WO2009037367A1

WO2009037367A1 - Coma-based image coding in optical systems

Info

Publication number: WO2009037367A1
Application number: PCT/ES2007/070161
Authority: WO
Inventors: Marta C. De La Fuente; Juan Manuel LÁZARO; José Manuel INFANTE HERRERO
Original assignee: Indra Sistemas, S.A.
Priority date: 2007-09-17
Filing date: 2007-09-17
Publication date: 2009-03-26

Abstract

The invention relates to an optical system for forming an image of an object, including an optical axis (1), an image-forming lens (2, 3) which reproduces images of the object and a light source which directs light through the image-forming lens (2, 3) toward the object being observed. The image-forming lens (2, 3) includes a decentration (4) with respect to the optical axis (1) of the optical system such that said decentration (4) generates axial coma across the field of the optical system, which is superimposed with the other characteristic aberrations of the optical system, whereby the magnitude of coma introduced into the system can be adjusted by varying the decentration (4) in the image-forming lens (2, 3).

Description

CODING OF IMAGE IN OPTICAL SYSTEMS THROUGH COMA

FIELD OF THE INVENTION

The present invention relates to an optical system for encoding an image by means of the generation of aberration inherent in said optical system.

BACKGROUND OF THE INVENTION

In optical systems, the comma (comatic aberration or comatic aberration) refers to the aberration inherent to certain optical systems due to some design defects or imperfections in the lenses or other components, which results in pointless off-axis sources that may appear distorted Aberrations in optical systems generally lead to degradation of the image, occurring when the light coming from a point of an object does not converge towards (or does not diverge from) a single point after it has been transmitted through the system. The players need to correct the optical systems to compensate for these aberrations. Specifically, the comma is defined as a variation in the increase on the entrance pupil. In refractive or diffractive optical systems, especially those that cover a wide spectral range, the coma may depend on the wavelength.

The comma is an inherent property of certain optical systems comprising mirrors in which the light of a point source in the center of the field is perfectly focused on the focal point of the mirror (not as in spherical mirrors, where the light of the parts outside the mirror focus closer to it than the parts coming from the center, a fact known as spherical aberration). However, when the light source does not come from the center of the field (off-axis) the different parts of the mirror do not reflect the light towards the same point. This results in a point of light that is not centered, appearing wedge-shaped. The more displacement of the center of the axis, the more noticeable this effect is.

The problem that arises is to design an opto-mechanical element that is capable of acting on the wavefront so that said front is coded to, as explained below, make the system insensitive to intrinsic aberrations or defects of the Image produced by blurring.

ATHEY 1995 (DOWSKI 95) combine to solve the above problem an optical system whose pupil has been modified by inserting phase sheets therein to obtain an intermediate image (encoded by the induced wavefront deformation), with a digital post-processing of said image to achieve the development of the complete system for a wide range of object distances. In summary, the response of an optical system is modified by a phase mask located in the exit pupil so that its PSF (Point Spread Function) remains invariable to blur and other aberrations while at the same time the OTF (Optical Transfer Function) does not have zeros in its frequency domain. This allows the subsequent restoration of the digital image after it has been captured by the detector. This procedure represents a new approach in the field of

The acquisition of images, since the use of filters in the pupils to improve the image quality of an optical system was traditionally carried out by means of "apodization", for example by placing amplitude sheets that partially obscured the exit pupil thus inevitably reducing the input of light in the optical system and the power resulting from the optical instrument [Mino 1971]. In the 80s, with the evolution of electronic processors, some authors [Cathey 84, Mati 89] proposed to combine the image formation process with the subsequent digital post-processing to improve the design of the optical system, considering both processes as only one. In this way, the acquisition of the image was treated by hybrid systems (opto-electronic).

Thanks to this, even the most stringent requirements that had traditionally been demanded in optical systems (corrected image quality up to the diffraction limit, aberration control, etc.) could be fulfilled more easily [Mait 03], because the Ia Image quality is not achieved only by the optical system but by the post-processing of the image. In this way, the optical part of the hybrid system can be designed to have a greater focus range and a great tolerance to its own intrinsic aberrations [Cathey 02]: the wavefront coding and post-processing, achieve that the optical system be insensitive to aberrations and blur. If the optical system is of high quality, the entire focus range can be used to keep the hybrid system from varying before a possible blur. Thus, the opto-electronic hybrid image system requires a new method for optimization, since the optical system, the sampling of intermediate images, the processing signal, etc. They are considered together. The procedure is more complex as more parameters are introduced than in a traditional optical design, although it has the advantage of allowing the range of focus to be distributed among the various factors.

The most used design for coding the image is the cubic phase sheet.

The problem that arises in the use of this type of known sheets is that, lacking revolution symmetry, they cannot be manufactured with diamond-tipped machines with three-axis control. It is therefore necessary to resort to control in five axes, being this type of machines very expensive and not extended.

On the other hand, adding an additional element in the optical path reduces the transmission of the optical system, which negatively affects its performance, especially in conditions of low illumination or low white contrast. Moreover, this additional element is added by permanently in the optical system, not being able to withdraw easily when convenient.

The present invention is oriented to the solution of these inconveniences.

SUMMARY OF THE INVENTION

Thus, the present invention proposes to achieve the desired coding of the wavefront in an optical system by decentralizing the optical elements that make up said system. If said runout is done in a controlled manner, it is possible to quantify how much the original wavefront has been deformed, and the wavefront can also be corrected electronically afterwards in the image.

The optical system of the invention achieves the desired aberration without using additional elements in the optical system, so that the transmission of said system is not influenced, while the offset of elements proposed by the invention can be removed when it is created. necessary or convenient.

Another advantage of the optical system of the invention is that it can be applied to small optical systems.

In addition, the type of machines used in the manufacture of the lenses of the optical system according to the invention is the conventional one, so that the cost of carrying out an optical system according to the invention does not become more expensive.

Other features and advantages of the present invention will be apparent from the detailed description that follows of an illustrative embodiment of its object in relation to the accompanying figures.

DESCRIPTION OF THE FIGURES

Figure 1 shows the lens offset of an optical system according to a first embodiment of the invention. Figure 2 shows the lens offset of an optical system according to a second embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention proposes an optical system that forms the image of an object comprising an optical axis 1, an image-forming objective, preferably formed by conventional lenses 2, 3, which reproduces the images of said object and a light source that directs natural or artificial light through the lenses 2, 3 and towards the object to be observed, in which the coding of the image of the object is carried out by means of an offset 4 of at least one of the lenses 2, 3, with revolution symmetry with respect to the optical axis 1 of the optical system to which it belongs. Said runout 4 generates an axis comma over the entire field of the system that overlaps with the other aberrations of the system. Using conventional elements (lenses 2, 3) and offsets 4 thereof, two advantages are obtained: o the phase coding developed according to the proposed procedure to correct the aberrations of the optical system does not require complex manufacturing techniques since it is formed by elements with revolution symmetry (replaces the coding sheet of conventional procedures); or the optical system is not expensive because it is no longer necessary to introduce additional elements, as in other solutions previously used.

The most interesting property of the optical system of the invention is that of allowing to adjust the magnitude of the comma introduced into the system by varying the displacement or offset 4 between the lenses 2, 3. This allows adjusting

The amount of comma necessary to keep the MTF (Modulation Transfer Function) or Transfer Function of the Modulation stable, which allows measure the resolution of an optical system based on the ability to solve or differentiate lines at a specific spatial frequency.

To evaluate the ability of the comma to encode the image and keep the MTF stable against aberrations, the invention proposes to design, as a technological demonstrator, an afocal system that can be put before any well-corrected image-forming objective. The design thus consists of a concave flat lens 2 plus a convex flat lens 3, the radii 5 of both lenses 2, 3 being equal and with the outer surfaces 6 of both flat lenses. The phase sheet effect is achieved with an offset 4 of each lens 2, 3 with respect to the optical axis 1 in equal and opposite magnitudes vertically, according to a first embodiment of the invention (Figure 1).

According to a second embodiment of the invention, the phase sheet effect is achieved with a rotation-shaped offset 7 of the convex flat lens 3 with respect to the optical axis 1 according to an angle 8 with respect to the normal one of said optical axis 1 ( Figure 2).

The phase variation introduced by the optical system of the invention can be calculated based on the offset or displacement 4 of each element 2,3. Being "R" the radius of curvature of the lenses 2, 3, "d" the displacement or offset 4 of said lenses, considering that the light goes from left to right and considering (X, Y) the coordinate system originating in The optical axis 1, the thickness through the first element or lens 2 is:

x ² + (y -df_ ₊ (x ² + (y -d) ² ) ²

2R 8iT

And in the same way the thickness of the second element or lens 3 is:

x ² + (y + df (x ² + (y + dff 2R SR ³ The optical path difference is then, where n is the index of refraction of the material in which the lenses are manufactured: y

So that the phase change in the plane (X, Y), that is, in the pupil is:

The first term introduces a lateral displacement of the image that can be easily compensated, while the second is, specifically, the comma term. Thus, the amount of phase variation introduced depends on "d" and the maximum values of x and y, that is, the size of the pupil for fixed values of n and R.

The main advantages of the optical system of the present invention for image coding are: o the phase sheet does not require complex manufacturing techniques since it is formed by elements with revolution symmetry; or the optical system is not expensive, since it is not necessary to introduce additional elements; or the magnitude of comma introduced into the system is variable and adjustable according to the working conditions of the equipment; or a wider range is used in manufacturing tolerances, which lowers costs in production; or by remaining invariant to blurring, the optical system has a clear application in IR optics, since it would not be necessary to terrify the system.

In a real optical system, the two lenses 2, 3 are part of the components of the optical system to which a movement is added, such as those described in the simple optical described in the previous points. The choice of lenses 2, 3 will depend on each specific optical system and the conditions that are applied in the design process.

In the preferred embodiments of the invention described above, those modifications within the scope defined by the following claims can be introduced.

Claims

1. Optical system for the formation of the image of an object comprising an optical axis (1), an image-forming objective (2, 3) that reproduces the images of said object and a light source that directs light through the image-forming objective (2, 3) and towards the object to be observed characterized in that the image-forming objective (2, 3) comprises an offset (4) with respect to the optical axis (1) of the optical system such that said offset (4) generates a comma in axis on the field of the optical system that overlaps with the other aberrations of the aforementioned optical system, being able to adjust the magnitude of the comma introduced into the optical system leaving the runout (4) in the image-forming objective ( 2. 3).

2. Optical system for the formation of the image of an object according to claim 1 characterized in that the image-forming objective comprises two lenses (2, 3) with revolution symmetry with respect to the optical axis (1).

3. Optical system for the formation of the image of an object according to claim 2 characterized in that one of the lenses (2) is a concave flat lens with a radius (5) and a flat outer surface (6), being

The other lens (3) a flat convex lens with a radius (5) and a flat outer surface (6).

4. Optical system for the formation of the image of an object according to claim 3 characterized in that the radii (5) of the concave flat lens (2) and the convex flat lens (3) are equal.

5. Optical system for the formation of the image of an object according to any of the preceding claims characterized in that the offset (4) of the image-forming objective (2, 3) is achieved by moving in equal and opposite magnitudes vertically with respect to the axis optical (1) the components of the image forming lens (2, 3). Optical system for the formation of the image of an object according to any of claims 1 to 4, characterized in that the offset (4) of the image-forming objective (2, 3) is achieved by a rotation-shaped offset (7) with respect to the optical axis (1) of one of the components of the image forming objective (2, 3) according to an angle (8) with respect to the normal one of said optical axis (1).