to demand a particular magnification factor; and the third input signal is provided on line 33 from an automatic temperature sensor 34 arranged automatically to sense and measure the temperature in the vicinity of the assembly 11. The signal on line 33 may in fact be derived from several sensors 34 mounted at different locations in the system 10 and is intended to indicate the temperature to which the several lenses of the assembly 11 are subjected.

Because the lenses E,F and G are predetermined in material and dimension the optical parameters of the assembly 11 are also predetermined at any one temperature and position of the movable lenses E,F as is already known. Accordingly the data preprogrammed into computing means 28 is in effect as illustrated in the graphs of Figures 2 and 3. Figure 2 illustrates that at one selected temperature (20"C in this case) lens E and lens F have predetermined positions for any one magnification factor and focal distance. In fact it will be observed that variation of focal distance demanded results in an extremely small variation of lens position and in order to illustrate this variation more clearly Table I is provided.In Figure 2 (and Figure 3) the lens position is taken with respect to the adjoining fixed lens in Figure 1 so that the position of lens E is related to lens D whereas the position of lens F is related to lens G. In each instance the separation, measured in millimeters, is the distance along axis 13 between the proximal refractive surfaces of the two lenses. Byway of example for lens F the refractive surface is 36 and for lens G the refractive surface is 37 and the position denoted in Figures 2 and 3 is the separation between surfaces 36 and 37 measured along axis 13.

Figure 3 illustrates the positional variation of lenses E,F with magnification factor variation for constant focal distance and at three separate temperatures. It will be noted that at the illustrated focal distance (infinity) lens position is markedly dependent upon temperature within the temperature range of -20 C to +60'C.

The graphs of Figures 2 and 3 are calculated to provide optional resolution for each position setting of lenses E and F at each focal distance, magnification factor and temperature and by way of illustration Figures 4 and 5 illustrate resolution graphs (expressed in terms of percentage of diffraction limit), one at constant temperature and the other at constant focal distance, each over the magnification range concerned. These resolution graphs are calculated polychromatically at three wavelengths (8.5; 10.0; and 11.5 micrometers) with respective weightings (0.63; 1.00; and 0.50) for the axial field pencil, the diffraction limit being chosen at a spatial frequency of approximately 5/32 of the diffraction cut-off spatial frequency.

In operation of the computing means 28 the operator inputs via controllers 30,32 demanded focal distance and magnification factor and sensor 34 automatically inputs measured temperature. Means 28 then calculates from these three inputs the selected two positions required for lenses E,F to provide the optimal resolution. As temperature changes and is automatically sensed by sensor 34 the changing temperature input signal causes the two output position signals to change automatically to conform to the data for example illustrated in Figure 3. Where the data is stored in discrete numerical value format the means 28 interpolates as required in a manner known per se on the basis of magnification factor and focal distance remaining constant during interpolation. Accordingly the positions of lenses E and F are automatically controlled to provide optimal resolution with varying temperature, varying focal distance demanded and varying magnification factor demanded.

The particular system 10 illustrated in Figure 1 and the graphs of Figures 2-4 is for a germanium/ZnSe lens design mounted on an aluminium support structure and accordingly the system operates in the infrared waveband (8-13 microns) where temperature variation is particularly significant due to the high thermal coefficient of refractive index possessed by germanium. The optical parameters of the design are set forth in Table II, which is taken at is taken at 20"C, all units being in millimeters. In this table the refractive surfaces of each lens is denoted by a subscript to the lens indicator (i.e. A,B,C, etc.) ordered with respect to pupil 17 so that for example surface A1 is the surface of lens A which is proximal to pupil 17 and surface A2 is distal from pupil 17.Surfaces D1 and E1 are aspheric having profiles governed by the known aspheric equation:

where Z=distance parallel to the optical axis 13 C inverse of the radius of curvature of a datum spherical surface H =radial distance perpendicular to the optical axis 13 and in the case of surface D1 H has a maximum value of 18.54 mm for the axial field and C = 1/(-95.37 mm); D = -2.79 x 10-8; G = zero; and D= zero and in the case of surface E1 H has a maximum value of 29.03 mm for the axial field and C =1/(-34722.22mm); B = -7.54 x 10-8; G = zero; and D = zero.

Separation (mm) Resolution % TABLE I Focus Temp. MAGNIFICATION m C X 6.0 X 7.0 X10.0 X12.5 X15.0 X17.5 X20.0 Lens 15.8 21.9 37.0 47.0 57.1 65.7 73.6 E # 82.5 84.5 88.7 90.9 92.5 93.6 94.5 F 15.3 21.5 36.7 47.4 56.9 65.9 65.6 73.5 E Fig. 2 200 20 82.6 84.7 88.8 91.0 92.6 93.7 94.6 F 14.8 21.1 36.5 47.1 56.7 65.5 73.5 E 100 82.8 84.8 88.9 91.1 92.7 93.9 94.7 F 15.8 21.9 37.0 47.6 57.1 65.7 73.6 E 20 82.5 84.5 88.7 90.9 92.5 93.6 94.5 F 22.5 26.6 39.4 49.3 58.3 66.6 74.4 E Fig. 3 # 60 79.6 82.4 87.4 89.8 91.5 92.7 93.6 F 6.4 16.5 34.6 45.9 55.8 64.7 72.8 E -20 85.9 86.7 90.0 92.0 93.4 94.5 95.3 F # 88.7 90.9 98.2 96.8 89.2 87.8 90.8 Fig. 4 200 20 88.7 90.6 98.1 97.3 96.1 91.7 84.9 100 87.8 90.4 97.8 97.8 93.5 94.3 77.4 20 88.7 90.9 98.2 96.8 89.2 87.8 90.8 Fig. 5 # 60 87.7 90.8 98.4 95.5 83.8 79.0 91.6 -20 87.1 90.4 97.8 97.8 93.4 93.5 83.0 TABLE II Radius of Lens Surface Separation Magnification Curvature Material Pupil 17 0 any Flat Air A A1 22.29 any -44.32 Air A2 4.50 any -36.37 Ge B B1 0.50 any 193.01 Air B2 4.25 any -1724.73 Ge C C1 0.50 any 35.33 Air C2 16.83 any 21.85 Ge D D1 69.21 any -95.37* Air D2 5.00 any -77.52 Ge 15.75 X 6 34722.22* E1 49.59 X13 Air 73.62 X20 E E2 5.50 any -243.31 Ge 78.45 X 6 F'1 35.83 X13 -331.31 Air 8.59 X20 F' F2 2.75 any 556.48 Ge F"1 6.00 any -407.05 Air F" F2 2.75 any 585.41 Ge F'''1 9.75 any -112.81 Air F"' F2 3.00 any -157.93 ZnSe 82.47 X 6 G1 91.25 X13 -263.19 Air 94.46 X20 G G2 22.50 any -182.97 Ge *Surfaces D1 and E1 have aspheric profiles

Claims

1. A zoom lens assembly having predetermined optical parameters comprising at least one lens which is fixedly mounted on a support which carries two movable carriages each incorporating a lens, the lenses of the assembly being aligned on a common optical axis and movement of said carriages being such as to effect movement of the pertaining lenses axially along said axis, wherein the carriages are independently axially positioned according to respective position signals generated by a computing means which is preprogrammed according to said predetermined optical parameters with data correlating lens position with focal distance, magnification factor and temperature, means being provided to input to said computing means demanded focal distance, demanded magnification factor and automatically-measured temperature in the vicinity of the assembly.

2. An assembly as claimed in claim 1, wherein said computing means comprises a microprocessor having a memory, the aforesaid data being preprogrammed in said memory.

3. An assembly as claimed in either preceding claim, wherein each said carriage is driven by an electric motor arranged in a servo-loop to which the pertaining lens-position signal generated by the computing means is delivered.

4. An assembly as claimed in any preceding claim, wherein each said lens is transmissive in the infrared radiation waveband.

5. A zoom lens assembly as claimed in claim 1, and substantially as hereinbefore described with reference to Tables I and II herein and the accompanying drawings.