RU2212044C1 - Method of formation of optical image in noncoherent light - Google Patents

Abstract

FIELD: methods and means converting optical radiation to form images of objects in noncoherent light. SUBSTANCE: technical result of method consists in rise of quality of optical images thanks to compensation for errors in focusing of optical system. This result is achieved as optical-electron conversion of formed intermediate distorted image of object and following compensation for distortions introduced by defocusing and optical system as whole are conducted in optical system with at least one lens. Compensation for distortions is carried out by means of matched spatial filtration of intermediate distorted image. In this case two intermediate distorted images are formed simultaneously in opposite bundles of rays. Compensation for above-mentioned distortions in process of matched spatial filtration is realized after match of these images and vector addition of signals in like points of matched images. EFFECT: rise of quality of optical images. 4 cl, 18 dwg

Description

 The invention relates to the field of optics, in particular to methods and means of optical conversion of electromagnetic radiation, and can be widely used in the formation of optical images of objects (mainly extended) in incoherent light to improve image quality.

 The method of forming an optical image of an object in incoherent light, which is based on the use of focusing optical elements (for example, the lens of the eye of a biological object or the lens of a movie, cameras), is widely known from the prior art (II Artobolevsky, Polytechnic Dictionary, ed. " Soviet Encyclopedia ", M., 1976, p. 323).

 The disadvantages of the method under consideration include the presence of aberrations of optical elements, as well as focusing errors in case of displacement of the original object from the plane of the best focusing of the optical system or in the case of the length of the said object along the optical axis of the system. These shortcomings lead to a deterioration in the image quality of the object (reduced sharpness, image deformation, the appearance of chromatism, etc.).

 One of the methods for partial elimination of the noted drawbacks is to reduce the numerical aperture of the used optical elements (i.e., to reduce the size of the entrance pupil of the optical system). However, this entails a decrease in illumination in the generated image in proportion to a decrease in the area of the entrance pupil, as well as a decrease in the resolution of the optical system as a whole.

 Another method known from the prior art for generating an optical image of an object in incoherent light is based on changing the transparency of the optical element in the direction from the center to the periphery of the entrance pupil. In particular, in the case of the Gaussian law of transparency change, the size of the diffraction image of a point object decreases and, therefore, the resolution of the optical system as a whole and, accordingly, the quality of the formed image of the original object increases (J. Ojeda-Castaneda at ol., "High Focal Depth By Apodiration and Digital Resporation ", Appl. Optics, 27, 12, 1988; M.Mino, Y. Okano" Improvement in the OTF of Defocussed Optical System Through the Use of Shaded Apertures ", Appl. Optics, 10, 10, 1971 .).

 However, as in the above cases, this method known from the prior art only partially reduces the effect of focusing errors (in the case of a displacement of the original object from the plane of the best focusing of the optical system or in the case of the length of the said object along the optical axis of the system) due to some decrease in the illumination of the image .

 There is also a method of forming an optical image of an extended object in incoherent light and a device for its implementation, providing the possibility of reducing the defocusing error by introducing a special phase mask into the light rays (US, 5748371, class G 02 B 5/18, 1998).

 The indicated technical solution uses one or several lenses to form an image on the surface of the optoelectronic converter, and in one of the main planes of the optical system a cubic phase mask is installed, by means of which the optical transfer function (i.e., distort the optical path of the rays) of the said system is changed (forming an image) in such a way that it (optical transfer function) remains almost unchanged when defocusing the optical system in a sufficiently wide range of ate. Next, the generated distorted image is fixed by means of an optoelectronic converter and the undistorted final image is restored from it by the method of matched spatial filtering using a digital filter implemented in hardware or numerically by means of a computer.

 The considered method allows, when implemented, to significantly reduce the effect of defocusing errors on the quality of the generated image of the object, and also allows you to take into account and reduce the residual aberration of the optical system.

 The disadvantages of this known technical solution include the high quality requirements of the optoelectronic converter due to the fact that to obtain a high-quality image, it is necessary to digitize the signal at each specific point in the image no worse than 10. ..12 bits. In the presence of receiver noise and / or a decrease in the number of quantization levels of the signal, the quality of the reconstructed image deteriorates significantly, and the effect of increasing the depth of the sharply imaged space decreases.

In addition, in the above-mentioned well-known technical solution, image distortion points introduced into the optical system by means of a cubic phase mask are determined by partial derivatives (with respect to pupil coordinates) of a function of the form: a (x ³ + y ³ ). Consequently, the directions of the mentioned distortions coincide to a large extent with the direction of the distortions caused by defocusing, which leads to the “smearing” of the reconstructed image of a point source in case of its displacement from the focus of the optical system.

 There is also a method of forming an optical image of an extended object in incoherent light, based on the combined use of amplitude and cubic phase masks (US, 6097856, class G 06 K 9/20, 08/01/2000).

 According to this method under consideration, in an optical system with at least one lens, the following are successively carried out: a predetermined distortion of the optical path of the rays (that is, a change in the optical transfer function of the said system) by means of an amplitude-phase mask introduced into the system; optoelectronic conversion of the distorted intermediate image of the object formed by the optical system and subsequent subtraction of the distortions introduced by the amplitude-phase mask and the optical system as a whole by means of coordinated spatial filtering of the said intermediate image.

 This known technical solution has the same disadvantages as the previously discussed method according to patent US 5748371.

In addition, the distortions in the image of a point light source caused by the introduction of a cubic phase mask (the phase delay function of which has the form: Δφ = k (x ³ + y ³ ) are determined by the partial derivatives of the function with respect to the pupil coordinates "X" and " Y ", that is, proportional to" x ² "," to ² ". At the same time, the distortions introduced by defocusing are proportional to" x "and" y ", respectively, and have the same direction. In this regard, the quality of the reconstructed image substantially depends on the amount of defocusing and, therefore, ANOVA ideal image of the source, particularly in the case of an extended object.

 Closest to the claimed subject matter of the invention is a method known from the prior art for generating an optical image of an extended object in incoherent light according to the application of the Russian Federation 2000132488, cl. G 06 K 9/20, 2001

 In this prior art method for generating an optical image of an object in incoherent light in an optical system with at least one lens, the following are successively carried out: a predetermined distortion of the optical path of the rays by means of an amplitude-phase mask inserted into the optical system; optoelectronic conversion of the generated distorted intermediate image of the object and the subsequent subtraction of distortions introduced by the amplitude-phase mask and the optical system as a whole, by means of coordinated spatial filtering of the distorted intermediate image. The specified distortion of the optical path of the rays is carried out in a direction close to or coinciding with orthogonal with respect to the distortions caused by defocusing and aberrations of the optical system, while ensuring the rotational symmetry of the function of the given distortion.

 The disadvantages of this technical solution under consideration include the following. Since during defocusing (arising in the process of changing the spatial position of an object), the response function of the optical system changes, compensation for defocusing is possible only to a limited extent, within which the mentioned response function is slightly different from what is included in the original mathematical model. If the response function of the optical system varies over a wider range, then there is a need to adjust the original mathematical model. As a result of this, the decryption process of the generated image is complicated, since the decryption algorithm cannot be enclosed in a rigid logical framework.

 The claimed invention was based on the task of expanding the functionality of a method for generating an optical image of an object in incoherent light by increasing the allowable defocusing range in which a real-time quality reconstructed image of the original object that is invariant with respect to defocusing can be obtained.

 The task is carried out by the fact that in the method of forming an optical image in incoherent light, according to which in an optical system with at least one lens, the optoelectronic conversion of the formed intermediate distorted image of the object is carried out sequentially and the subsequent compensation of distortions introduced by defocusing and optical system as a whole, through the implementation of a consistent spatial filtering of the intermediate distorted image, according to The invention forms two intermediate distorted images of the object in the opposing beam of rays, and the compensation for the above distortions in the process of matched spatial filtering is carried out after combining these intermediate images and vector addition of signals at the same points of the combined images.

 It is advisable to first ensure in the optical system that the defocusing values of the aforementioned intermediate images obtained in oncoming beams of rays are close, vector addition of signals at the same points of the combined images is carried out with the opposite sign, and coordinated spatial filtering is carried out taking into account the absolute values of defocusing values without taking into account possible object displacements in the process of image formation.

 To further increase the permissible range of defocusing during the formation of an intermediate distorted image of an object in each of the said oncoming beams of rays, a predetermined distortion of the optical path of the rays is additionally carried out in a direction close to or coinciding with orthogonal to the distortions caused by defocusing and aberrations of the optical system, providing rotational symmetries of the specified function of a given distortion by means of amplitude-phase m introduced into the optical system asok.

 It is also advisable before projecting the generated intermediate distorted image of the object on the plane of the image receiver, each of the said oncoming beams of rays (carrying the said image of the object) to undergo aperture.

 It is optimal to form intermediate distorted images of an object at the same time, while using counterpropagating beams of rays with mutually perpendicular polarization.

 The invention is illustrated in graphic materials.

 Figure 1 gives a variant of the optical system for implementing the inventive method with two light sources.

 Figure 2 illustrates the intermediate distorted image of the object formed by the optical system of figure 1.

 FIG. 3 is an embodiment of an optical system for implementing the inventive method with one light source and a mirror system.

 FIG. 4 is an embodiment of an optical system for implementing the inventive method with a single light source, a system of mirrors and phase masks, through which a given distortion of the optical path of the rays in each of the oncoming beams of rays is carried out.

 FIG. 5 illustrates intermediate distorted images of an object formed by the optical systems of FIG. 3 and FIG. 4.

 FIG. 6 illustrates the change in the response function of the optical system (forming an image) with a change (increase and decrease) in defocusing relative to the initial state of the optical system (i.e., the state of the optical system at a given value of defocus).

 7 illustrates a source object.

 FIG. 8 illustrates the response function of an optical system with a phase mask for a technical solution adopted as a prototype.

 FIG. 9 and 11 - distorted intermediate images of the original object of FIG. 7, implemented by means of a technical solution adopted as a prototype, with a defocus value of 2 relative units and 6 relative units, respectively.

 FIG. 10 and FIG. 12 are reconstructed images of the original object of FIG. 7, implemented by means of a technical solution adopted as a prototype, with a defocus value of 2 relative units and 6 relative units, respectively.

 Fig - response function of the optical system according to the claimed invention in the case of vector addition with the opposite sign of the signals at the same points of the combined images (according to claim 2).

 Fig.14 is a perspective view of the response function of Fig.13.

 FIG. 15 and FIG. 17 are distorted intermediate images of the original object of FIG. 7, implemented by the claimed technical solution, with the defocus value equal to 3 relative units and 6 relative units, respectively (option implemented in accordance with claim 3 of the claims).

 FIG. 16 and FIG. 18 are reconstructed images of the original object of FIG. 7, implemented by the claimed technical solution with a defocus value equal to 3 relative units and 6 relative units, respectively (option implemented in accordance with claim 3 of the claims).

 The physical principle of the implementation (implementation) of the patented method of forming an optical image of an object in incoherent light is as follows.

In an optical system with at least one lens 1, the optoelectronic conversion of the formed (at the image receiver 2) intermediate distorted image 3 and 3 ^{1 of the} studied object 4 is carried out sequentially and the subsequent compensation of distortions introduced by defocusing and the optical system as a whole. Compensation of the aforementioned distortions is realized by means of coordinated spatial filtering. At the same time, at the receiver 2, the images form (as a rule, simultaneously) two intermediate distorted images 3 and 3 ^{1 of the} object 4 in the opposed beam beams (mainly mutually polarized at an angle of 90 ^{° by} polarizing beam splitters 5) (generated by or two autonomous sources of incoherent radiation 6 / Fig. .1 /, or one source 6 of incoherent radiation and a system of rotary mirrors 7 / Fig. 3 and Fig. 4 /). When projecting intermediate distorted images 3 and 3 ¹ onto the surface of the receiver 2, said oncoming beams of rays are passed through the diaphragm 8, which eliminates the overlap of images formed in transmitted and reflected light. To do this, either axial or off-axis beams of light rays are diaphragmed (overlapped), which corresponds to either a shadow image or an image in a bright field.

Further, the mentioned images 3 and 3 ¹ are read by means of known means from the image receiver 2, are combined and vector-added, and the resulting total image is subjected to a consistent spatial filtering. With this (double) formation of an intermediate distorted image 3 and 3 ^{1 of} object 4, the response functions of the optical system for each of the mentioned images 3 and 3 ¹ will change in opposite directions (i.e., for one of the images / for example, image 3 / the response function will decrease, and for another / correspondingly image 3 ¹ / increase) depending on the direction of defocusing, i.e. the direction of displacement of the object under study 4. In this case, the first derivatives of the response functions (characterizing its change) will be opposite in sign for each of the considered images 3 and 3 ^{1 of} object 4. And it follows that in the total image these first-order components will strictly compensate each other (i.e., the total distortion of the final image of object 4, due to the contribution of the first derivatives of the corresponding response functions of the optical system, will have a zero value). Graphically change in the response function φ _(ρ) (where: ρ = (x-x ₀ ) ² + (y-y ₀ ) ² , x, y are the coordinates in the plane of the image receiver, x ₀ , y ₀ are the coordinates of the center of the image of the point source ) optical system (relative to a given initial state of the system at a given value of defocusing) depending on the direction of defocusing shown in Fig.6 graphic materials. Here: curve "O" characterizes the initial state of the optical system at a given (initial value of defocusing); curve "I" characterizes the state of the optical system with a decrease in the amount of defocus; curve "II" characterizes the state of the optical system with increasing defocus value.

 Thus, according to the claimed method, the change (uncompensated) of the response function of the optical system (and, accordingly, the quality of the final image of the object under study 4) will be affected exclusively by even derivatives of this function with respect to the defocus parameter. Therefore, to the extent necessary, adjustment of the optical system will only be necessary with respect to the second derivative of the response function, which significantly expands (approximately 2-3 times with respect to the prototype) the allowable range of defocusing of the optical system without noticeable (visually perceptible) distortion in the quality of the final image of the studied object 4 (which is clearly illustrated in Fig. 10 and Fig. 12 / prototype /, as well as Fig. 16 and Fig. 18 / claimed technical solution /).

 For optoelectronic conversion of a distorted intermediate image, it is advisable to use an optically addressable spatial light modulator (AOSLM). At the same time, it is possible to carry out a consistent spatial filtering of this image directly in coherent light using the double optical Fourier transform (not conventionally shown in graphic materials).

It is advisable in the optical system (see Fig. 3) to preliminarily ensure that the defocusing values of the said intermediate images 3 and 3 ¹ obtained in the opposing beams of rays are modulo close, vector addition of signals at the same points of the combined images is carried out with the opposite sign, and spatial filtering is performed taking into account the absolute value of the defocus values without taking into account the possible displacements of the object 4 in the process of image formation 3 and 3 ¹ . This embodiment of the inventive method provides a significant increase in the quality of the filtered image due to the mutual compensation of the noise components of the image, and also helps to simplify and stabilize the algorithm of spatial matching filtering due to the possibility of reducing to zero some integral moments of the response function of the optical system for the total image.

An optical circuit that implements such an embodiment of the patented method is shown in figure 3 of graphic materials. In this optical scheme, the closeness modulo the values of the defocusing of the mentioned intermediate images (obtained in the opposing beams of rays generated by a single source 6 of incoherent radiation and a system of inverse optical mirrors 7) is ensured due to the fact that the value (l ₁ + l ₂ ) in the direct path of the rays is close by the value of (l ₃ + l ₄ + l ₅ ) in the reverse ray path. This scheme makes it possible to superimpose (in the plane of the image pickup 2) distorted images 3 and 3 ^{1 of} object 4 (in the forward and backward rays) on each other with a complete combination of their adequate points and eliminates the electron-optical noise that occurs, for example, in the optical scheme of figure 1.

To further increase the permissible range of defocusing during the formation of an intermediate distorted image 3 and 3 ^{1 of the} investigated object 4, in each of the said oncoming beams of rays, a predetermined distortion of the optical path of the rays in the direction close to or coinciding with orthogonal to the distortions caused by defocusing and aberrations is additionally carried out optical system, with the provision of the aforementioned function of a given distortion of rotational symmetry by means of introduced into the optical system amplitude-phase masks 9 and 9 ¹ . In this case, to implement the patented method, it is necessary to use amplitude-phase masks with opposite strokes in beams with direct and reverse paths of rays, which provides an opposite sign in changing the response function of the optical system when defocusing images obtained in oncoming beams of rays.

 Since, according to this embodiment of the claimed method for forming an optical image of an object (mainly extended) in incoherent light, a given distortion of the optical path of the rays (i.e., the optical transfer function of the optical system) is carried out in a direction close (mainly coinciding) with orthogonal with respect to distortions, caused by defocusing and aberrations of the optical system, the intermediate distorted image of a point source practically does not change even with errors ah defocusing, reaches half the values of the distortions introduced by the amplitude-phase mask, since the light beams are shifted along a tangent to the spot scattering while maintaining substantially unchanged their position relative to the image center. In this case, the distribution of illumination in the spot (due to the rotational symmetry of the distortion function) remains almost unchanged and, when a coordinated spatial filtering is performed, a virtually undistorted image of the source (source object) is restored. As the amplitude-phase mask in the proposed method, any known means can be used that causes deviations of the corresponding beam of rays in the direction orthogonal to the aberrations of the image-forming system (which generally performs a projective transformation of the information signal).

 Thus, the claimed method can be widely used to convert electromagnetic and other types of radiation in the case when the imaging of the object in the corresponding field requires display invariance with respect to defocusing or other aberrations of the optical system. This area includes, first of all, high-resolution scanning converters, devices for reading information from a mobile medium, various devices for measuring the physical and geometric characteristics of three-dimensional objects, pattern recognition devices, film, photo, television equipment, introscopy tools in x-ray, optical and acoustic ranges, etc.

 It should also be noted that the above considerations do not in any way limit the scope of the proposed method exclusively to the optical range due to the use of optoelectronic conversion, since any other means sufficient to convert the information signal acting on the system into a form accessible for subsequent action and perception.

Claims (5)

 1. A method of forming an optical image in incoherent light, according to which in an optical system with at least one lens, the optoelectronic conversion of the generated intermediate distorted image of the object and subsequent compensation of distortions introduced by defocusing and the optical system as a whole is carried out sequentially by spatial filtering of an intermediate distorted image, characterized in that two intermediate distorted images are formed the object in the oncoming beams of rays, and the compensation for the above distortions in the process of coordinated spatial filtering is carried out after combining these intermediate images and vector addition of signals at the same points of the combined images.  2. The method according to p. 1, characterized in that preliminary in the optical system they provide modulus proximity of the defocus values of the said intermediate images obtained in the opposing beam of rays, the vector addition of signals at the same points of the combined images is carried out with the opposite sign, and the spatial filtering is carried out taking into account the absolute values of defocusing values without taking into account possible object displacements in the process of image formation.  3. The method according to p. 1 or 2, characterized in that in the process of forming an intermediate distorted image of the object in each of the said oncoming beams of rays additionally carry out a given distortion of the optical path of the rays in a direction close to or coinciding with orthogonal to the distortion caused by defocusing and aberrations of the optical system, providing rotational symmetry of the specified function of a given distortion by means of amplitude-phase masks introduced into the optical system.  4. The method according to any one of paragraphs. 1-3, characterized in that before the formation of an intermediate distorted image of the object, each of the said oncoming beams of rays is subjected to aperture.  5. The method according to any one of paragraphs. 1-4, characterized in that the intermediate distorted images of the object are formed simultaneously, using counterpropagating beams of rays with mutually perpendicular polarization.

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