WO1993000766A1

WO1993000766A1 - Dual lens borescope measurement device

Info

Publication number: WO1993000766A1
Application number: PCT/US1992/005408
Authority: WO
Inventors: John C. Lafeber; William J. O'neill; Randy A. Motyka
Original assignee: Lafeber John C
Priority date: 1991-06-26
Filing date: 1992-06-26
Publication date: 1993-01-07
Also published as: AU2305792A

Abstract

A dual lens borescope measurement device comprising a pair of substantially identical lenses (16, 18) having the dual lens axes fixed a specified distance apart (26). A feature such as an internal engine crack (68) can be viewed through each lens simultaneously and upon selection of a single point on the crack (72), the length of the crack in a plane normal to the dual lens axes and distance of the crack from the dual borescope lenses (16, 18) can be determined. In the preferred embodiments a video viewing system (36, 38, 46) in combination with a microprocessor (42) based calculation system enables the viewer to rapidly determine crack length. Each borescope lens (16, 18) includes a computer generated 'reference line' (64, 66) comprising a single reference for the dual borescope. With viewing of two or more points on the crack the full length of the crack in a non-normal plane can be accurately determined. Further, with a three-dimensional stereoscopic viewing mode as a part of the system the viewer can quickly ascertain the relative depth of objects and features in view. The system includes modes for viewer gaging and computer gaging of objects in view.

Description

DUAL LENS BORESCOPE MEASUREMENT DEVICE

Background of the Invention

The field of the invention pertains to borescopes and endoscopes and, in particular, to the visual inspection of internal engine parts with a borescope to discover defects and damage and in the medical field for visual internal inspection within the human body.

With current technology borescopes visible defects internal to an engine can be viewed but not easily measured. The determination of crack length in turbine blades and discs, for example, is critical to the decision to tear down a turbofan engine at considerable cost or to continue the engine in service. Regulations prescribe that if crack length is above a specific size the engine must be torn down.

With a single borescope the viewer can see the crack, however, the distance to the crack is generally unknown or inexact and the three dimensional length and shape of the crack not clear. The viewer is repeatedly confronted with the decision to ignore a questionable defect with possible disastrous consequences until an obviously over specification crack is found, if any. Otherwise, the engine passes the inspection and returns to service. Currently, disassembly of the engine is required to accurately measure crack length.

Internal inspection borescopes can be constructed in a variety of optical configurations but basically all borescopes perform the same functions. The basic functions are to bring light into the interior of an engine or other device and to transmit an image out of the engine. An inspector - viewer peers through an optical eyepiece to observe the critical internal parts in performing the inspection or, if so equipped, watches a television monitor upon which the image is projected. Such a borescope and color monitor system is illustrated in the November 1988 brochure of Optronics Engineering, Goleta, California. The typical engine specifically includes inspection ports at strategic locations for the insertion of the borescope. Enough clearance and freedom is provided to enable the inspector to move the borescope tip about and to enable the entire inspection area to be observed. Borescopes are equipped with fixed focus objective lenses which provide good depth of focus along with a wide field of view. The typical borescope might have a depth of focus ranging from 5mm to infinity while maintaining a field of view of 60". These optical properties contribute to comfort of use as well as convenience of use. However, these properties interfere with the inspector's ability to accurately judge the size of objects such as cracks within the field of view. A 60" field of view produces an image diameter substantially equal to the distance from the object to the borescope tip divided by cosine 30° (more accurately cotangent 30°). Without the ability to focus the objective lens to produce a distance dimension and otherwise not having the distance from an object, it is not possible to make measurements of the object to sufficient accuracy.

To overcome the lack of measurement accuracy, various borescope measurement methods and borescope modifications have been developed. One method uses a very narrow depth of focus lens. This lens requires the viewer to place the borescope tip a precise distance from the object before a sharply focused image can be observed. With the object in sharp focus the viewer knows the distance of the tip from the object as well as the field of view. With this information the object can be measured with a reticle. Unfortunately, the narrow depth of focus lens results in protracted inspection time, viewer eyestrain and fatigue, and a greatly increased likelihood of missed defects when the lens is out of focus.

Another shortcoming is the range of acceptable focus. The very narrow depth of focus lens produces an image of maximum sharpness at one precise distance but produces images of acceptable sharpness only at slightly less and greater distances. The size of the field of view also alters thereby introducing errors in the measured values of the object.

A further shortcoming is introduced by geometrical distortion in the lens. Borescopes typically exhibit barrel distortion producing curved lines when viewing straight lines. This is an inherent source of error when measuring sharply defined geometrical objects. Accurate measurements of objects such as small cracks require distortion free computations. U.S. Pat. 4,207,594 discloses a single borescope modified with a depth scale and computer generated cross-hairs. By measuring the object twice over at differing borescope depths and applying a mathematical relationship, the object can be measured and displayed on a monitor. The foregoing comments assume the object such as a small crack lies on a plane perpendicular to the optical axis of the borescope. In actuality, most objects of interest to viewers, in particular, inspectors of turbofan engines, are features of complex surfaces not perpendicular to the optical axis of the borescope tip. This introduces further inaccuracy into a very narrow depth of focus lens.

Flexible borescopes are fitted with an articulating tip which is controlled by the viewer with a "joy stick" mechanism. The "joy stick" - moving tip combination permits the viewer to point the tip at objects lying off the optical axis of the borescope. Some typical moving tip borescopes have an off-axis angular range substantially through a full 4 spheradians of rotation about the borescope axis.

The flexible borescope, although of increased value to the viewer, introduces additional errors. The articulating tip virtually assures off-axis viewing will result in distortion induced by the relationship of the angle of the object relative to the angle of the borescope. With converging angles this distortion is minimized. If the angles are diverging distortion will be increased. Furthermore, as angular divergence increases distance from the object may increase introducing a possible apparent foreshortening by perspective. This decrease in apparent length of an object introduces a serious error by masking the true length.

For example, if a crack within an engine lies on a plane at 45° to the borescope optical axis an error of 30" will result in any measurement of crack length. If the crack lies on a plane at 25" the measurement will be in error by about 10%. Thus, these measurements can create a false sense of security by minimizing true crack length. The result could be serious failure with catastrophic consequences in the case of a turbofan airliner engine.

Another source of error in the measurement and inspection process is damage to the borescope tip. Commonly, in rotating machinery such as turbine engines the rotating parts are revolved to bring portions into view of the borescope. As noted above the borescope extends through an inspection port, usually a single port. If the viewer fails to retract the borescope before rotating the turbine the tip may be unknowingly damaged and measurement accuracy affected.

The various measuring errors noted above can and do accumulate and may be additive. Measurement error could exceed 50%, a magnitude that drastically affects the value of the measurement process. In summary, additive measurement errors result when the following three conditions exist: 1. The borescope tip is close to the object which exaggerates barrel distortion.

2. The plane of the object (crack) is not perpendicular to the optical axis of the system.

3. The distance of the borescope tip from the object is not exactly known by the viewer.

These conditions exist in virtually every inspection involving complex machines.

In order to better view objects such as cracks and defects dual borescope and other stereoscopic inspection means have been developed. U.S. Pat. 4,588,294 discloses a dual endoscope having a wide angle, fixed focus lens with a large depth of field for searching in one bore tip and a narrow angle, fixed focus lens with a small depth of field in the other tip. The narrow angle lens has a predetermined working distance and magnification for measuring. U.S. Pat. 4,826,317 discloses a dual borescope wherein one of the pair is used as a directed light source providing light on the object of interest at an angle of incidence different from the angle of incidence directed to the viewing lens of the pair. U.S. Pat. 3,806,252 discloses a mathematical correlation method in combination with a pair of light sensing devices to measure depth and diameter of microscopic holes. In response to different amounts of light sensed by the device the mathematical method calculates the depth and diameter. This device, however, does not physically penetrate the hole. U.S. Pat. 3,894,802 discloses a stereoscopic measurement technique for gaging a contoured surface. The technique permits surface inaccuracies to be measured from a distance, thus the inaccuracies can be corrected on the surface without disturbing the relationship of the surface to the stereoscopic gaging system. U.S. Pat. 4,637,715 discloses an optical distance measuring apparatus having a lens and a reflector to reduce the size of the device in comparison with a stereoscopic dual lens configuration. The apparatus relies upon triangulation with the direct and reflected rays both passing through the single lens. The use of a borescope as a remote vision device has obvious advantages over disassembly for inspection. In the inspection of aircraft turbine engines the borescope permits the inspector to easily observe large defects. Small defects, however, are easily overlooked or misinterpreted due to the presence of the above noted errors in borescope measurements. Unfortunately, a misinterpreted small defect can have results as catastrophic as a large defect.

Summary Of The Invention

The principal object of the invention is to measure accurately defects such as cracks within turbine engines regardless of the curvature and depth orientation of the surface exhibiting the crack. The device comprises a dual lens borescope with a known fixed distance between the identical objective lenses positioned in parallel at the borescope tip. Preferably the borescope lens pair are identical with each lens being a fixed focus wide angle objective lens. Included in the dual borescope configuration is a light guide supplied by an external light source and a protective sheath to prevent inadvertent damage to the internal optical components.

The dual borescope in a basic configuration transmits images from the pair of objective lenses through relay lenses or fiberoptic bundles to a pair of eyepieces fitted with a pair of cameras. The dual video signals are transmitted to a dual video digitizer forming a part of a computer system for the storage of the video data and the real time presentation of the video images on one or more monitors. In more sophisticated form the eyepieces are eliminated with the relay lenses or fiber optic bundles directly coupled to the pair of video cameras. Other systems are also applicable such as:

1. Simultaneous projection of both images to one video camera ("split screen); and

2. Sequential alternating projection of each image to one video camera utilizing liquid crystal light shutters, polarizers or optical filters.

The computer program for the computer system provides multiple functions for multiple capabilities as follows. With the system in live display mode and one monitor, a right or left display image is presented. Once an area of interest is located, the computer is instructed to retain the right and left images in memory for three differing modes of further analysis.

In the first mode an "analog" measurement approach is taken by recording with a cursor the X, Y position in each image of one point on the object of interest. In response the computer program produces a dot grid on the monitor and displays the distance from the borescope tip to the point on the object. The dot grid can then be used to make a preliminary estimate of the object size. Although all the image data are in digital form, the approach is considered analog because of the viewer judgment in selecting and recording the image point position. The stereoscopic visualization provides the ability to make depth measurements and to correct for optical and geometrical aberrations in the images.

With the stereoscopic borescope foreground objects can be distinguished from background. Thus, objects such as dirt which projects outwardly from a surface can be distinguished from holes, pits and cracks which project inwardly. The stereoscopic effect in the first mode provides the viewer with the ability to identify curved surfaces and planes non-normal to the axis of the dual borescope. Therefore, erroneous crack length estimates are less likely during the inspection process. The experienced viewer can also better estimate object size and distance without actual computer analysis if necessary, in particular for relatively large defects. The second and third modes comprise computer gaging modes. The second mode, best applied to gaging for objects in a plane normal to the axis of the borescope, uses a three point sample of the object. Each of the three points are selected to identify the extremities of the object and the computer program calculates the exact distance of each point from the borescope tip and the distances between the point. In this manner as applied to a crack, the crack length and basic curvature can be digitally characterized. The third mode applies to objects in surfaces not normal to the axis of the borescope.

Description Of The Drawings FIG. 1 is a schematic diagram of the dual borescope and associated hardware;

FIG. 2A is a partial cross-section plan view of the dual borescope tip; FIGs. 2B, 2C and 2D show alternate forms of the tip of the dual borescope; FIGs. 2E, 2F and 2G show alternate forms of the video camera and relay lens arrangement for the dual borescope;

FIG. 3 is a block diagram of the analysis and recording hardware for the borescope;

FIG. 4 is a sample dual view through the dual borescope; FIGs. 5 and 6 illustrate two sample generated reticles or grids;

FIG. 7 illustrates in partial section plan view an articulated dual borescope tip;

FIG. 8 is a schematic diagram of a dual borescope single camera alternate configuration; FIG. 9 is a flow chart for the dual borescope; and

FIG. 10 is an illustration of the effect of lens barrel distortion.

Description Of The Preferred Embodiments

Illustrated in FIGs. 1 and 2A is a dual borescope having a tubular metal sheath 10 enclosing a pair of optical wave guides 12 and 14 fitted with fixed focus wide angle objective lenses 1 and 18 at the borescope tip 20.

The objective lenses are positioned optically in parallel to the axis 22 of the borescope tip and in the same plane 24 at the tip 20. The objective lenses 16 and 18 are mounted a specified distance apart 26. Also included within the tubular sheath is an optical wave guide 28 communicating with a light source 30 and through the tip 20 to provide illumination within a cavity. As shown in FIG. 2A the light source optical wave guide 28 may be located in the same radial plane with the two objective lenses 16 and 18 or may be located above or below the objective lenses and their respective wave guides 12 and 14 as illustrated in FIB 2B at 28'. FIG. 2C illustrates a concentric or surrounding light source optical wave guide 28" and FIG. 2D illustrates the use of the entire cavity within the sheath surrounding the lenses 16 and 18 and their respective optical wave guides as a light source optical wave guide 28'". The alternate light source optical wave guides of FIGs. 2C and 2D provide a more diffuse illumination to overcome the "spot light" or "head lights" effect of the light source optical wave guides in FIGs. 2A and 2B. The wave guides 12, 14 and 28 may be fiber optic or multiple lens system wave guides. Fitted to the opposite end of the dual borescope tubular sheath 10 are a pair of eyepieces 32 and 34 in optical communication with the wave guides 12 and 14. Each eyepiece is fitted with a color video camera 36 or 38, preferably a charge coupled device (CCD) sensor camera. FIGs. 2E, 2F and 2G illustrate alternative arrangements for the camera(s) by providing optical systems for a single camera. In FIG. 2E relay lenses 29 and 31 for the optical wave guides 12 and 14 direct the images to a single video camera 36'. The video camera 36' provides a split screen image signal which may be viewed as a split screen image. FIG. 2F illustrates a movable prism or mirror 33 which may selectively direct the images from optical wave guides 12 or 14 to the single video camera 36' for a full screen image. In this arrangement the seen and recorded images are not strictly simultaneous, however, for most purposes the dual borescope and object in view do not move between images. TIG. 2G further illustrates liquid crystal shutters or filters 35 and 37 to selectively direct the images from optical wave guides 12 or 14 to the single video camera 36'. In each embodiment the relay lenses 29 and 31 direct the images onto the sensing chip of the video camera. Illustrated in FIG. 3 is a schematic of the analysis and recording hardware associated with the dual borescope. The camera(s) 36 and 38 (Sony or equivalent) feed electrically into a dual video digitizer board 40 (True Vision, Inc. Indianapolis, Indiana) inserted in a suitable personal computer 42 (IBM PC Compatible). The input and/or output of the dual video digitizer board 40 comprises an RS-1 70, NTSC, RGB or SVIDEO compatible video signal and digitized video data which is stored in internal memory but accessible via the computer 42. A multiplexer board 44, also preferably in the computer 42, communicates directly with the digitizer board 40 and a video display monitor 46 (Sony or equivalent). Optionally two display monitors or a split screen technique may be employed to simultaneously display live and separately the views from both objective lenses 16 and 18. For most purposes, however, a single monitor that can be switched from lens to lens is sufficient. In addition the computer 42 is equipped with "mouse" 50, hard disc memory 52, floppy disc memory 54, keyboard 56, printer 58 and an optional text monitor 48. The computer 42 controls all system operations except manual movement of the borescope probe.

Each of the objective lenses 1 and 18 has a finite effective field of view. Preferably the lenses have a 60" field of view to provide a large change in image size as a function of distance to a surface. More specifically the change in image size follows the inverse square law with distance. Illustrated in FIG. 4 are the right 60 and left 62 views as seen through the dual borescope positioned with both objective lenses 1 6 and 18 in a horizontal plane. Each view includes a computer generated vertical reference line or cross hair 64 and 66 respectively and as shown a view of a crack 68 in a surface 70. In the live display mode either the right 60 or left 62 view is displayed on the video monitor 46.

Using a curser any point 72 on the crack 68 may be selected and the horizontal distance 74 from the reference line 64 in the right view 60 measured. Likewise the horizontal distance 76 from the reference line 66 in the left view 62 can also be mea.ured. The difference in the distances 74 and 76 is defined as the "separation". Further, the separation may be divided by the distance 26 between the centers of the objective lenses 1 6 and 18 to arrive at a picture element separation size or "pixel" size. Preferably the separation in pixel size is given in millimeters (mm).

The field of vision and geometry of each objective lens 16 and 18, here identical, defines a constant "K". The constant K further may be defined as equal to the separation in pixels times the distance "Z" from the lenses or borescope tip 20. Thus, the distance Z to the point 72 on the crack 68 can be quickly and easily calculated. More importantly, by entering the two distances 74 and 76 into a computer program the distance Z can be quickly calculated and displayed on the video monitor 46 with either view 60 or 62. The calculation of the separation in pixels moreover defines a reticle or grid size 78 for display on the video monitor 46 as illustrated in FIG. 5. For example, if the separation and separation in pixels are defined in millimeters then the computer program can immediately display on the video monitor 46 a grid representing millimeter spacing. Thus, within the working distance range of identical fixed focus wide angle lenses the crack 68 can be viewed on the video monitor, the point 72 selected, the distances 74 and 76 measured by the program and almost instantaneously a distance Z and millimeter grid caused to appear on the video monitor 46. FIG. 6 illustrates that with a lesser depth of view Z the grid size 78 increases nevertheless representing millimeter spacing btHween the grid lines. Although shown as a net of lines, the grid or reticle can be a "dot" matrix wherein only the intersections of the lines show on the screen as dots.

As shown in FIG. 7 the dual borescope stereoscopic tip can be applied to an articulated tip. The protective sheath 10 includes flexible bellows 11 enclosing the articulation joint 13 and articulation cables 15.

Fiber optic cables are used for the objective lens' optical wave guides 12' and

14' and the concentric light source wave guide 28".

To obviate the need for dual cameras, the single camera 36' configuration illustrated in FIG. 8 is an alternative. The optical wave guides 12" and 14" leading from the objective lenses 16 and 18 intersect mirrors 17 and 19. Mirror 17 is half silvered to permit light reflected from mirror 19 to pass through. The optical path between mirror 19 and mirror 17 is intersected by a liquid crystal shutter 21. With the shutter 21 closed the camera 36 receives only the left image. With the both optical images transformed into digital form a simple computer routine may be used to subtract the left from the left plus right image and obtain the right image.

With the equipment illustrated in FIG. 3, the flow chart in FIG. 9 and the program described below more detailed analyses can be performed. Firstly, real time calculations can be performed and accurate estimates of minimum crack length ascertained. As a part of the program, however, the video data from each view can be digitized and stored in computer memory thereby providing three dimensional information about the crack. To avoid errors due to vibration and other time related disturbances, the program, upon a command from the viewer, "grabs" simultaneously the images from both cameras 36 and 38 and stores the images "frozen" in memory to create a set of right and left views. In FIG. 9 the views are frozen by initializing the video frame "grabber" 80, prompting the user to properly focus and frame the desired image 82 on the television monitor 46, freeze the image in digitized form in memory 84 and repeat 82 for the other image in memory 86.

With the frozen views, the computer program provides three modes of analysis. In the first mode the curser can be used to enter a point or location on an object in view into the program from both views as described above in FIG. 4. Curser positioning can be accomplished in any conventional way such as with the mouse 50, key board 56 or with a light pen, joystick or grid table.

The calibration coefficients for the particular borescope optics are retrieved from memory 88. With both curser locations entered from the two views, the program at 90 produces either view with the grid superimposed and the grid lines representing a known dimension such as millimeters as described above. The size of the object can thus be estimated by comparison with the grid. The distance from the borescope tip 20 is displayed 92 on the screen therebelow as shown in FIGs. 5 and 6. The field of view also can be estimated from the grid display on the screen. This method of estimating object size can be considered "analog" because of the visual judgments of dimensions on the part of the viewer. No attempt is made in the calculations or visual estimates to correct for optical or geometric aberrations in the simplest mode although all data exist in digital form in the computer memory.

As further "analog" analysis, the viewer may view both views simultaneously superimposed on the video monitor 46 for stereoscopic three- dimensional view. With the stereoscopic view foreground objects can be distinguished from background objects. Thus, the view can distinguish holes and cracks from dirt particles and surface streaks, the latter being typically caused by combustion processes.

Furthermore, of more importance are the clues to the orientation of the object or surface in view relative to the plane 24 of the tip 20. Since a two-dimensional view as above provides only an estimate of minimum crack length, actual length may be considerably larger, a factor that becomes apparent with stereoscopic viewing. As with unaided human vision, the experienced viewer of three-dimensional stereoscopic views can estimate depth distances and thereby obtain gaging estimates of object size and distance without the use of the computer gaging program described above or the more detailed computer gaging modes described below. Subtle perspective differences to help distinguish between inward tending and outward tending surfaces are a clear advantage to stereoscopic viewing. The viewer has clues when encountering angular surfaces, compound curve surfaces and non-normal planes relative lo the borescope axis. These clues are of important advantage and significance in identifying possible erroneous crack length estimates. Thus, the viewer can quickly determine visually whether additional points on a crack should be calculated for distance Z from the borescope tip. Significant changes in Z point to point can thus be used to better estimate crack length on the surfaces noted above.

Referring to FIG. 3 a pair of viewing glasses 80 are illustrated with an electrical connection to the computer 42. To provide stereoscopic viewing on the video monitor 46 the three-dimensional stereoscopic module of the program includes means to alternate right and left views on the video monitor screen, these views being coordinated with liquid crystal "shutters" in the respective right and left lens of the viewing glasses 80. With sufficient rapidity to overcome flicker the viewer see_ a sιereυ_coρic view . Other means such as split screen or dual screen with polarized filters and polarized glasses can be used as an alternative.

The primary reason to acquire two different perspectives simultaneously is to compute differences between images in order to make accurate measurements as noted above. Having the images in digital form provides for calculation of measurements and for correction of optical and geometrical aberrations contained within the digitized images. Barrel distortion as illustrated in FIG. 10 is an example of such an optical aberration.

Barrel distortion causes the apparent physical displacement of an object within a scene relative to the centerline of the optical system. The image of the object is distorted or lacking in linearity. Stiaight lines become curved 94 and a rectangle takes on the appearance of a truncated ellipsoid 96. Clearly, barrel distortion has a negative effect upon the accuracy of an ordinary measurement system. Barrel distortion is commonly found in simple lens systems.

Various methods are available for correcting barrel distortion in images. The basic optical correction system utilizes a compound lens system which comprises lenses of equal and opposite distortion thereby cancelling the effect. For digitized images various computer programs achieve correction by applying "warp" programs to the images. In particular, an image is "flattened" by reassignment of pixel locations within the image digital matrix.

In practice, borescopes use simple lenses therefore barrel distortion appears. The experienced viewer is accustomed to the distorted image therefore it is not necessarily desireable to remove barrel distortion from the digital image because it would appeal different from the optical image the viewer would expect.

Calculated measurements howevei can be more seriously affected unless barrel distortion and other distortions are digitally corrected. These corrections, based upon the size and geometry of each of the borescope lenses for example, can greatly improve the accuracy of the calculated measurements. The experienced viewer can make the visual determinations almost instantly and then with the mode υt operation above quickly obtain "Z"'s and grid sizes or perform digital gaging in a more organized fashion as follows:

Three point digital gaging (second mode) requires a three point sample basis and is well applied to gaging on a surface or plane normal to the axis of the borescope. The algorithm that performs the measurement calculations above is relatively simple. The algorithm is based upon knowledge of the angular field of view, the center to center distance between the borescope lens pair and of the magnification of one at only one precise distance from borescope tip to the object in the plane. With the angular field of view θ and the magnification of one, the distance Z is equal to the cotangent 9/2 multiplied by half the field diameter. For most lenses cos θ/2 may be substituted for cot θ/2. The field diameter is related to the video frame of the monitor by a matrix of picture elements or pixels. The pixels represent distance in proportion to the distance across the field diameter.

The distance between the axes of the lens pair is fixed. Taking the same point on an object in both the left and right borescope images produces a difference measurement or displacement in pixels relative to the reference line. By dividing the pixel lens separation by the displacement and multiplying by the video frame dimension in pixels, the field diameter can be calculated in mm for example and Z in turn calculated. The grid size and Z are therefore determined from the displacement and fixed geometric quantities.

Performing the same calculation and measurement twice for points at opposite ends of a crack in the plane gives a calculated value for crack length. Comparing Z values assures that the plane of the crack is normal to the borescope axis. Moreover, the barrel distortion correction lies within the fundamental algorithm as follows: The operator or viewer moves the cross hairs or cursor to the left point of the crack or object being measured in the left image and then moves the cross hairs to the right point of the crack or object in the left image. The process is repeated in the right image for both extremes of the crack or object. T hus, the calculation of the separation in pixels for the left point includes a subtraction as above that cancels the barrel distortion in each lens (assuming the dual lenses are identical in the borescope). Likewise, the calculation of the separation in pixels for the right point also includes a subtraction as above that cancels the barrel distortion from the result. By taking a third point and possibly more points along the crack the plane can be checked for flatness or the true shape of the surface containing the crack and the true crack length ascertained to any accuracy desired. The latter is the four point digital gaging (third mode) calculation from the digital data.

Claims

ClaimsWe claim:

1. A dual borescope comprising a long slender tube enclosing a plurality of light guides, said tube extending to a tip, a pair of substantially identical objective lenses at the tip, said objective lenses each in optical communication with a separate light guide, both lenses mounted in the same plane with optical axes in parallel with the axis of the borescope tip, and optical means in optical communication with each of said separate light guides to direct images to viewing and image recording means.

2. The dual borescope of claim 1 wherein at least one video camera is in optical communication with the optical means and means to monitor and record the optical images sensed by the camera are in electrical communication with the camera.

3. The dual borescope of claim 2 wherein the means to video monitor and record include means to digitize the images and apply quantitative measurement algorithms to the digitized images.

4. The dual borescope of claim 3 including means to generate a reference location for each image viewable on the video monitor.

5. The dual borescope of claim 3 including means to generate a grid for each image viewable on the video monitor.

6. The dual borescope of claim 3 including means to digitally compensate for optical lens distortion.

7. The dual borescope of claim 2 wherein the means to video monitor and record include a dual path digitizer in electrical communication with the video camera, a computer in electrical communication with the dual path digitizer and a computer program specifically including means to calculate, record and provide for display of quantitative data derived from said optical images.

8. The dual borescope of claim 2 wherein the means to video monitor and record include a dual path digitizer in electrical communication with the video camera, a multiplexer board and a display monitor in electrical communication with the dual path digitizer whereby the live view of either or both of the pair of objective lense*. may be displayed on the monitor.

9. The dual borescope of claim 1 wherein the optical communication means include a pair of relay lenses that simultaneously direct both images to a single video camera.

10. The dual borescope of claim 1 including selectably movable optical means to sequentially direct each image to a single video camera.

1 1 . The dual borescope of claim 1 incl tiding selectably actuatable liquid crystal means to sequentially direct each image to a single video camera.

12. The method of measuring the size of an object viewed through a dual stereoscopic lens borescope comprising the steps of: a) viewing the object in focus through both of the lenses; b) generatinga single reference location viewed through each lens, c) selecting a point on the object and measuring the distances to each reference location in a direction parallel to the plane containing the axes of the dual lenses and, d) calculating the difference between the distances and dividing by the distance between the axes of the lenses to find the picture element separation.

1 3. The method of claim 12 further including the step of: dividing the lens constant "K" by the picture element separation to calculate the distance "Z" of the point on the object from the dual lens tip of the borescope.

14. The method of claim 13 further including the steps of: a) selecting a second point on the object spaced from the first point and measuring the distance to each reference location in a direction parallel to the plane containing the axes of the dual lenses, b) repeating the calculation of the picture element separation and the distance "Z" to the borescope dual lens tip, c) comparing the values of "Z" for each point.

15. The method of claim 14 further including the step of calculating the straight line distance between the points.

16. The method of claim 14 further including the steps of selecting additional points on the object and repeating the calculation of the picture element separation and the distance "Z" for each point selected.

17. The method of claim 16 further including the steps of calculating the straight line distances between selected pairs of points on the object.

18. The method of claim 12 further including the steps of: a) calculating the viewing grid size from the picture element separation and b) displaying dual grids superimposed over the dual views, the grids being sized in proportion to the magnification of the object.

19. The method of claim 12 further including the step of

"freezing" in digital memory both views substantially at one instant.

20. The method of claim 12 including the step of calculating the compensation required to compensate for optical lens distortion.

21. The dual borescope of claim 3 including means to digitally measure the difference in depth from borescope tip to two differing locations on an object in the images.

22. The dual borescope of claim 21 including means to digitally measure the Cartesian coordinates for any locations on an object in the images to correct for off-horizontal axis locations.

23. The dual borescope of claim 21 including means to calculate a length value between two locations on an object in the images both in a plane perpendicular to the axes of the borescope lenses and in a plane passing through both actual locations on the object.

24. The dual borescope of claim 3 including means to retain the images in memory to perform multiple calculations for multiple locations on an object in the images.