A SYSTEM AND METHOD TO MEASUREDEVIATIONS OF INCIDENT LIGHT THROUGH OBJECTS

Field of the invention

[0001] The present disclosure relates to a system and a method to measure angular and binocular deviations of incident light beams, passing through a transparent object, without resorting to shifting of the object. The present disclosure also provides a system and method to measure refractive indices of media, based on the deviations of incident light beams.

Background of the invention

[0002] Objects viewed through transparent objects or sheets with residual non- parallelism and irregularity, appear shifted and distorted. This distortion is measured in terms of angular and binocular deviation of an object viewed through the transparent sheet. The angular and binocular deviations introduced are particularly important in the context of windscreens and canopies of vehicles, used in air or underwater, as they can interfere with decision making of persons operating these vehicles, resulting in accidents and other untoward incidents.

[0003] There have been several attempts to develop devices and systems based on different methodologies, which can precisely and consistently measure angular and binocular deviations introduced by windscreens so as to qualify them. For instance, a pair of orthogonal incoherent light rays are combined and projected through a test specimen and the combined images are then separated for arriving at the azimuthal and tangential components of the angular deviation.

[0004] Another system to measure the angular deviation in which the measurement of the difference in position of the focus with and without a transparent specimen is used to measure the angular deviation.

[0005] The American Society of Testing and Materials (ASTM) also specify the standard test method for measuring the vertical and horizontal shift introduced by the specimen to a suitably designed L-shaped, incoherently trans-illuminated slit. However, in this method, there is a need for movement of the test specimen during the test, which can lead to errors arising out of the lateral re-positioning of the specimen.

[0006] According to Federal Aviation Administration (FAA) the vision of pilots is altered when a windscreen is scratched, pitted or windscreen made up of translucent. Regular testing or maintenance of windscreen is very important especially in military aircraft otherwise pilots will have problem with visibility through windscreen. Therefore, the problems encountered by military pilots with windscreen visibility are more serious than general aviation. Various methods have been attempted at quantizing the degree of degradation (generally known as haze) in windscreen transparencies. Typically, the measurement of the haze in a windscreen is accomplished by removing the windscreen from the aircraft, cutting it into small pieces, and placing one piece of it in one of several measuring devices. This creates problems for the FAA, since they prefer to evaluate aircraft windscreens with an in situ, non-destructive test.

[0007] It is therefore, preferable to have an apparatus, system and method to measure the angular and binocular deviations introduced by the object, without having to reposition the test object in a test run, accurately, by inferring displacement minutely. This is achieved by recording a shifted suitable test pattern, shifted owing to the specimen, and cross-correlating locally with an original pattern prerecorded and stored.

Objects of the present invention

[0008] The primary object of the present invention is to provide a system and method to measure angular and binocular deviations of incident light beams, passing through a transparent object, without resorting to shifting of the object, while measuring the angular and binocular deviations.

[0009] An object of the present invention is to provide a system and method to measure angular and binocular deviations of incident light beams, passing through a transparent object, with an enhanced sensitivity and accuracy, by adopting a silhouette or s shadow pattern, whose geometric silhouette, cast by the deviated light beam coming through the transparent object, is cross-correlated with its original (reference silhouette pattern).

[0010] Another object of the present invention is to provide a system and a method to measure refractive indices of media, based on the deviations of incident light beams, while passing through an object.

Summary of the invention

[0011] The present invention provides a system to measure, qualitatively, angular and binocular deviations, introduced by a transparent object to an incident light. The present invention also provides a system to measure refractive index of transparent materials. An image pattern with a matrix of small opaque structures and transparent areas is adopted, which corresponds to a pre-selected standard transparent material, where the transparent material does not induce angular and binocular deviation of the incident light or induces deviations that fall under the applicable or permissible limits. The geometrical silhouette of the image pattern is recorded. A test specimen, which is a transparent material for which the measurement of angular and binocular deviation is to be measured, is arranged in between the light source and the image pattern so that geometrical silhouette caused by an incident light while passing through the test specimen falls on the image plane. The deviation, if any, of the incident light is measured by comparing the position of silhouettes generated by the test specimen with that of the image pattern.

Brief description of the drawings

[0012] FIG.l is broad system architecture of the present invention for measuring angular and binocular deviations of an incident light beams traversing through an exemplary transparent object.

[0013] FIG.2 is block drawing of the control unit of the system.

[0014] FIG.3 is broad system architecture of the present invention for measuring haze characteristics of an exemplary transparent object.

[0015] FIG.4 is broad system architecture of an exemplary embodiment for the remote measurement of angular and binocular deviations in aircraft windscreens using geometrical silhouette casting technique of the present invention.

[0016] FIG.5 is broad system architecture of an exemplary embodiment for the measurement of angular and binocular deviations at multiple portions of a single exemplary transparent object using geometrical silhouette casting technique of the present invention.

[0017] FIG.6 is broad system architecture of an exemplary embodiment for the measurement of angular and binocular deviations in a multiple transparent objects using geometrical silhouette casting technique of the present invention.

[0018] FIG.7 is broad system architecture of an exemplary embodiment for the measurement angular and binocular deviation testing of a transparent object from various directions and angles of the transparent object.

[0019] FIG.8 is a graphical representation of the system of the present invention to measure refractive index (RI) of a single solid medium.

[0020] FIG.9 is a graphical representation of measurement of refractive indices of liquid media such as water, acetone and kerosene.

[0021] FIG.10 is a graphical representation of the silhouette pattern of various media while measuring their refractive indices.

[0022] FIG.ll is a flow drawing depicting a method of implementation the present invention to determine angular and binocular deviations of an incident light traversing through an object.

[0023] FIG.12A Geometrical silhouette-casting of transparent specimen or windscreen

[0024] FIG.12B Geometrical silhouette-casting of reference or thick transparent slab with parallel sides.

[0025] FIG.12C Close up of geometrical silhouette-casting with the transparent specimen.

[0026] FIG. 12D Close up of geometrical silhouette-casting using the reference slab.

[0027] FIG. 13A is an illustrative representation of geometrical silhouette-casting, where an incident light at normal incidence through a transparent medium matrix of opaque structures producing parallel silhouette patterns.

[0028] FIG. 13B is an illustrative representation of geometrical silhouette-casting, where an incident light at angle of incidence passing through a matrix of opaque structures producing shifted silhouette pattern.

[0029] FIG.14A is an illustrative representation of geometrical silhouette-casting, depicting shifted patterns, where silhouette patterns are superimposed on the un-shifted silhouette pattern.

[0030] FIG.14B is an illustrative representation of geometrical silhouette-casting, where single pair of dots selected from FIG. 14A are shown.

[0031] FIG.14C is a graphical representation depicting the relationship between the original and shifted patterns.

[0032] FIG.14D is a graphical representation depicting measurement of angular deviation in terms of angular deviation 0=(0x+0y), which is related to displacement of silhouette, Ad=(Adx,Ady).

[0033] FIG.15 Cross-correlation between the two images after bilinear interpolation, (a) Non-linear fit of amplitude of cross-correlated matrix, (b) Bilinear fit of cross-correlated matrix, (c & d) The bilinear cross-correlated peak in planar view along x & y axis, (e) The error between (a) and (b).

Detailed description of the invention

[0034] The subject matter of the invention will now be described in the form of specific embodiments to measure angular and binocular deviations that are introduced by transparent objects to an incident light.

[0035] Accordingly, the present invention provides a system to measure angular and binocular deviations introduced by transparent objects to an incident light. Referring now to the accompanied drawings and more particularly to FIG.l, there is shown schematically, a preferred embodiment of the angular and binocular deviation measurement system of the present invention, designated as 10. The system 10 includes an illumination section (A) that is provided with multiple light sources 11a, lib and lie to generate visible light beamsl2a, 12b and 12c respectively. The light beams 12a, 12b and 12c possess desired divergence and preferably of equal intensities. The light sources 11a, lib and lie that are exemplified in the present invention are quartz-halogen lamps of 50W rating. It is understood here that other suitable light sources (Laser, laser diodes, Light Emitting Diodes (LEDS), Incandescent lamps, Gas discharge lamps) in the visible spectral range about 380 to 740 nanometers, can be suitably adapted in the illumination section (A) of the system. Laser emitting devices can also be used as the light sources for the system 10 of the present invention. The light sources 11a, lib and lie, are arranged at different positions and are spatially apart, to generate parallel light beams 12a, 12b and 12c of desired divergence, as shown in FIG.l. The light sources 11a, lib and lie can be arranged or mounted on a suitable platform (not shown in FIG.l) to facilitate the generation of parallel light beams 12a, 12b and 12c. In this exemplary embodiment, the total number of light sources is shown as three in number, which are arranged emit to visible light beams 12a, 12b and 12c of desired wavelength and diameter, to pass or traverse through an object or a test specimen 13, as here in after described.

[0036] The parallel light beams 12a, 12b and 12c, with the desired divergence characteristics are permitted to pass through the different regions of the object 13, particularly, through the mid-point 14b and the laterally shifted regions of the object 13 designated as 14a.

[0037] The light beam 12b is arranged to pass through the center region 14b of the object 13 where angular deviation, suffered by the light beam 12b, if any, is measured. Whereas, the light beams 12a and 12c are arranged to pass through the laterally shifted regions 14a of the object 13, so as to mimic the eye position of an observer in order to measure, the binocular deviations, suffered by the light beams 12a and 12c, while traversing through the object 13.

[0038] It is also within the purview of this invention, to vary the number of light sources 11a, lib and lie, in accordance with the requirement of total number of light beams, in the illumination section (A).

[0039] In the present invention, as an exemplary embodiment, the light beams 12a, 12b and 12c having diameter of about 25 mm is preferred, considering the size and characteristics of the sample object 13 that is used to measure the deviations. It is understood here that the diameter (2 mm to 25 mm) of the light beams 12a, 12b and 12c can be suitably varied by altering the light sources 11a, lib and lie. The parallelism of the light beams 12a, 12b and 12c is also verified using a parallel plate interferometer after ensuring quasi-monochromaticity of the illumination with an interference filter.

[0040] A heat shield (not shown in FIG.l) can also be incorporated in the illumination section (A) of the system 10, to protect the optical elements from the effects of heat that is normally emitted by the light sources, while they are in use.

[0041] In another aspect of the present invention, suitable optical lenses 15a, 15b and 15c are arranged in the optical pathways of the light beams 12a, 12b and 12c.In this embodiment, the condensing lenses are preferred, primarily to concentrate or focus the onward transmission of the light beams 12a, 12b and 12c.

[0042] Apertures or pin holes 16a, 16b and 16c are positioned in the condensed pathways of the light beams 12a, 12b and 12c, to permit the passage of the light beams through the apertures 16a, 16b and 16c.The apertures 16a, 16b and 16c are arranged to determine the extent of collimation of the light beams 12a, 12b and 12c and their cone angles. During configuration of the illumination section (A) of the system 10, the parallelism of the illumination of the light beams 12a, 12b and 12c is checked using devices such as a parallel plate interferometer after ensuring quasi-monochromaticity of the illumination with interference filters (not shown in FIG.l).

[0043] Optical lenses such as collimating lenses 17a, 17b and 17c are arranged to receive the condensed light beamsl2a, 12b and 12c from the condensing lenses 15a, 15b and 15c through the apertures 16a, 16b and 16c.The arrangement of the collimating lenses may be dispensed with in case the light source is equipped to emit collimated light beams.

[0044] The aforementioned light sources and lens arrangement of Section-A can be suitably placed in an enclosure, with an opening that is covered with a transparent material such as glass or metal plate shown in FIG.l.

[0045] The object 13 is arranged in the optical pathways of the condensed and collimated light beams 12a, 12b and 12c, as shown in FIG.l. The arrangement of the object 13 in the optical pathways of the light beamsl2a, 12b and 12c can be performed by incorporating a suitable platform or a holder and physically connecting the object 13 to the platform or holder. The object 13 can also be suspended in the optical pathways of the light beams 12a, 12b and 12c. In other words, the placing of the object 13 in the optical pathways of the light beams 12a, 12b and 12c, can be performed in any suitable manner.

[0046] The object 13 as used in the system 10 of the present invention is a transparent or a translucent object that can permit the passage of incident light beams. The types of objects that can be used to measure the angular and binocular deviations, in the system 10 of the present invention include slabs, prisms, lenses, mirrors, films, wedges, sheets, screens, containers with media such as liquids, windscreens, canopies, etc., (non-limiting examples). The selected objects can be of any shape including linear, parallel, non-parallel, angular and curvature shapes. The term "linear" as used denotes a shape of an object that is straight or flat; the term "angular" denotes a shape of an object having angular edges; and the term "curvature" denotes a shape of an object that is curved at an angle from the straight surface the object.

[0047] The object 13 can be made of any transparent and translucent materials, preferably, glass, polymer, ceramic, quartz or a combination thereof.

[0048] Therefore, the object 13 that can be used in conjunction with the system 10 of the present invention can be of any desired transparent material with variable thickness, size and shape.

[0049] The geographical orientation of the object 13 can be perpendicular (vertical) or angular to the optical axes of the incident light beamsl2a, 12b and 12c.

[0050] In the system of the present invention as shown in FIG.l, a windscreen of an aircraft is used as an object 13 or test specimen, to measure the deviations of the incident light.

[0051] In further aspect of the present invention the light beams 12a, 12b and 12c are permitted to pass through passing through the mid-point regionl4b and laterally shifted regions 14a of the object 13. The light beam 12b passing through the midpoint region 14b of the object 13 is preferably used to measure the angular deviation suffered by the light beam 12b and whereas the light beams 12a and 12c passing through the laterally shifted regions 14a of the object 13 are preferably used to measure the binocular deviations suffered by the incident light beams.

[0052] In contrast to the known systems, in the present system 10, once the object 13 is positioned in its desired position and orientation, it is not required to displace or shift the object 13, in order to measure the angular and binocular deviations of the incident light, concurrently. This is achieved by permitting the passage of parallel light beams through the midpoint 14b and laterally shifted regions 14a of the object 13.

[0053] In another aspect of the present invention the arrangement of Section-B, which is a detection section of the system of the present invention is now described. A complex of optical devices such as beam splitters 18a, 18b and mirror 20 are arranged along the focal plane of the condensed light beams 12a, 12b and 12c, as shown in FIG.l, to permit the incidence of light beams passing through from the object 13 in order to orient and direct the light beams on to a silhouette or a shadow caster 19.These light beams 12a, 12b and 12c, after traversing through the object 13 and glass window 26, are combined by using the beam splitters 18a and 18b and the mirror 20.The light beams 12a, 12b and 12c that originate from the light sources 11a, lib and lie are steered using two 50:50 beam splitters 18a and 18b so that the light beams from the mid-point region 14b and laterally shifted regions 14a of the object 13, fall on the silhouette caster 19. The optical alignment or adjustment of the optical devices viz., beam splitters and mirrors is controlled by a control unit 25. The movement of the beam splitters 18a, 18b and mirror 20 is effected in order to steer the light beams 12a, 12b and 12c to illuminate the shadow caster and is performed by the control unit.

[0054] In further aspect of the present invention, the silhouette caster 19, as shown in FIG. 1, is positioned on a platform or a holder (not shown in FIG.l) along the focal plane of the combination of the light beams 12a, 12b and 12c, which are the light beams passing through the mid-point region and laterally shifted regions of the object 13, as shown in FIG.l.

[0055] The silhouette caster 19 is arranged in the optical pathways of the light beams as focused from the complex of optical devices viz., beam splitters 18 and mirrors 20 as shown in FIG.l. The arrangement of the silhouette caster 19 in the optical pathways of the light beam scan be performed by incorporating a suitable platform or a holder and physically connecting the silhouette caster 19 to the platform or holder. The silhouette caster 19 can also be suspended in the optical pathways of the light beams. In other words, the placing of the silhouette caster 19 in the optical pathways of the light beams can be performed in any other suitable manner.

[0056] In further aspect of the present invention the silhouette caster 19 is a transparent member made of glass, polymer, ceramic, quartz or a combination thereof. The size, number, thickness and shape of the silhouette caster 19 can be varied suitably, depending upon the user requirements. In this specific preferred embodiment, geometrical shape of the silhouette caster 19 is shown as in rectangular shape. However, it is understood that the silhouette caster 19 with other geometrical suitable shapes and sizes that are not limited to round, triangular, oval, elliptical shapes can be suitably adapted. The geometrical shape and size of the silhouette caster 19 can also be determined by reckoning the shape of a test specimen 13, which is subjected to angular and binocular deviation measurements, by using the system of the present invention.

[0057] In yet another aspect of the present invention, the silhouette caster 19 is configured with a pre-determined matrix of opaque 21 and transparent 22 areas or regions. The opaque areas 21 are formed on the surface of the silhouette caster 19 with an opaque coating material. The opacity of the coating material is neither transparent nor translucent. In other words, the opaque areas 21 do not permit the incident light falling on them to pass or traverse and produce silhouettes corresponding to the matrix of opaque areas 21, behind the silhouette caster 19.The opaque areas 21 are formed by coatings on the surface of the silhouette caster 19 with coating materials such as electrochromic, epoxy, or paint, silicon-glass, silicon-ceramic etc.

[0058] The opaque areas 21 that are formed the silhouette caster 19 are interspersed with transparent areas 22. In other words, the transparent areas of the silhouette caster 19 permit the passage of light beams to traverse through them.

[0059] The arrangement of the opaque areas 21 and the transparent areas 22 form a matrix on the silhouette caster 19 and render a pattern of combination of the opaque areas 21 and transparent areas 22.

[0060] The arrangement of the opaque areas 21 and the transparent areas 22 of the silhouette caster 19 cast a silhouette or shadow when a light beam traverses through it.

[0061] In yet another exemplary aspect of the present invention, as shown in FIG. 1, the silhouette caster 19 is provided with a matrix of opaque areas 21, arranged in the form of an array of 12 x 9 dots with a diameter of 0.5mm and populated with adjoining transparent areas 22.The shapes of opaque areas 21 are shown having a dot pattern. It is within the purview of this invention to use other non-limiting patterns of shapes such as rectangle, triangle, semi-circular, curved and non-curved shapes.

[0062] In further aspect of the present invention, the silhouette caster 19 is provided with matrix of opaque areas 21 and transparent areas 22, the light beams that pass through the silhouette caster 19 produce a silhouette or shadow pattern 23, behind the silhouette caster 19, corresponding to the matrix of opaque areas 21 and transparent areas 22 of the silhouette caster 19.

[0063] In yet another aspect of the present invention at least a silhouette-capturing member 24 is arranged downstream of the silhouette caster 19 and in plane parallel (geometrical silhouette plane), to capture the generated silhouette pattern 23. The silhouette-capturing member 24 is a camera or a Charge-Coupled Device (CCD). In this exemplary embodiment, a camera (AVT Guppy F-503B), having about 5 mega pixel image sensors, to capture the silhouette pattern 23 with H:2592 V:1944 resolution. The focal number (f/number) of the lens of the silhouette-capturing member 24 is chosen as f/1.0, so that it has a very small depth of focus, to ensure that while it is imaging the silhouette pattern, the original silhouette pattern is out of its range to get very clear shadow silhouette pattern image when the object is placed.

[0064] In another aspect of the present invention, Section-C of the system of the present invention is now described. The Section-C of the present invention includes a control unit 25, as shown in FIG.2, which controls the components as arranged in Sections A & B of the system. Section-C includes at least a digital processor, a power regulator module and a display unit. The control unit 25 is connected to the silhouette capturing member 24 to receive the captured silhouette pattern 23 and process the same further to determine the extent of binocular and angular deviation of the light beams, while passing through the object 13.

[0065] The power regulator module apart from providing the desired power supply to the system 10 of the present invention also regulates varies the intensity of the light that is required for the system 10 of the present invention, to a cast a shadow or silhouette of the silhouette caster 19.The digital processor controls the light sources, for example quartz-halogen lamps, as used in the system of the present invention, with the help of semiconductor devices such as power Metal-Oxide Semiconductor Field-Effect Transistors (MOSFETs). The port of the processor is connected to the gate pin of MOSFET, to act like a switch. The drain terminal of MOSFET is connected to the power supply source (voltage supply) and source terminal of MOSFET is connected to the light source. In this arrangement, all the drain pins of MOSFETs (in this case three in numbers) connected to same regulated power source. Since, each source pin of MOSFET is connected to corresponding light source, whenever port pin of the processor sends an active signal to the Gate terminal, the light source is switched ON. In normal state, when the port pin of the processor is low, the light source will be turned OFF. So as long as the port pin of the processor is in high state, the light source will be ON or else the light source will be OFF state. The duration of active high state signal is controlled by timers with counters that are connected to the processor. This duration of active signal state can be fixed, for instance around five seconds, which can vary depending on the time of ON-OFF requirement of the light sources. Indicator LEDs are connected to port pins of processor to indicate ON-OFF status of light sources. The instructions to turn on the light sources are issued by the digital processor and processor acts to turn on the light sources by sending an active high signal to the Gate terminal of MOSFET and turns on indicator LED for that duration. The processor is also arranged to control heat emission characteristics of the light sources. The light sources are also arranged to switch on just before the silhouette-capturing member 24 captures the silhouette pattern 23 and to switch off automatically once the task of capturing of the silhouette pattern 23 is completed. The silhouette-capturing member 24 is connected to the digital processor through an interface card such as IEEE 1394 High Speed Serial Bus Interface Card (Firewire™).

[0066] The aforementioned embodiments describe a system to measure angular and binocular deviation of an incident light through an object, where the selected object is new and free from any physical damages. However, when transparent objects such as windscreens or canopies that are used in aircraft are subject to various forms of structural stresses such as tension, compression, torsion, wind shear etc., act on the aircraft when it is flying and when it is static. These stresses may cause damage in the form of development of haze characteristics such as cracks, scratches, in canopies and windscreens, over a period of time, resulting in reduced visibility and pose a serious threat to the aircraft while flying. Therefore, it is necessary to provide a system, which can measure such defects occurring in the canopies or windscreens of the aircraft. Accordingly, the system of the present invention as shown in FIG.3 provides for the measurement of defects such as haze characteristics, in the windscreens or canopies of aircraft, by measuring the deviation of the incident light while passing through such areas of the objects. The broad system configuration as shown in FIG.3 includes the elements of Sections A, B & C, as shown in FIG.l. However, in Section-A, as shown in FIG.3, the object 13 or the test specimen that is used for the measurement of angular and binocular deviation, is a windscreen or canopy of an aircraft, where the haze characteristics of the object 13 are to be measured. In this aspect, in order to measure the angular and binocular deviations of the light beams 12a, 12b and 12c passing through the object 13, a plurality of silhouette casters 19a, 19b and 19c are used. The structural configuration of the silhouette casters 19a, 19b and 19c are as described above. In this arrangement, the light beams passing through the object 13, through its mid-point 14b and laterally shifted regions 14a and 14c are permitted to pass through a complex of optical devices such as beam splitters 18a, 18b and mirror 20 are arranged along the focal plane of the condensed light beams 12a, 12b and 12c, as shown in FIG.l, to permit the incidence of light beams passing through from the object 13 in order to orient and direct the light beams on to a silhouette caster 19. These light beams 12a, 12b and 12c, after traversing through the object 13, are combined by using the splitters 18a and 18b and the mirror 20. The light beams 12a, 12b and 12c that originate from the light sources 11a, lib and lie are steered using three 50:50 beam splitters 18a and 18b so that the light beams from the mid-point region 14b and laterally shifted regions 14a and 14c of the object 13, fall on the silhouette caster 19. The optical alignment or adjustment of the optical devices viz., beam splitters and mirrors is controlled by a control unit 25.

[0067] In further aspect of the present invention, the silhouette casters 19a, 19b and 19c, as shown in FIG. 3, are positioned on at least a platform or a holder (not shown in FIG.3)along the focal plane of the combination of the light beams 12a, 12b and 12c, which are the light beams passing through the mid-point region and laterally shifted regions of the object 13, as shown in FIG.3.

[0068] The silhouette casters 19a, 19b and 19c are arranged in the optical pathways of the light beams as focused from the complex of optical devices viz., beam splitters 18 and mirrors 20. The arrangement of the silhouette casters 19a, 19b and 19c, in the optical pathways of the light beams can be performed by incorporating at least a suitable platform or a holder and physically connecting the 19a, 19b and 19c to such platform or holder. The silhouette casters 19a, 19b and 19c can also be suspended in the optical pathways of the light beams. In other words, the placing of the silhouette casters 19a, 19b and 19c in the optical pathways of the light beams can be performed in any other suitable manner. Silhouette-capturing members 24a, 24b and 24c are arranged downstream of the silhouette casters 19a, 19b and 19c and in plane parallel (geometrical silhouette plane), to capture the generated silhouette patterns 23a, 23b and 23c. Silhouette-capturing members 24a, 24b and 24c can be a camera or a Charge-Coupled Device (CCD), as described above. Control unit 25 is connected to the Silhouette-capturing members 24a, 24b and 24c to receive the captured silhouette patterns 23a, 23b and 23c and process the same further to determine the extent of binocular and angular deviation of the light beams, while passing through the object 13. Thus by using the system of the present invention as shown in FIG.3, in situ measurements of haze in windscreens of an aircraft can be measured by using shadow or silhouette casting technique by measuring the deviation in the geometrical shadow cast by a captured silhouette patterns that are trans-illuminated by the distorted multiple light beams from the transparent test specimen or object by comparing to the reference pattern. The shift in the pattern is obtained by cross-correlating the reference shadow pattern with the specimen shadow pattern and measuring the location of the correlation peak. Therefore, by using the system of the present invention haze characteristics on aircraft windscreens can be measured in a nondestructive method by using geometrical silhouette or shadow casting technique.

[0069] In another aspect of the present invention a system for the measurement of angular and binocular deviation of a windscreen of an aircraft, while the aircraft is airborne, is now described by referring to FIGAThe system includes all the sections as shown in FIG.l, that are coupled to communication protocols to facilitate onboard processing of data and communicating the same with a base station. The system as shown in FIG.4 facilitates onboard measurement of angular and binocular deviation of a light beam passing through the windscreen of the aircraft and communicating the resultant data to a base station. Since, the system is compact and can be carried easily, enables a user of the system, to test the windscreen of the aircraft regularly and communicate the angular and binocular deviation data, if any, of the tested windscreen to the base station for further necessary action and decision making. The data communication between the aircraft and the base station can be performed through any data communication protocols, including satellite communication protocols.

[0070] Hitherto, as described above, the arrangement of the system architecture 10, in which a single object 13 is used for measuring angular and binocular deviations of an incident light, through the mid-point 14b and laterally shifted regions 14a of the object 13 is described.

[0071] In yet another aspect of the present invention, the system of the present invention can be suitably adapted to measure angular and binocular deviations of the incident light through a plurality of regions (1, 2, 3 n) of the object 13, as exemplified in FIG.5. The system as shown in FIG.5 includes all the sections as shown in FIG.l, with the object 13 as used in instant case is lengthier and the areas that are subjected to deviations measurements are spread over the length and breadth of the object 13.In this aspect, a plurality of light beams 12a, 12b, 12c ... 12n and silhouette-capturing members 24a, 24b, 24c, .... 24n are incorporated on either side of the object 13, to measure the angular and binocular deviations of the incident light. Therefore, the instant system architecture can be suitably used to measure angular and binocular deviations of the object 13, where the size of the object 13 is lengthy and the areas of the deviations that required to be measured as spread over the object 13.

[0072] In yet another aspect of the present invention as shown in FIG.6, a pluralityof objects 15a, 15b and 15c can be arranged contiguously or in sequence, to measure their angular and binocular deviations of the incident light beams. It is also within the purview of this invention to adapt a series of objects 13a, 13b and 13c, to perform the measurement of angular and binocular deviations, through batch processing. In this aspect of the present invention, the arrangement of light sources, optics and electronics can be as shown in FIG.l, with a silhouette caster 19, to capture the corresponding silhouette patterns 23a, 23b and 23c. Silhouette-capturing member 24 is arranged downstream of the silhouette caster 19 and in plane parallel (geometrical silhouette plane), to capture the generated silhouette patterns 23a, 23b and 23c of each of the objects 13 as arranged in series, individually and in batches. Control unit 25 is connected to the silhouette-capturing member 24 to receive the captured silhouette pattern 23 and process the same further to determine the extent of binocular and angular deviation of the light beams, while passing through the series of objects 13a, 13b and 13c.

[0073] In yet another aspect of the present invention, the angular and binocular deviations of an object 13 can be measured from multiple angles and directions as shown in FIG.7. In this aspect of the present invention, the arrangement of light sources, optics and electronics can be as shown in FIG.l, with a silhouette caster 19, to capture the corresponding silhouette patterns 23a, 23b and 23c.The instant system architecture can be used for the detection of angular and binocular deviations with 360 degree rotational capabilities.

[0074] It is also an aspect of the present invention, in which the broad system architecture of the present invention, which is used to measure angular and binocular deviations, can also be suitably adapted to measure Refractive Index (RI) of media, such as any transparent or translucent materials. In this aspect, the deviations suffered by an incident light while traversing through a medium is used to measure the refractive index (RI) of the medium for a given wavelength of the incident light. In other words, the system of the present invention can also suitably used as refractometers.

[0075] Accordingly, the system architecture as shown in FIG.8 as an exemplary embodiment, an optical wedge (of known subtended angle) is used as an object 32 or test specimen 32 to measure the RI, using the system of the present invention, as shown in FIG.6. The illumination, optical and electronic components of the system as shown Sections-A, B and C of FIG.8 are as shown and described in FIG.l.The parallel light beams are passed through the optical wedge 32 and the deviations are measured, repeatedly, to ascertain the accuracy of the system for repeated measurements. The results thus obtained are tabulated in the following Table 1.

[0076] In further aspect of the present invention, a system for the measurement of refractive indices (RI) of plurality objects is now described by referring to FIG.9. The primary configuration of the arrangement of the Sections-A to C, are same as shown in FIG.l. A collimated light beam 12 is permitted to pass through the objects 13a, 13b and 13 and the silhouette caster 19, as shown in FIG.9. In this aspect, initially, the matrix of opaque 21a and transparent areas 21b of reference silhouette caster 19 is obtained, while permitting the passage of light beam 12, through a medium such as air, preferably still air, at an ambient temperature. Thereafter, objects 13a, 13b and 13c are arranged between the reference silhouette caster 19 and the collimated light beam 12 and then the geometrical silhouettes of the collimated light beam passing through the reference silhouette caster 19 recorded are by the silhouette capturing member 24.The collimated light beam 12 while passing through the objects 13a, 13b and 13c undergo deviation, while passing through them and also while passing through the reference silhouette caster 19. By measuring the separation of the centroid of the cross-correlation peak from the origin of coordinates, the angular deviation or refractive index introduced by the objects 13a, 13b and 13c is measured. The objects 13a, 13b and 13c for which refractive index is to be measured can be any transparent objects such as slabs, lenses, or wedges etc., as described in previous embodiments. In order to measure refractive index of multiple objects 13a, 13b and 13c, geometrical silhouette of the silhouette pattern 23 is captured by silhouette-capturing member 24, when the plane wave traverses through air. The first object 13a is mounted between the silhouette pattern 23a and collimated beam, and then the geometrical shadow of the silhouette pattern 23a is recorded by the silhouette-capturing member 24. Cross-correlation of the reference silhouette pattern and the silhouette pattern 23a as casted by the first object 13a, gives a shift in the cross-correlation peak, which is proportional to the deviation of the light beam 12 caused by the introduction of the first object 13a in the light pathway of the light beam 12. This deviation is the directly calibrated against the refractive index of the first object 13aobjecti- Then the second object 13b is placed in the light pathway of the light beam 12 and the silhouette pattern 23b is cast and captured. Thereafter, by cross correlating the reference silhouette pattern with the silhouette pattern produced by the air and first object 13a gives a relative shift of peak from which the refractive index of the second object 13b and refractive index of the first object 13a and second object 13b. By measuring the difference between of above results the refractive index of the second object 13b can be obtained i.e., RI 0bject2 = RI object2 + objecti - RI objecti-Like this we can measure keep multiple objects in between collimated beam and silhouette pattern to find refractive indices of multiple objects.

[0077] In yet another aspect of the present invention, a system for the measurement of refractive index (RI) of a medium such as a liquid, is described by referring to FIG.9 and 10.The broad system configuration for the measurement of refractive index (RI) of a liquid medium is as shown in FIG.9, with the test object or specimen 27 is a beaker filled with a liquid for which RI is to be measured. As described above, the silhouette pattern 23 (reference pattern) is captured by the silhouette capturing member 24, when the collimated light beam 12 traverses through still air. Thereafter, a corresponding silhouette pattern is obtained for an empty transparent container such as made of glass. By cross correlating the reference silhouette pattern and the silhouette pattern as generated by the container, in which a shift in the cross-correlation peak is observed, which is proportional to the deviation of light introduced by the material of the container (glass) appearing in the path. This deviation is the directly calibrated against the n of the beaker nB. Thereafter, the beaker is filled with a liquid of interest without disturbing the beaker and the corresponding silhouette pattern is captured. By cross correlating the reference silhouette pattern with the silhouette pattern produced by the liquid and the container gives a relative shift of peak from which the refractive index of liquid and material of the beaker is calibrated, ne+i refractive index of beaker + liquid. By measuring the difference between of above results the refractive index of liquid can be obtained i.e., RI Liquid = RI Liquid + Beaker -RI Beaker- The functional aspects of the system of the present invention are illustrated by measuring the refractive indices of water, acetone, and kerosene wherein the en route measurements of silhouettes and correlation are shown in FIG.10.

[0078] In order to compare the accuracy of the measured values of RI using the system of the present invention and the standard RI values, initially the shift pattern of silhouettes corresponding to air is measured and is denoted by Adajr. Thereafter, the shift pattern of silhouettes for the liquids of known RTs are calibrated, by assuming a linear variation of An (change in RI) with Ad and assuming nair= 1.008. By using the system as shown in FIG.10, the measurement RI of various liquids is performed by calibrating shifts in the silhouette patterns and the results are tabulated in Table 2. Table 2 provides a comparative account of the refractive indices of liquids as obtained by using the system of the present invention (Silhouette casting technique) and as obtained by a standard (Abbe's refractor) technique.

Table 2

[0079] It can be seen from the above Table 2 that RIs of liquids can be measured accurately by using the system of the present invention.

[0080] In yet another aspect of the present invention, a system for the measurement
In yet another aspect of the present invention, as shown in FIG. 10, the system of the present invention can be used for bio-chemically sensing, based on refractive index. Initially, geometrical silhouette image of the silhouette pattern is captured by the silhouette capturing member without any test sample, which acts a reference image. The cuvette/beaker is mounted between the light source and silhouette capturing member, and then the geometrical silhouette pattern is recorded by the silhouette capturing member. By cross-correlating the reference silhouette image and the silhouette image produced by a cuvette gives the refractive index of a cuvette (RI cuvette)- Then the cuvette is filled with a bio-chemical sample of interest without disturbing the cuvette and geometrical silhouette image of the silhouette pattern is captured continuously, by cross correlating the reference silhouette image with the silhouette images produced by the bio-chemical sample + cuvette gives the refractive index of bio-chemical+cuvette (RI bio-chemicai + cuvette)By measuring the difference between of above results i.e., RI bio-chemical = RI bio-chemical + cuvette - RI cuvette jthe bio-chemical sensing based on refractive index can be measured by the variation in refractive index changes the shift in silhouettes, which is monitored by the peak in the cross-correlation.

[0081] In further aspect of the present invention the method to determine angular and binocular deviations of an incident light traversing through an object, is now described, by referring to FIG.ll, which is a flow drawing, depicting steps of method of measurement angular and binocular deviations of incident light beams, passing through a transparent object, without resorting to shifting of the object.

[0082] Initially, parameters such as height and distance between the components of
Section-A and Section-B are adjusted and optically aligned. Desired number of light sources is switched onto generates visible light beams, by using the control unit. The system of the present invention is then calibrated or checked using standard transparent optical objects such as wedges.

[0083] A shadow caster with a geometrical pattern is arranged in the Section -B, to cast a shadow or silhouette as the light beams through it. Once the system is calibrated, the components of Section-A and the detection Section-B are actuated, a reference object is placed and the corresponding reference shadow or silhouette image is captured, which is an un-shifted dot pattern of the shadow caster, of the reference object, and a corresponding geometrical silhouette or shadow of the silhouette caster or shadow caster of the reference image is captured by the image capturing member such as a CCD camera. Subsequently, a test specimen, which is a windscreen of an aircraft, is placed between the illumination and detection sections and the corresponding geometrical shadow of the silhouette caster of the specimen image is captured by CCD camera at various desired angles, as prescribed by American Society for Testing and Materials (ASTM) standards. Considering the fact of non-uniform illumination of the silhouette or shadow caster, an initial step is required to find the centroid of the brighter region in the acquired silhouette pattern. Towards this, specimen image is cropped to a 3x3 dot matrix size, in a location where the background is uniform. The cropped specimen image is cross-correlated with the reference image (cropped at the same location). Thereafter, bi-linear interpolation is carried out on these cropped 3x3 dot matrix reference and specimen images to determine correlation peak location. The images of reference object and the test object are as shown in FIG.12a, 12b, 12c, and 12d, where 12a is a shadow or silhouette image of the test or specimen object and 12b is a close-up view of the reference object. Similarly, 12c is a shadow or silhouette image of the test or specimen object and 12d is a close-up view of the test object.

[0084] An incident light is permitted to pass through a standard specimen and permitted to fall on a selected a matrix of opaque and transparent areas (reference pattern) to produce corresponding silhouette or shadow pattern. As shown in FIG.13A, where a silhouette pattern is generated, which is parallel to the reference pattern and hence no deviation of the incident light. In other words, the incident light at normal incidence on the transparent object producing a silhouette that is a parallel to reference silhouette pattern.

[0085] Now, the reference object is replaced with a test object (windscreen) and measured for angular and binocular deviation of the incident light. The light beams pass through the test object intercepting its mid-point region as well as two laterally shifted regions. The test object introduces a small angular shift in the beam. The central light patch is used to measure the angular deviation introduced by the specimen, whereas the two symmetrically located side patches are used to measure the binocular deviation.

[0086] Whenever the incident light deviates, at an angle from its normal incidence, while traversing through the test object, the geometrical silhouette pattern gets shifted or displaced with respect to the original or the reference silhouette pattern. This shift in the silhouette pattern is related to the angle of incidence of the light through tan 0 ~ 0 =Ad/Az or the components 0X and 0y through 0x=Adx /Az and 0y=Ady/Az. As shown in FIG.13B the silhouette pattern obtained from the incident light passing through a transparent medium is shifted (by Ad) because of the angle of incidence 0. The shifted silhouette pattern (Ad) is accurately measured by cross-correlating the original un-shifted pattern with the shifted one.

[0087] A typical peak in the correlation of shifted and parallel pattern is schematically shown in FIGs.l4a-d. If there is no tilt in the illumination, the peak in the plane will be centered at (0, 0). However, the incidence at angle 0 causes the peak to shift to by Ad, which is measured by finding the centre of mass of the correlation peak. FIG. 14a depicts a dot pattern suffering a shift caused by an incident light where the shifted dot pattern or silhouette pattern is superimposed on the un-shifted one. FIG.14b depicts a single pair dots as extracted from FIG.14a. FIG.14c depicts how much shift in x-axis and y-axis (Adx and Ady) of shadow pattern with respect to reference which helps to find angular deviation 0 as shown in FIG.14d.

[0088] The method of the present invention determines angular deviation by accurately determining the pixel shift between a reference and sample image (with shift caused by specimen). The shift is determined using the cross-correlation between the two images, the original and the shifted (sample image). Further, in order to improve the resolution of the measurement, a bi-linear interpolation is performed (as shown in FIG. 15) on the cross-correlated output and the peak is determined to a high accuracy which provides sub-pixel resolution of the estimated peak of the correlation function. FIG.15a depicts a non-linear fit of amplitude of cross-correlated matrix and whereas FIG. 15b depicts a bilinear fit of cross-correlated matrix. FIG.15c and FIG.15d depicts bilinear cross-correlated peak in planar view along x & y axes. FIG.15d depicts error between FIG.15a and FIG.15b.

[0089] The invention is now described by the following non-limiting example. In order to illustrate the usefulness of the methodology developed, four windscreen specimens have been tested. The first two have been tested for angular deviation which changes when moved from the centre to the edges i.e. position 1 to 3 depend on ASTM standard. The other two specimens have been tested for both angular deviation and binocular deviation. The results are presented in Table I and II.

TABLE I: Angular deviation measurements of two sample specimens

TABLE II: Angular and binocular deviation measurements on two sample specimens

[0090] As seen from Table II, except for two readings, all the other readings confirm to the predicted accuracy with repeatability of the equipment of less than ± 0.1 milliradian with an accuracy of ± 0.036 milliradian. There are two primary sources of error with this procedure, the first arising from the instrument, and the second owing to the positioning of the object to be measured. An accuracy test is conducted by interposing the standard wedge in the path of light beam between transmitter and receiver. The percentage error in the measurement of angular deviation is found to be 1.80 as shown in Table III.

TABLE III: Angular Deviation of the optical wedge

[0091] If the angular deviation is less than the maximum value specified by the

ASTM standard then the windscreen or specimen is accepted otherwise it is rejected.

[0092] In the case of measurement of binocular deviation, the light beams 12a and 12c and the above-stated steps are used to obtain the shadow pattern. The resultant vector magnitude of the angular deviation obtained at the ends of the semi-minor axis of the circle (Head up display) is the binocular deviation. If the binocular deviation is less than the maximum value specified by the ASTM standard then the windscreen or specimen is accepted otherwise it is rejected.

[0093] By using this method, the intensity peak can be measured from (Adx, Ady), (0X, 0y) with an accuracy in the range of less than (l/128)th of a pixel, in the silhouette capturing member.

[0094] The system and method of the present invention compute both angular and binocular deviation with an accuracy of less than ± 0.1 milliradian («0.036 milliradian) and with an enhanced repeatability factor with an error scale of less than about 2 percent.

[0095] Another aspect of the present invention is the accurate quantification of the
shift in the pattern which is obtained by cross-correlating the reference silhouette pattern with the specimen silhouette pattern and measuring the location of the correlation peak.

Advantages of the present invention:

[0096] The system and method of the present invention provide for better sensitivity and accuracy in the measurement of angular deviation, by adopting a periodic dot pattern whose geometric silhouette or shadow cast by the deviated light beam coming through the transparent test specimen is cross-correlated with its original (reference pattern).

[0097] The present disclosure provides a system and method to measure both angular and binocular deviation of transparent objects placed in the line of sight of an observer, without moving the objects.

[0098] By adopting the system of the present invention it is possible to achieve repeatability in angular deviation measurement with accuracy of ± 0.036milliradian, which is lower than the ± 0.1 milliradian obtainable in the ASTM standard method.

[0099] The system and method of the present invention provides for measurement of the refractive indices of media based on the angular deviations of the incident light.

[00100] It is also understood that the following claims are intended to cover all the generic and specific features of the invention herein described and all statements of the scope of the invention, which as a matter of language might be said to fall there between.

We Claim:

1. A system for measuring angular and binocular deviations of incident light beams,
comprising:

(a) a plurality of light sources to generate incident light beams;

(b) a reference and a test object disposed in the optical pathways of the light beams;

(c) a silhouette caster with a matrix of opaque and transparent areas, disposed to receive the light beams passing through the test object, to generate an object-silhouette pattern of the reference and test objects respectively;

(d) a silhouette-capturing member operably connected to the silhouette caster and arranged to capture the object-silhouette patterns; and

(e) a control unit functionally connected to the silhouette-capturing member, to detect a shift in the object-silhouette pattern between said reference object and test object and to measure angular and binocular deviations of the incident light beams.

2. The system as claimed in claim 1, wherein a light source is disposed to generate a single light beam.

3. The system as claimed in claim 1, wherein a lens, prism mirror complex is disposed in the optical pathways of the light beams.

4. The system as claimed in claim 1, wherein the test object is solid and transparent and made of plastic, glass or a combination thereof.

5. The system as claimed in claim 1, wherein the test object is linear or curved.

6. The system as claimed in claim 1, wherein a plurality of test objects disposed in the optical pathways of the light beams.

7. The system as claimed in claim 1, wherein the test object is an air or liquid stored in a container.

8. The system as claimed in claim 1, wherein the light beams are incident on the central and lateral portions of the test object.

9. The system as claimed in claim 1, wherein the light beams are incident on multiple portions of the test object.

10. The system as claimed in claim 1, wherein the image capturing members are disposed to capture deviations from various angles.

11. The system as claimed in claim 1, wherein the matrix of opaque and transparent areas of the silhouette caster is symmetric or asymmetric.

12. The system as claimed in claim 1, wherein the opaque areas are circular, triangular, rectangular, polygonal, or a combination thereof, preferably circular.

13. The system as claimed in claim 1, wherein the control unit is remotely located.

14. The system as claimed in claim 1, wherein the silhouette capturing member is a charge-coupled device (CCD) or a camera.

15. The system as claimed in claim 1, wherein a plurality of silhouette capturing members connected to the silhouette caster.

16. The system as claimed in claim 1, wherein the silhouette capturing members are arranged co-axial to the optical pathways of the light beams.

17. A method for measuring angular and binocular deviations of light beams, comprising the steps of:

(a) directing light beams from light sources to pass through central and peripheral portions of a reference object;

(b) disposing a silhouette caster with a pre-determined matrix of opaque and transparent areas, in the optical pathways of the light beams and generating a reference-silhouette pattern;

(c) substituting the reference object with a test object and generating corresponding test-object silhouette pattern;

(d) measuring the shift in the silhouette positions of the test-object silhouette pattern; and

(e) determining angular and binocular deviations of the light beams.

18. The method as claimed in claim 17, wherein a plurality of light sources arranged to generate silhouette patterns at various points of reference and test objects.

19. The method as claimed in claim 17, wherein a light source is disposed to generate a single light beam.

20. The method as claimed in claim 17, wherein the test object is a liquid stored in a container.

21. The method as claimed in claim 17, wherein a plurality of test objects disposed in the optical pathways of the light beams.

22. The method as claimed in claim 17, wherein the deviations are measured at multiple points of the test object.

23. The method as claimed in claim 17, wherein the deviations are measured remotely.
24. The method as claimed in claim 17, wherein the deviations are measured at multiple angles of the test object.