GB2201782A

GB2201782A - Measurement of transparency deflection

Info

Publication number: GB2201782A
Application number: GB08704744A
Authority: GB
Inventors: Ian Schofield
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1987-02-28
Filing date: 1987-02-28
Publication date: 1988-09-07
Also published as: GB8704744D0

Abstract

Apparatus for, and a methed of, measuring distortions of vehicle transparencies, particularly aircraft windscreens, are described wherein an optical projector (21) and a wide angle camera (22) mounted on a common base (20) are used to project and record a grid-like image focussed on an internal surface of the transparency (11) before and during vehicle use. The grid pattern is projected in two distinct wavebands so that the composite coloured primary reflection image remains conspicuous against a background of different coloured multiple reflections due to internal reflections within the transparency which may be of a complex aerodynamic shape. <IMAGE>

Description

MEASUREMENT OF TRANSPARENCY DEFLECTION Transparencies (for example windscreens) fitted to vehicles must not significantly distort the vision through them. Particularly in aircraft there are stringent optical requirements imposed to ensure that this is so. However, although in static conditions it is relatively easy to ensure that the transparencies meet the stipulated requirements, in dynamic conditions it has hitherto proved difficult or even impossible. This is because such transparencies are subject to distortion from such causes as dynamic pressure due to the speed of the vehicle, static pressure due to pressure differentials between the interior or the vehicle and the exterior, and also flexing of the supporting structure. All these causes are particularly evident in aircraft windscreens.In particular the distortion due to dynamic pressure resulting from aircraft forward motion can be severe and, moreover, it can be asymmetric because the aircraft's motion may have a yaw component. Similarly, distortion from cabin pressurisation can be severe particularly at altitude.

It is thus desirable to provide apparatus which can form the basis for an in use distortion measurement system. Such a system should desirably be usable: (a) with little or no disturbance to existing structure and particularly no modification to the transparency itself, (b) on different vehicle types, and, (c) with relative ease such that expeditious processing of the results can be achieved.

From the results, mathematical details of distortion can be obtained and used for redesigning transparencies and/or their supporting structure so that distortions can be minimised.

Furthermore, transparencies can be checked subsequently to such redesign.

According to one aspect of the present invention apparatus for detecting distortion of vehicle transparencies during use of the vehicle comprises optical projector means arranged to project a grid-like image upon an inner surface of the transparency in two discrete wavelength bands, and image recording means arranged to record said image whereby changes in the grid-like image can be measured and distortion of the transparency determined.

By this arrangement it is found that by using an image from a dual wavelength beam, depending upon the material of the transparency, the composite colour of the primary reflection from the inner surface of the transparency remains conspicuous against a background of different coloured multiple reflections due to internal reflections within the transparency itself.

One embodiment of apparatus according to the invention is described by way of example and with reference to the accompanying drawing. In this drawing, which is a diagramatic side view of a fighter * rPraft cockpit region, the cockpit region is provided with an upper skin or decking 10 and a windscreen panel 11 lying at an acute angle to the decking 10. Aft of the decking lies the cockpit proper; this is shown generally at 12. Shown in broken outline is a cockpic cXogy 13 whose leading edge abuts the trailing edge of the windscreen panel at 14 when the cockpit is closed.

Encompassed by the windscreen panel 11 and the decking 10 is a region 15 in which apparatus for detecting distortion in the windscreen panel 11 during flight can be mounted. This region normally houses a gunsight and/or a head-up display which for test purposes can be removed and replaced by apparatus according to the present invention.

Such apparatus comprises a base 20 adapted to be temporarily fixed to the aircraft structure adjacent the decking 10 by means of mountings provided for the gunsight and head-up display and/or by other fixing points (e.g. on bulkheads) as necessary, and mounted upon the base, an optical projector 21 and a wide angle camera 22. The projector 21 is set to project a grid like image onto an inner surface of the windscreen panel at an acute angle 23, whilst the camera is set to receive the reflection from that image at an acute angle 23a.

The projector is provided with a light source (not shown but of known design) which projects and focusses a composite image of two distinct wavebands of light onto the windscreen. This image, depending upon the optical dispersion characteristics of the material of the windscreen, due to the composite colour of its primary reflection remains conspicuous against a background of different coloured multiple reflections irrespective of the deflection and distortion of the windscreen.

In operation, the projector 21 is arranged to project its beam so as to focus an image upon the internal surface of the windscreen, the image extending over that portion of the windscreen to be tested.

With the windscreen in a quiescent state (i.e. prior to flight) a calibration photograph is taken to define the grid-like image position on the recording medium which in this case is photographic film.

Subsequently, a series of photographs of in flight photographs are taken under the required conditions of height, speed, yaw etc. After flight, movement of the grid-like image due to the in-flight loads exerted on the windscreen can be readily measured from a comparison of the photographs and an analysis made of the windscreen design. Linear displacements of the grid lines will indicate translational movement of the windscreen, whilst opening up or closing of the grid lines in any one plane indicates angular movement in that plane. Relationship of grid movement to windscreen movement and hence optical distortion is then a mathematical process.

Claims

1 Apparatus for detecting distortion of vehicle transparencies during use of the vehicle comprises optical projector means arranged to project a grid-like image upon an inner surface of the transparency in two discrete wavelength bands, and image recording means arranged to record said image whereby changes in the grid-like image can be measured and distortion of the transparency determined.

2 Apparatus as claimed in Claim 1 and wherein the image recording means is a wide angle camera.

3 Apparatus as claimed in Claim 1 or Claim 2 and wherein the optical projector means and the image recording means are mounted on a common base adapted to co-operate with existing mountings, such as mountings for gunsights or head-up displays, on decking adjacent to a forward windscreen panel.

4 A method for detecting distortion of vehicle transparencies during use of the vehicle includes the steps of; a) mounting apparatus as claimed in any preceding claim adjacent to a vehicle transparency, b) prior to use of the vehicle, using the optical projector means to project and focus a grid-like image upon the internal surface of the transparency in two discrete wavebands of light and recording the image by means of the image recording means, c) during use of the vehicle, repeating the above step, and d) comparing the composite coloured primary reflection images recorded in the above two steps thereby to obtain a measure of the distortion of the transparency during use of the vehicle.

5 Apparatus for detecting distortion of vehicle transparencies during use of the vehicle substantially as hereinbefore described with reference to the accompanying drawing.

6 A method of detecting of vehicle transparencies during use of the vehicle substantially as hereinbefore described and with reference to the accompanying drawing.