GB2256712A - Measurement of optical angular deviation caused by a transparency - Google Patents

Abstract

A measuring apparatus for determining deviation of radiation through a transparent member (12) comprises a collimated target device (34) and an infinity focus viewing device (36). The viewing device and target device are disposed on a carrier (30) with their optical axes coincident or parallel so as to be locatable one on either side of the transparent member. Means (31') associated with the viewing device is provided for determining the deviation of the viewed image after passage through the transparent member. The member comprises a windscreen or canopy of an aircraft. <IMAGE>

Description

MEASUREMENT OF OPTICAL ANGULAR DEVIATION CAUSED BY A TRANSPARENCY This invention relates to the measurement of optical angular deviation caused by a transparency and more particularly but not solely to measurement of such deviation through the cockpit of an aircraft.

Radiation to be transmitted by the transparency may be visible, or infra-red, or ultraviolet, radiation. The term optical angular deviation as employed in this specification, and the accompanying claims, refers to angular deviation in relation to visible, or infra-red, or ultraviolet, radiation.

According to the invention there is provided a measuring apparatus for determining deviation of radiation through a transparent material, the apparatus comprising a collimated target device and an infinity focusing viewing device disposed on a carrier with their optical axes coincident or parallel so as to be locatable one on either side of a transparent material and means associated with the viewing device for determining the deviation of the viewed image after passage through the transparent material.

Optical angular deviation caused by a transparency comprising a windscreen, or canopy, of an aircraft can be important and the invention has specific application to such measurement in which case the carrier may be adapted for mounting on the aircraft with the target device outside the windscreen and the viewing device inside the windscreen.

The mounting of the carrier may be arranged to permit movement of the carrier such that the target device can be moved in Azimuth and elevation thereby to permit measurement of deviation in any viewing direction through the windscreen. The mounting of the carrier may be by means of rotational measuring devices defining the azimuth and elevation axes of the viewing direction.

The target device may comprise a graticule at the rear focal plane of a collimating lens.

The viewing device may comprise a telescope or an electronic camera and may comprise a graticule. Such a graticule may be calibrated such that the relative position of the image of the target with and without the transparent material therebetween can be observed to determine the deviation.

Optical angular deviation is required to be measured if a head-up-display (HUD) is to be provided on the aircraft, and parts of the HUD symbology, provided under the control of the HUD's computer, are required to be in register with points in space outside the aircraft.

Accordingly, when the measuring apparatus is mounted on an aircraft the head-up display may be arranged to provide a marker for alignment with the image of the target device by adjustment of head-up display controls thereby to permit calibration of the head-up display to compensate for deviation resulting from viewing through the screen.

In a refinement of the invention there is provided a method of measuring the optical angular deviation of a portion of a transparency, which comprises securing a rigid assembly of a test target device and a viewing device so that the transparency portion is between the devices, the test target device has a graticule at the rear focal plane of a collimating lens, the viewing device is arranged to form an image at infinity of a distant object, and is also provided with a graticule, the centres of the graticules being on the optic axes, respectively, of the test target device and the viewing device, initially the optic axis of the viewing device being arranged to be aligned in an appropriate manner with the optic axis of the test target device, the optic axes being either coincident, or parallel, with each other, also initially, calculating the bearings from the viewing device of the transparency portion in relation to a predetermined orientation of the viewing device when aligned with the test target device in said appropriate manner, displacing the rigid assembly so that the optic axis of at least one of the test target device and the viewing device is coincident with the calculated bearings, with the transparency portion between the two devices, any angular separation between the optic axes caused by the transparency portion, and comprising optical angular deviation caused by the transparency portion, being measured.

Advantageous design criteria, associated with equipment relating to the method in accordance with the present invention, relate to the compactness, simplicity, portability, flexibility in use, and accuracy, of the equipment.

If the equipment is employed with a windscreen mounted in situ on an aircraft, then optical angular deviations caused by distortion of the windscreen induced by mounting stress; or changes in the optical angular deviations caused by the windscreen during its lifetime, say, as a result of removing scratches by polishing; can be determined in an appropriate way.

In order that the invention and its various other preferred features may be understood more easily, some embodiments thereof will now be described, by way of example only, with reference to the drawings, in which: Figure 1 is a diagrammatic representation of a lateral shift, and an angular deviation, caused by a transparency, comprising an aircraft's windscreen, Figure 2 is a perspective view of one form of equipment constructed in accordance with the present invention, and capable of measuring optical angular deviations caused by the windscreen when mounted in situ on an aircraft, Figure 3 is a more detailed perspective view of a device similar to Figure 2 mounted on an aircraft only the windscreen of the cockpit of the aircraft being shown, and Figure 4 is a perspective view of a modified form of such equipment constructed in accordance with the invention.

Inevitably within an aircraft's windscreen, or canopy, there are regions in which the surfaces are not parallel, or the regions do not have a uniform composition. Such variations may be caused by any curvature of the windscreen, especially because the windscreen is typically 2.5 centimetres thick. The effect of any such variation within the windscreen is that constituent portions of these windscreen regions each act as a prism, capable of causing optical angular deviation, and different windscreen portions cause different amounts of optical angular deviation.

Such optical angular deviation is shown in Figure 1, in which a pilot's eye 10 is illustrated as viewing a remote point 11 in space, through a windscreen 12. The portion 13 of the windscreen 12 through which the pilot observes the point 11 has non-parallel surfaces 14 and 15, and these surfaces are inclined at acute angles to the initial part 16 of the path along which the pilot looks in order to see the point 11. Thus, at the windscreen 12, the path along which the pilot looks is subjected to a small lateral shift L, and to a small angular deviation D, with respect to the initial path part 16 between the eye 10 and the windscreen 12. The subsequent, deviated, path part remote from the pilot's eye 10, is indicated at 17. The point 11 appears to the pilot to be located at 18, on the extension 19 of the initial path part 16 between the eye 10 and the windscreen 12.

Because the point 11, and the corresponding, apparent point 18, in space are remote from the pilot's eye 10, the effect of the small lateral shift L can be ignored; and the deviated path part 17 can be considered as being coincident with the direct line 21, 17 between the pilot's eye 10 and the point 11 in space. Thus, there is considered to be an angular displacement D between the point 11, and the corresponding point 18 apparently viewed by the pilot when, in fact, looking at the point 11.

A head-up-display (HUD), is required to be provided and a part of the HUD symbology, provided under the control of the HUD's computer, comprises a marker. The marker is associated individually with the remote point 11 in space and is employed to enable the aircraft to be aimed in relation with this point.

In relation to Figure 1, it is assumed that the aircraft is correctly aimed with respect to the point 11 in space. It is required that the marker is located so as to appear to the pilot to be in registration with the associated point 11 in space. However, the path between the pilot's eye 10 and the point 11 in space traverses the windscreen 12, at a portion 13 thereof, which portion 13 causes the optical angular deviation D, as described above.

If the HUD's computer does not compensate for the angular deviation D, the marker of the KUD associated with the point 11 in space is incorrectly located in direction 21, 22 where it is otherwise expected to be required, and where the direct line 21, 17 between the pilot's eye 10 and the point 11 in space intersects the windscreen 12. Thus, the marker appears to be angularly separated, by an amount D, from the point 11 in space.

The required direction 16, 22' for the marker is where the initial part 16 of the path along which the pilot looks, to see the point 11 in space, intersects the windscreen 12.

Hence, when the pilot is required to look at the point 11 in space, the HUD's computer is required to compensate for the angular deviation caused by the portion 13 of the windscreen 12, and to move the marker from its otherwise expected direction 21, 22 by an angle D, and in the appropriate direction, to its compensated direction 16, 22'.

In order to be able to make this desired compensation, the HUD's computer, in addition to generating a symbol equating to the point 11 in space, there is required to be stored in the HUD's computer information corresponding to the information that the appropriate portion 13 of the windscreen 12 causes the amount D of angular deviation, in a particular direction.

The HUD's computer requires to know such information about the angular deviations caused by the windscreen in relation to each point in space required to be in registration individually with markers of the display provided by the HUD's computer. Thus, it is desired to know, inter alia, and therefore to measure, the amount of angular deviation caused at each of several portions of the windscreen, so that this information, in an appropriate form, can be stored in the KUD's computer.

In order to determine the optical angular deviation of, say, the portion 13 of the windscreen, it is necessary to know the angular separation D of the point 11, and the corresponding apparent point 11, and the corresponding apparent point 18, in space, or an equivalent such angular separation referred to below, using equipment including angular separation measuring means.

In addition, it is convenient to know any such angular separation, between the point 11 in space and a corresponding apparent point in space, in terms of an angle in azimuth and an angle in elevation, and their senses. The angular separation comprises the appropriate form of summation of the corresponding angle in azimuth, and the corresponding angle in elevation.

It may be necessary to know the angular separation of the point 11 in space in relation to a reference line, the reference line being referred to in detail below; and the angular separation of a corresponding apparent point in space, such as the corresponding apparent point 18 in space in relation to the same reference line; the optical angular deviation to be measured being the difference between these two angular separations.

An equipment constructed in accordance with the present invention which can be used to measure the optical angular deviations of portions of the windscreen 12 of an aircraft, comprises, as shown in Figure 2, a rigid framework 30, generally rectangular in shape, and a single support therefore, indicated generally at 31. The support 31 includes a known form of goniometer 31', for the sake of clarity, the goniometer being shown enlarged, and separated from the framework 30. The required coupling between he support 31, and, in particular, the goniometer of the support, and the framework 30, is indicated by dotted lines 32.

On one short end 33 of the rectangular framework 30 is a test target device 34, on a mounting 33'. On the other short end 35 of the framework 30 is a viewing device 36, on a mounting 35', the viewing device to enable an observer to view the test target device 34. Conveniently, the viewing device 36 comprises a telescope. The telescope 36 and the test target device 34 are, at least substantially, in alignment with each other. Neither the windscreen 12, nor any other part of the aircraft is shown in Figure 2, However, the windscreen 12 is capable of being mounted in situ on the aircraft, with the equipment fixed to the aircraft, and with the windscreen between the test target device 34 and the telescope 36.

The single support 31 for the framework 30 is required to be adapted to enable the framework to be mounted readily, in a required orientation, on an aircraft. The mounting of the framework 30 on the aircraft also is required to be such that an observer is capable of viewing the test target device 34 through the telescope 36, with the path between the telescope and the test target device, and traversing the windscreen 12, being coincident with the path 16, 17 along which the aircraft's pilot looks to view the point 11 in space. Usually, the observer using the telescope 36 is in the pilot's position in the cockpit of the aircraft.

Further, the single support 31 is required to be coupled to the framework 30, so that the framework, and hence also the telescope 36 and the test target device 34, can rotate in azimuth and elevation, respectively, about an axis 38, and an axis 39. In particular, the manner in which the framework 30 rotates is required to be such that the path between the telescope 36 and the test target device 34 can be coincident with each path along which the pilot looks to view individually each a plurality of points in space.

There is an operable range of possible displacement of the framework in azimuth, and a corresponding operable range in elevation. Any angular displacement of the framework 30, in azimuth and elevation, can be determined by comparing initial and subsequent settings of the goniometer.

Desirably the equipment has a readily portable form.

Hence, the rigid framework 30 may be formed from lightweight, hollow, aluminium sections, which both are readily secured together, and are readily detached from each other, in any convenient way.

In addition to being required to be compact in order to be portable, the telescope 36 is required to be compact to enable it, and an observer to use the telescope, to fit within the confined space of an aircraft's cockpit.

The test target device 34, whilst also required to be compact in order to be portable, also desirably, does not increase the moment of inertia of the rotating framework 30 about the single support 31 by an unacceptable amount, nor causes the framework to be distorted. Otherwise, because the test target device 34 is normally located outside the aircraft, there are no restrictions as to its size.

The test target device 34 may comprise an illuminated graticule, not shown, in the rear focal plane of a collimating lens 40. Thus, the graticule, may comprise a cell, carrying a graticule in the form of a grid, or a rectangular array of intersecting cross-hairs. When viewed only through the lens 40, the grid appears to be located at infinity. This grid of the graticule of the test target device 34 is required for measurement purposes, and maybe calibrated in angular terms in a known manner.

Alternatively the graticule of the test target device 34 may comprise a pair of cross-hairs.

The telescope 36, which is arranged to form an image at infinity of a distance object, or of an apparent distant object, such as the graticule of the test target device 34, may also have a graticule, not shown, positioned at the focal plane of the telescope. The graticule of the telescope usually comprises a pair of cross-hairs.

A more detailed view of a device similar to Figure 2 is shown in Figure 3 and similar reference numerals are used for similar parts to these shown in Figure 1 and 2 so will not be further described. The construction of Figure 3 has a d.c. supply connected to input lines 40 of the target device 34 which has a collimator. The framework 30 has arm extensions 41, 42 provided with counterweights 43, 44. The goniometer 31' is mounted on a tilt table 45. The assembly is mounted on an aircraft with the curved windscreen of the cockpit between the viewing device 36 and test target device 34.

Instead of including a rigid framework 30, the equipment may be modified, as shown partially in Figure 4, in which modification the test target device 34 and the telescope 36 are mounted on a stiff beam 50. The beam 50 is coupled to the support 31 (not shown in Figure 3), in the same manner as the rigid framework 30 of Figure 2.

In Figure 4 the windscreen 12 is shown between the test target device 34 and the telescope 36. Whilst the remainder of the aircraft is not shown in the Figure, the windscreen 12 is capable of being mounted in situ on the aircraft, with the equipment fixed to the aircraft.

In order to determined the optical angular deviation of a portion of the windscreen it is convenient to consider Figure 1 in combination with Figure 2, 3 or 4.

In the operation of the equipment, of the form shown in Figure 2, 3 or 4, and when the equipment is mounted on an aircraft, initially it is required to calibrate the equipment, in order to compensate for any bending of the framework 30, or of the beam 50. Subsequently, only the embodiment of Figure 2 will be considered. Thus, the centre of the graticule of the test target device 34, on the optic axis of the test target device, and as viewed by an observer through the telescope 36, is brought into registration with the centre of the graticule of the telescope, on the optic axis of the telescope. Any such required adjustment is caused by the orientation of the telescope 36 on its mounting 35', and in azimuth and elevation, being altered in a known manner. When the calibration is completed the telescope 36 is secured, against rotation, to its mounting 35'.In particular, such calibrating may be performed without the windscreen 12 between the test target device 34 and the telescope 36; or with the windscreen 12 in a position, between the test target device and the telescope, such that the, possibly deviated, path along which the observer looks traverses a predetermined reference portion, not shown, of the windscreen. With either such form of calibration there can be considered to be a straight path, along which path the observer looks, between the centre of the telescope 36 and the centre of the test target device 34, for convenience, this path being considered to be the reference line referred to above.If the calibration takes place with the windscreen 12 between the test target device 34 and the telescope 36, it is required that the azimuth and elevation bearings of the frame work 30, and hence of the reference line, are to be obtained by reading the settings of the goniometer included in the support 31, with their path along which the observer looks traversing the predetermined reference portion of the windscreen.

Alternatively, the HUD's computer stores the previously calculated azimuth and elevation bearings of the predetermined reference portion of the windscreen 12, and when the equipment is required to be calibrated, the HUD's computer provides a marker at the predetermined reference direction. The framework 30 is then displaced until the centre of the graticule of the test target device 34 is in register with the marker. The framework 30 is clamped, and the telescope 36 is adjusted until the centre of the graticule of the telescope also is in register with the marker. The telescope then is secured, against rotation, to its mounting 35', and the equipment is calibrated. It is required that the HUD's computer previously has been harmonised to other equipment of the aircraft when providing the marker, and, thereby, the other equipment may predetermine the reference portion of the windscreen.

If the equipment, whilst fixed to the aircraft, is calibrated with the windscreen 12 not between the test target device 34 and the telescope 36, initially the windscreen has to be placed in its operable position on the aircraft between the test target device and the telescope.

It is required that the centre of the graticule of the test target device 34 is on the part 17 of the path along which the pilot looks in order to see the point 11 in space, the path part 17 being beyond the windscreen 12. Because the test target device 34 provides a collimated beam of radiation therefrom, it is unimportant where along the path part 17 the centre of the graticule of the device is position.

Thus, it is required to calculated initially the angles of separation, in azimuth and elevation, of the point 11 in space from a reference orientation of the framework 30.

The reference orientation of the framework 30 may be when the framework is arranged both to be horizontal, and to be such that the longer sides 37 of the framework are parallel to the fore-and-aft axis of the aircraft.

Conveniently, such a reference orientation of the framework 30 can be considered to correspond to the zero settings, in both azimuth and elevation, of the goniometer included in the support 31. The framework 30 is arranged to be in the reference orientation by employing any conventional surveying technique.

Employing the goniometer included in the support 31 for the framework 30, the framework is then moved from the reference orientation, so that the centre of the graticule of the test target device 34 is on the path part 17, by setting the goniometer in accordance with the calculated bearings, in azimuth and elevation, of the framework 30. If the portion 13 of the windscreen 12 does not cause any optical angular deviation, when the test target device 34 then is viewed by the observer using the telescope 36, the centre of the telescope graticule is in register with the centre of the graticule of the test target device.However, if the portion 13 of the windscreen 12 caused optical angular deviation D, when the test target device 34 then is viewed by the observer using the telescope 36, the centre of the telescope graticule is not in register with the centre of the graticule of the test target device. The centre of the graticule of the telescope 36 is on a path part 21, extending between the pilot's eye 10 and the location 22 on the windscreen 12. However, the eye 10 looks along a path having a deviated part 60 beyond the windscreen 12, and views a point 62 in space, in stead of the point 11. The direct line 64, 60 between the eye 10 and the point 62 in space is angularly separated by an amount D from the direct line 21, 17 between the eye 10 and the point 11 in space.

This is because the same assumptions can be made in relation to points 11 and 62 in space, and the paths 21, 17 and 64, 60, as were made above in relation to the points 11 and 18 in space, and the paths 21, 17 and 16, 19. Hence, the point 61 in space in angularly separated from the point 11 in space, by the same amount, but of opposite sense, as the point 18 in space is angularly separated from the point 11 in space. The optical angular deviation D can be determined by using the measurement graticule grid of the test target device, to determine the displacement of the centre of the test target device graticule from the centre of the telescope graticule.Alternatively, if a measurement graticule grid is not provided in the test target device 34, and a goniometer, of some form, not shown, is included in the mounting 35' for the telescope 36, the telescope can be displaced from its calibration alignment, so that the centre of the telescope graticule is brought into register with the centre of the graticule of the test target device. The telescope 36 is then aligned with the direct line 16, 19.

The differences in the settings, in azimuth and elevation, of the gonimeter included in the telescopes mounting 35' provide the amount D of optical angular deviation of the portion 13 of the windscreen 12.

If the equipment, whilst fixed to the aircraft, is calibrated with the windscreen 12 between the test target device 34 and the telescope 36, it is required to calculate initially the angles of separation, in azimuth and elevation, of the point 11 in space from the bearing of the reference line traversing the predetermined reference portion of the windscreen 12.

From the initial calibration orientation of the framework 30, the framework is displaced to the calculated bearing of the point 11 in space, using the goniometer included in the support 31 of the framework 30. If the portion 13 of the windscreen 12 does not cause any optical angular deviation, when the test target device 34 is viewed by the observer using the telescope 36, the telescope graticule and the graticule of the test target device are in registration with each other. However, if the portion 13 of the windscreen 12 causes optical angular deviation D, when the test target device 34 is viewed by the observer using the telescope 36, the telescope graticule and the graticule of the test target device are not in registration with each other.The optical angular deviation D can be determined by using the measurement graticule grid of the test target device to determine the displacement from registration of the telescope graticule and the graticule of the test target device. Alternatively, if a measurement graticule grid is not provided in the test target device 34, and a goniometer, of some form, (not shown), is included in the mounting 35' for the telescope 36, the telescope can be displaced from its calibration alignment, so that the centre of the telescope graticule is brought into register with the centre of the graticule of the test target device. The telescope 36 is then aligned with the direct line 16, 19. The differences in the settings, in azimuth and elevation, of the goniometer included in the telescope's mounting 35' provide the amount D of optical angular deviation of the portion 13 of the windscreen 12.

Any optical angular deviation D, measured in any one of the ways described above, and for the portion 13 of the windscreen 12, can be stored in the HUD's computer in an appropriate form, and in addition to the storage therein of the initially calculated bearings of the point 11 in space.

Hence, when the pilot is required to look at the point 11 in space the bearings of this point, and the corresponding optical angular deviation, in the form stored in the HUD's computer, can be operated upon within the computer in the required manner, so that the computer provides the associated marker at the compensated direction 16, 22' instead of the direction 21, 22.

Instead of moving the framework 30 initially, to set the goniometer, including in the support 31 for the framework, in accordance with the calculated bearings of the point 11 in space, because information in relation to the bearings is required to be stored in the HUD's computer, the computer can be used initially to provide a marker at the uncompensated location 22 on the windscreen. The framework 30 then is displaced so as to be orientated with the centre of the graticule of the telescope 36 in register with the provided marker, and with the telescope aligned with the path part 21 extending between the eye 10 and the marker.

The centre of the graticule of the test target device 34 is not in register with the marker, but is in register with the deviated path part 60.

Alternatively, the framework 30 is orientated with the centre of the graticule of the test target device 34 in register both with the provided marker at the location 22, and with the deviated path part 60. The telescope's graticule is in register with the compensated location 22' for the marker.

With either arrangement, the framework 30 then is clamped. The amount D of angular separation is measured by using a measurement grid comprising the graticule of the test target device 34. Alternatively, the telescope 36 is re-aligned from its calibration orientation, so that the graticules of the test target device 34 and of the telescope are in register, the telescope being realigned so that the centre of its graticule is in register with the marker location 22. The amount D of optical angular deviation of the portion 13 of the windscreen 12 is obtained by reading the differences in the settings of the goniometer included in the telescope's support 35'.

The HUD's computer can also be employed as an optical angular deviation compensating device, obviating the need for the observer to determine any angular separation D, in any of the ways described in the preceding paragraph.

In one such mode of the computer, the framework 30 is orientated so that the centre of the graticule of the test target device 34 is in register with the provided marker, located in direction 21, 22 and the centre of this graticule is on the deviated path part 60, whilst the centre of the graticule of the telescope 36 is in register with the compensated direction 16, 22'. The framework then is clamped, and the computer drives the marker's angular position, in response to movement by the observer of a joystick inputting to the computer. The observer moves the joystick until the marker is in register with the centre of the telescope graticule, the marker then being positioned in its required direction 16, 22'.The compensated direction 16, 22' is then stored in the HUD's computer, to be retrieved instead of the direction 21, 22 for the display of the marker associated with the point 11 in space, within the subsequently provided HUD symbology.

Usually, the HUD symbology has a plurality of parts each comprising a marker associated individually with a point in space. It is required that each marker is to be in registration with the associated point, but not necessarily simultaneously. In respect of each marker, compensation of its location on the windscreen, for any optical angular deviation caused by the associated portion of the windscreen, may be obtained in any one of the ways previously described.

It is desired that the radiation beam from the test target device 34 is accurately collimated over the full aperture of the lens 40 of the device, and, especially, it is desired that the lens does not introduce a significant amount of spherical aberration. Further, if the radiation beam is polychromatic, it is desired that the lens 40 does not introduce a significant amount of chromatic aberration.

Chromatic aberration can be compensated in a conventional manner by providing, say, a lens 40, in the form of a doublet. In order to reduce spherical aberration it is usual to decrease the aperture of a lens. However, if the aperture of the lens 40 is reduced, it is more difficult to arrange that at least a part of the collimated beam from the test target device 34 is received by the objective lens of the telescope 36. In addition, as stated below, it is desirable that the objective lens of the telescope 36 has a small aperture. Further, it may be convenient to arrange that the optic axes of the test target device 34 and of the telescope 36 are not co-linear, in order to try and compensate for the lateral shift L normally encountered because of the presence of the windscreen therebetween.

Thus, the lateral shift L may be estimated. If the estimate is not sufficiently accurate, for example, because the lateral shift L is different for different paths therethrough; and the apertures of both the lens 40 and the telescope objective lens are small; the collimated beam from the test target device 34 may not be received by the telescope objective.

If the windscreen 12 is significantly curved at the portion 13 through which the path 16, 17 from the pilot's eye 10 to the point 11 in space traverses, it can act as a weak lens, possibly acting to decollimate the parallel beam of radiation from the test target device 34. The effect of such decollimation can be reduced if the telescope 36 has a sufficient depth of focus, by having a large value for the ratio of the focal length of the telescope objective lens to the aperture size of this lens. Increasing the focal length of the objective lens can be disadvantageous, because it causes the length of the telescope, which is to fit into a confined cockpit space, to increase. Thus, there is conflict between providing a small aperture for the lens 40, and between providing a small aperture for the telescope objective lens, and a compromise solution is required.It may be advantageous in this respect to provide a telescope objective lens arrangement whereby the lens has a variable aperture.

It is required that the manner of operation of the telescope 36, and its mounting 35', simulates as much as is possible how a pilot views the different points in space associated with the HUD markers. Different pilots may view the different points in different ways, in relation to movements of their heads, and/or of their eyes, and it is required that the KUD's computer can make either the appropriate adjustment in this respect, or a suitable average adjustment. Thus, in a preliminary step, the pilot's head positions when viewing the different points in space may be monitored by sensors, and the required adjustments calculated.

Further, an average adjustment may be required to be taken into account by the HUD's computer because of bioptical vision of the pilots. In addition, it may be desirable that the telescope's mounting 35' enables the telescope to be translated horizontally, and/or vertically, in order to simulate, as close as possible, how a pilot views the different points in space.

The viewing device, instead of comprising a telescope, may comprise a camera, focused at infinity. Such an arrangement can be advantageous in that a monitor displaying an electronic camera's field of view can be provided remote from the aircraft, obviating the need for an observer to be in the confined space of the aircraft's cockpit.

A further advantage in relation to the use of an electronic camera is that, the photosensitive detectors provided in the camera can be sensitive to radiation outside the visible part of the spectrum, for example, being sensitive to infra-red, or ultraviolet, radiation. thus, the optical angular deviation of the appropriate portions of the windscreen can be determined for infra-red, or ultraviolet, radiation, if desired.

It may be required to measure the optical angular deviations of constituent portions of a windscreen, or canopy, of an aircraft, other than in order to be able to compensate for misplacement of markers associated with points in space. Such an alternative requirement to measure the optical angular deviations of constituent portions of a windscreen of an aircraft may be irrespective of whether a HUD is provided in the aircraft, or not.

Instead, of comprising a windscreen, or canopy, of an aircraft, the present invention may relate to any other form of transparency of which it is required to measure the optical angular deviations caused by constituent portions thereof.

Claims (12)

1. A measuring apparatus for determining deviation of radiation through a transparent material, the apparatus comprising a collimated target device and an infinity focusing viewing device disposed on a carrier with their optical axes coincident or parallel so as to be locatable one on either side of a transparent material and means associated with the viewing device of determining the deviation of the viewed image after passage through the transparent material.

2. A measuring apparatus as claimed in claim 1 for determining the deviation of radiation through the windscreen of an aircraft, wherein the carrier is adapted for mounting on the aircraft with the target device outside the windscreen and the viewing device inside the windscreen.

3. A measuring apparatus as claimed in claim 2, wherein the mounting of the carrier is arranged to permit movement of the carrier such that the target device can be moved in Azimuth and elevation thereby to permit measurement of deviation in any viewing direction through the windscreen.

4. A measuring apparatus as claimed in claim 3, wherein the mounting of the carrier is by means of rotational measuring devices defining the azimuth and elevation axes of the viewing direction.

5. A measuring apparatus as claimed in any one of the preceding claims, wherein the target device comprises a graticule at the rear focal plane of a collimating lens.

6. A measuring apparatus as claimed in any one of the preceding claims, wherein the viewing device comprises a telescope.

7. A measuring apparatus as claimed in any one of claims 1 to 5, wherein the viewing device comprises an electronic camera coupled with a remote monitor.

8. A measuring apparatus as claimed in claim 6 or 7, wherein the viewing device comprises a graticule.

9. A measuring apparatus as claimed in claim 8, wherein the graticule of the viewing device is calibrated such that the relative position of the image of the target with and without the transparent material therebetween can be observed to determine the deviation.

10. A measuring apparatus as claimed in claim 6 when dependant from claim 2 mounted in an aircraft with a head up display arranged to provide a marker for alignment with the image of the target device by adjustment of head up display controls thereby to permit calibration of the head up display to compensate for deviation resulting from viewing through the screen.

11. A method of measuring the optical angular deviation of a portion of a transparency, comprises securing a rigid assembly of a test target device and a viewing device so that the transparency portion is between the devices, the test target device has a graticule at the rear focal plane of a collimating lens, the viewing device is arranged to form an image at infinity of a distant object, and is also provided with a graticule, the centres of the graticules being on the optic axes, respectively, of the test target device and the viewing device, initially the optic axis of the viewing device being arranged to be aligned in an appropriate manner with the optic axis of the test target device, the optic axes being either coincident, or parallel, with each other, also initially, calculating the bearings from the viewing device of the transparency portion in relation to a predetermined orientation of the viewing device when aligned with the test target device in said appropriate manner, displacing the rigid assembly so that the optic axis of at least one of the test target device and the viewing device is coincident with the calculated bearings, with the transparency portion between the two devices, any angular separation between the optic axes caused by the transparency portion, and comprising optical angular deviation caused by the transparency portion being measured.

12. A measuring apparatus for determining deviations of radiation through a transparent material substantially as described herein with reference to the drawings.