JP2528921B2 - Distance measurement by diffraction - Google Patents

Description

FIELD OF THE INVENTION The present invention relates generally to the concept of distance measurement utilizing the mechanism of an optical rangefinder, and more particularly to the relationship of the distance of a diffraction grating from the surface of a target illuminated with monochromatic light. And a method of measuring a distance by correlating each relative displacement of a higher-order diffraction image from each diffraction image of order 0 observed through a grating.

BACKGROUND AND OBJECT OF THE INVENTION Optical rangefinders are cameras that display distance information. A conventional two-dimensional camera displays the brightness of an object with incident light or reflected light. Range finding camer
a) connects the images, but the brightness of the image is determined by the distance from the camera to the image.

There are essentially two types of optical range finding cameras: active and passive. The difference between active and passive depends on how the target is illuminated. Active rangefinders control the light source that illuminates the target, while passive rangefinders rely only on ambient lighting.

Three main principles apply to passive range finding: shape analysis, multiple (generally stereoscopic) views, depth of field or optical focus analysis. All implementations of the three principles of passive range finding are implemented with conventional two-dimensional cameras. For example, one form of shape analysis is accomplished by observing and displaying (imaging) a target of known size and determining the distance of the target from the camera by simply measuring the displayed size. Or, if two horizontally offset views of one target are taken and the two photos are registered at a point with known depth, any other distances that are related to each other are registered. It can be measured by measuring the difference in the horizontal dimension between the two. Such multiple view method is a variation of the triangulation procedure and is also used for active range finding. The last principle of passive range finding is depth of field or focus analysis. Since the radius of the circle of confusion of a defocused lens is proportional to the distance from the lens to the subject, when the target object is placed at the position where the focus is the best, the distance of the target object The case of the lens can be calibrated for readability.

Active light-based distance measurement requires a light source controlled by a rangefinder. The areas where the most intense research is conducted are triangulation, time of flight (LADAR), and projection patterns (Moir
e) and focus calibration. In triangulation, the second camera of the stereoscopic photography system is replaced by a light source with a well-defined structure, like a floodlight. Usually, the light is emitted from the projector in the form of a sheet. Looking straight at the projector axis, the sheet of light looks straight, whatever the depth of the surface on which the light is incident. Also, when viewed from a position off the axis, the light surface looks curved. The shape of the curvature of the light surface is easy to correlate with depth.

Another method, using a well-defined light source, must emit a pair of two-dimensional patterns that are evenly spaced over the subject. These two patterns interfere with each other to form a moire pattern, which allows easy photographing. The topographical shape of the moire pattern is proportional to the change in the distance from the camera to the subject. LADAR is similar to electromagnetic range finding with radar. The difference is that a pulse-modulated laser is used as the active light source. A specially designed rangefinder sensor measures the time of flight from the laser back through the target to the rangefinder. Although LADAR systems are expensive, cumbersome and inaccurate, they have the very important advantage of having the light source coaxial with the sensor over triangulation and moire pattern based systems. As a result, the image being measured is not interrupted or shadowed.

Another coaxial system of measurement with active light sources is focus calibration (analysis). In such a system, a pencil beam is emitted from the camera. The radius of the circle of confusion of the beam seen by the calibrated lens is a submultiple of the distance to the target.

When using an optical rangefinder, whether it is active or passive, it is generally clear that there is a single common drawback. All known systems require a large number of sophisticated optics, cumbersome calibrating, biaxial light sources and display systems, and very sophisticated equipment for solving fairly high order numerical models. The conventional technique of optical distance measurement is a technique that obtains real-time distance data while avoiding the first three of the above-mentioned drawbacks. Studies in this area by P. Shable and TC Strand, Institute of Image Processing at the University of Southern California, Los Angeles, California, have shown that distances can be measured using the Talbot diffraction image of a grating. March 1984 15
In an article in Applied Optics of the Date, Volume 23, No. 6, Shable and Strand discuss one method. The method differs from many of the previous methods in two fundamental aspects, the ability to measure in real time and the underlying physical development and measured variables of the method. The method utilizes diffractive development and measures the light-dark ratio variable. The Shable and Strand method relies on the contrast ratio of the Fresnel diffraction pattern of the grating varying as a function of distance from the grating and the frequency of light. However, measuring the light-dark ratio clearly increases the drawbacks. To obtain arbitrary distance information, monochromatic coherent light illumination must emit the entire electromagnetic field spectrum at discrete frequencies,
Such a method is unnecessarily expensive and the measuring distance
If it is less than 10 meters, it must be unnecessarily complicated. Applicants appear to have overlooked a significant element of the Shable and Strand method, and it appears that diffractive development must be investigated further. Diffraction gratings are optical elements that have been used in a wide range of applications for over 100 years. The optical device consists of evenly spaced straight lines centered near the wavelength of the light by the main optical elements. The nature of light reflected or transmitted by diffraction gratings has been the subject of numerous dissertations, majors, editorials, and texts. The remaining literature has a theoretical explanation of development at the heart of the invention of distance measurement by diffraction, but except for the aforementioned study of Shable and Strand, the current literature available to the applicant is also a thorough examination of the patents. Does not give a clear suggestion to this subject. Diffraction development is often divided into two general types: Fraunhofer diffraction and Fresnel diffraction. The former is a simple case where the light passing through the grating is treated as a plane wave. On the other hand, if the wavefront exhibits a curvature with respect to the spacing of the grating, then by using a more complex Fresnel analysis,
Secondary diffraction effects must be calculated. In particular, the illuminated point light source emits light having a spherical wavefront. In finely spaced gratings such as those produced by holography, the effects of curved wavefronts are noticeable at distances of a few meters, an effective range for some distance measurements.

The Applicant has attempted to develop a method of distance measurement by utilizing the diffraction phenomenon and more particularly by expressing the angular displacement of the order of the diffraction image as a function of the distance from the diffraction grating to the point source. When looking at a point source of monochromatic light through a diffraction grating, multiple images of the point source are visible. The diffracted image of order 0 appears at the exact position of the point light source, but the diffracted image of high order is obliquely arranged at an angle determined by the frequency of the grating, the distance from the grating to the point light source, and the color of the light emitted from the point light source. . In particular, the effect of the angular displacement of the high-order diffracted image as a function of the distance of the point source is only observed in the case of Fresnel diffraction, ie when the wavefront from the point source is curved with respect to the size of the grating. Measurements by this method are very accurate when the point source is close to the grid. The rate of angular displacement diminishes as the point source moves further away from the grid. (The first experiment was 1m
A 10 mm grating engraved with 600 lines per m clearly shows the measurable displacement of the displacement angle of the first order diffraction of the target with a red point source at a distance of 1 m. The grating was focused on a half-inch vidicon tube and confirmed at a distance of 10 cm with a 100 mm focal length lens. 3.) The simplest embodiment of the above method is intended for emitting a pencil beam of monochromatic light to a target, observing the target through a grating, and measuring the displacement of a high order diffracted image from a zero order position. To do. Such rangefinders can be calibrated by simply measuring a calibrated target and establish a correlated scale of displacement and distance. The displacement readings can be read visually by a calibrated viewer or by a linear array of photocells or an electrical sensor such as a television camera. The device is very effective when the emitted beam and the viewfinder are optically coaxial. This makes it possible to measure all visible points.

A more sophisticated implementation of this method is to use one or two moveable mirrors to change the coaxial emission and viewing angle of the horizontal and vertical fields.
This makes it possible to obtain distance information for each picture element on the three-dimensional surface. Although such scanning devices are known in the art, the inclusion of a diffraction grating in the scanning device to obtain distance information has not been published in the Applicant's available literature.

Many different pitches of the diffraction grating can be manufactured on a single substrate. Techniques for its manufacture include electron beam lithography and holographic optical elements (HO
E'S) is included. An array of diffraction gratings of different orders allows distance measurement at different stages. In such an embodiment, as the distance increases discontinuously, the light undergoes a unique displacement in the individual gratings.

To extend the measurement distance beyond the limits of Fresnel diffraction, diffraction gratings are used to measure the size of distant objects. If a straight line or square of light is emitted, the light will appear as a continuous object until the grating is at a distance sufficient to separate the first-order diffraction pattern from the zero-order diffraction pattern. As in the continuous array above, the distance of the target is such that at which pitch of the grating there are three consecutive images of two straight or solid objects (two first-order diffraction images and the zeroth order of the center of the field). It can be measured by examining whether one image is separated). This type of distance measurement is similar to shape analysis, but it becomes more convenient by using the method described above.

The distance measuring method and the above-mentioned examination of the diffraction pattern have various advantages and disadvantages, and the applicant made efforts in avoiding the drawbacks by incorporating the respective advantages in the present invention.

The object of the present invention is therefore to develop an accurate distance measuring method which can be used for measurements at very short distances of 0 to 3 meters with maximum accuracy close to the observed object or target.

Another object of the invention is to avoid overuse of the optical device.

Yet another object of the present invention is to develop an easy calibrating system.

Yet another object is to avoid the need for bi-axial lighting and secondary display systems.

Yet another object of the invention is to realize with devices that are not normally required to resort to elaborate analytical or synthetic techniques and equipment, but rather to universities or small companies that manufacture visual aids, or equipment normally available in optical computer laboratories. It is to provide a distance measuring system that can perform.

Some of the objects and advantages of the invention have been set forth above and, in part, will be apparent from or apparent from practice of the invention. The invention consists of items, figures, examples and combinations shown and described herein or inferred from reading this document.

SUMMARY OF THE INVENTION It is an object of the invention to emit a pencil beam of monochromatic light,
It has been found to be realized by looking at the illuminated target through the diffraction grating and measuring the change in the higher order diffraction pattern from the position of the zeroth order diffraction pattern. Distance measurement calibrating is to measure a target at a known distance and to establish a correlated scale between the diffraction pattern and the target distance. If the emitted beam and the viewfinder are optically coaxial, there is no shading. In such an arrangement, the optics may be minimal, easy to calibrate, the scale for coaxial illumination and secondary display systems is simple, and there is no sophisticated mathematical analysis and synthesizer.

BRIEF DESCRIPTION OF THE FIGURES FIG. 1 depicts Fresnel diffraction of a curved wavefront.

 FIG. 2 is a diagram of a preferred embodiment of the distance measuring device.

Description of the Preferred Embodiments The accompanying drawings, FIGS. 1 and 2, are touched in more detail. FIG. 1 illustrates a point where the wavefront emitted from the point light source So5 propagates and approaches the diffraction grating 7. S _{1 of the} diffraction grating 7,
The curved wavefront 6 is diffracted at points S and ₂ . As a result, the wavefront pattern on the other side of the diffraction grating interacts with the wavefronts propagating through the adjacent apertures, thereby interfering with or enhancing each other. I ₀ 11 strengthening point of the wavefront is diffracted image and the first-order diffraction images of the respective orders 0
And end with 1 ₁ 12, 12 '.

FIG. 2 of the preferred embodiment is a simple apparatus utilizing the Fresnel diffraction phenomenon depicted in FIG. Monochromatic light source 1
Concentrates the beam 2 of monochromatic light on the surface of the beam splitter 3. The beam reflects off the target 5 along another path 2 '. The illuminated S ₀ or similar point source is thus propagated back along the path 6'in which the light was originally generated, passes through the beam splitter 3 and emerges as beam 6 '. The beam 6 ′ is curved just before hitting the diffraction grating 7, as depicted by the stylized arrow. The beam is diffracted by the diffraction grating 7 as described and depicted in FIG. For clarity, the wavefronts or beams associated with only the zeroth order diffracted image 8 and the first order diffracted image 9 are drawn. The diffracted image passes through the objective lens 10 in a concentrated manner and appears,
It is shown as a diffraction image 11 of order 0 and a diffraction image 12 of first order.
The phenomenon that can be observed and repeated experimentally is that the distance 13 from the diffraction grating 7 to the point source S ₀ 5 and the diffraction images 11 and 1 of order 0
It is shown by the correspondence of the displacement 13 'with respect to the next diffraction image 12.

It will be readily apparent to those skilled in the art that the objective lens 10 can be replaced by a video camera and the diffraction images 11 and 12 traditionally emitted in the plane will appear on the video screen. From this point onwards it will be easy to calibrate the device so that the distance 13 will be correlated with the displacement 13 '.

From a broader perspective, the invention should not be limited to the particular embodiments discussed in the drawings and text, but it departs from the scope of the appended claims in accordance with the principles of the invention and for its benefit. Without thinking, various changes and new ideas can be considered.

Claims (7)

(57) [Claims] 1. The relationship between various distances of a diffraction grating from the surface of a target irradiated with light, the position of a zero-order diffraction image observed through a lens and a light receiving sensor after passing through the diffraction grating. A distance measuring method for measuring the distance by correlating with a relative position change of a high-order diffraction image with respect to 2. The distance measuring method according to claim 1, further comprising the step of irradiating the target with monochromatic light. 3. Light from a point source, which is incident from a distant point source through a diffraction grating and is decomposed into a plurality of wavefronts corresponding to a plurality of diffraction images of different orders, is observed by using a lens and a light receiving sensor. A method for measuring the distance of a high-order diffraction image, which relates an observed positional change from a zero-order diffraction image to a physical distance of the point light source from the diffraction grating. 4. The distance measuring method according to claim 3, including a step of irradiating a point light source with monochromatic light. 5. A diffractive distance measuring device, comprising a monochromatic light source used to illuminate an object for which distance data is sought, and a wavefront for receiving light reflected by the object. A diffraction grating for decomposing into a higher-order diffraction image, and a means including a lens and a light-receiving sensor for observing and identifying the position change of the diffraction image of each order with respect to the position of the 0th-order diffraction image of the diffraction image. Then, the distance measuring device by diffraction in which the position change of the diffraction image of each order with respect to the position of the 0th-order diffraction image is associated with the absolute distance from the target to the diffraction grating. 6. The distance measuring device according to claim 5, wherein the diffraction grating has lines drawn so as to have adjacent line intervals of at least 2 which are equal to an integral multiple of one wavelength of the monochromatic light. . 7. The distance measuring device according to claim 5, wherein the means including the lens and the light receiving sensor comprises a video camera.