FIELD OF THE INVENTION

The present invention relates to the field of imaging systems. More particularly, the present invention relates to an optical system for inspection of objects.

BACKGROUND

Human vision is a powerful tool for surrounding world exploration. It has, however, certain well-known limitations: limited spectral bandwidth, limited optical resolution, limited Field of View (FOV) and response time, insensitivity to light polarization state, etc.

To overcome these shortcomings, various sophisticated optical techniques and corresponding devices were developed over centuries. Nowadays, it is hard to imagine contemporary science and industry without microscopes, telescopes, photo cameras, Machine Vision systems and the like.

Inspection may be required in production line for various types of objects. Inspection may be required to identify defects of various sizes. For example, in automotive glass production line and Liquid Crystal Display production lines, inspection systems are used to locate defects.

To obtain the most comprehensive information for inspection purposes, several different optical techniques may be needed, such as Bright Field Illumination, Dark Field Illumination, Polarization Contrast and Luminescence. The selection of the most appropriate optical technique is generally based on the reflective and transmissive properties of the object.

Numerous optical systems for object observations and inspections, employing combined optical techniques, were developed in the past. US Patent 6,437,357 describes a glass inspection system including transmissive directional Bright Field Illumination and transmissive Dark Field illumination. US Patent Application 2008/316,476 describes a system for inspecting glass panes, using transmissive diffused Bright Field Illumination, Moire effect, reflective and transmissive Dark Field Illumination. Prior art optical systems combining optical techniques define for each optical technique a distinct optical channel. Each optical channel includes a light source and imaging components. The imaging components may include one or several of the following: objective lens, camera, data acquisition hardware, etc.

In an attempt to minimize optical systems costs, some manufacturers have proposed combined optical techniques, which share one or several of the optical components.

Many modern microscopes use the same light source, tube lens and ocular lens(s), and provide Bright Field Illumination, Dark Field Illumination, Phase Contrast imaging, Nomarsky Contrast imaging by changing objective lens and spatial illumination filtering for the various optical systems. In such optical systems, the optical techniques are used independently and if desired sequentially. To optically image an object, a viewer has to select one of the optical techniques and corresponding imaging components are then actuated either manually or automatically.

In many cases, however, a Region of Interest (ROI) of the object to be optically imaged is often small in comparison with the optical system Field Of View (FOV) and image background around the ROI is uniform for all optical techniques of interest.

There is therefore a need for an optical system for efficient optical imaging of defects in objects.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an optical system that overcomes or mitigates one or more disadvantages of known optical systems, or at least provides a useful alternative.

In accordance with a first aspect, the present invention relates to an optical structure for providing concurrent optical images of an object, where each one of the optical images corresponds to a different optical technique. The optical structure comprises a plurality of substantially superposed and spatially separated light reflecting components. Each one of the light reflecting components has at least one different optical characteristic and directs light reflected from the object for a particular one of the optical techniques. The plurality of light reflecting components concurrently directs light into a plane, whereby providing the concurrent optical images.

In accordance with another aspect, the present invention relates to an optical system for providing concurrent optical images of an object. The optical system comprises at least one light for lighting the object, and a plurality of substantially superposed and spatially separated light reflecting components. Each one of the light reflecting components has at least one different optical characteristic and directs light reflected from the object for one of a plurality of optical techniques. The plurality of light reflecting components concurrently directs light into a plane, thereby providing the concurrent optical images.

BRIEF DESCRIPTION OF DRAWINGS

These and other features of the present invention will become more apparent from the following description in which reference is made to the appended drawings wherein:

Figure 1 is a schematic view of an optical system using multiple optical techniques combined in one optical channel for visual observation and inspection.

Figure 2 is a schematic view of an optical system using multiple optical techniques combined in one optical channel for camera-based observation and inspection.

Figure 3 is a schematic view of an optical system using multiple optical techniques combined in one optical channel for observation and inspection with selective optical structures positioned inside an imaging assembly.

DETAILED DESCRIPTION OF THE INVENTION

Various industries rely on optical imaging for performing quality reviews of manufactured objects. Optical imaging offers good detection threshold, while being a non-destructive method of detection and identification of possible defects. Examples of such industries include glass industries, electronics industries, high precision products, etc. Nowadays, multiple optical imaging techniques exist for performing observation and inspection of objects. Examples of optical imaging techniques include Bright Field illumination, Dark Field illumination, interference techniques such as Moire effect, etc. Typically, to identify defects in objects, one or several optical techniques are used. The optical techniques are used independently, in sequence, or simultaneously. Each optical technique produces an object image, which is then treated and analyzed independently.

Although sufficient for detecting defects in some types of objects, the prior art optical techniques often lack precision and are considered somewhat slow. The present invention alleviates these problems by providing an optical structure and an optical system adapted to increase precision and speed of observation and inspection.

Figure 1 schematically illustrates an optical system 100 for observing and/or inspecting an object 101. The object 101 may be still or in movement, as for example in a production line. The object 101 may reflect light, absorb light, generate light, allow light to pass there through, or be composed of a combination thereof. One or a plurality of light sources 109 may be used to illuminate the object 101 , or a Region of Interest (ROI) thereof. The light sources 109 may consist of one of several light sources 109 of the same type, or of multiple different light sources such as for example, white light, colored light, Moire pattern, Bright Field transmissive light, Dark Field transmissive light, stroboscopic light, Polarization contrast, Luminescence or any other type of light. Those skilled in the art will recognize that various light sources 109 correspond to different optical techniques. The selection of light sources 109 is directly dependent on the type of object 101 being illuminated, and the type of defect in the object to be observed or identified. The light sources 109 are distributed in such a manner as to efficiently illuminate the object 101 , in areas of interest. Depending on the type of object being observed or inspected, the light source 109 could be optimized to provide different types of illuminations from specific positions, or could consist of directional light, diffused light or a combination thereof. In another embodiment of the present system, the light source 109 could consist of a light source adapted to move with respect to the object 101 so as to ensure optimal reflection of light for various aspects of the object 101.

Light reflected from the object 101 , shown as 102, reaches an optical structure composed of a plurality of optical components 103. The optical components 103 are substantially superposed and spatially separated from one another. The optical components 103 are mounted on a supporting device (not shown for clarity purposes), to ensure proper alignment of the optical components 103 with respect to the object 101 and an imaging assembly 104. The supporting device may fix the optical components 103 with respect to one another, or may alternatively include adjustment mechanisms to adjust the position of each optical component 103 independently. The optical components 103 may consist of dichroic mirrors, thin film plate polarizers, wire-grid polarizers, or any other type of optical structure having appropriate optical characteristics. The optical components 103 may further be deposited on one or both surfaces of optically transparent substrates. When the optical component 103 is deposited just on one substrate surface, it is recommended to deposit an antireflective coating on an opposite surface of the transparent substrate.

Each of the optical components 103 has at least one optical characteristic that is different from the other optical component. However, all optical components 103 share one optical characteristic: all reflect a portion of the reflected light

102 in accordance with their intrinsic optical properties (spectral band, polarization state, etc.) towards the imaging assembly 104, while allowing the rest of the reflected light 102 to pass there through. The optical components

103 are selected so that each optical component reflects light corresponding to one of the optical techniques used by the optical system 100 to the imaging assembly 104. The optical components 103 are separated in space with distances D ₁ (i=1 ,... ,N-1) and are positioned under angles 0,,/3, O=I ,...,N) with respect to the light reflected 102 from the object 101 , where N corresponds to a number of optical components 103. The distances D ₁ and angles α,,/3, are adjusted in the present embodiment of the system to create concurrent distinct object images on a plane of the imaging assembly 104, where each distinct object image corresponds to a particular optical technique. Desired object image separation for different optical techniques is thus achieved by adjusting the distances D ₁ and angles ct _j ,β _j of the optical components 103.

The optical structure may further comprise a reflective surface such as a regular metal or a dielectric mirror installed behind the plural optical components 103. The reflective surface reflects all remaining reflected light 102 towards the imaging assembly 104. However, the remaining reflected light 102 may not be desirable for all applications, and may adversely affect the object images.

In the present embodiment of the system, the imaging assembly then combines the multiple object images, so as to allow visual observation or inspection by an observer 105. The resulting image corresponds to N spatially shifted and overlapped object images of the object 101 , created by different optical techniques.

To separate light reflected 102 into multiple object images where each object image corresponds to one of the optical techniques, certain limitations on the light sources 109 and the optical components 103 must be considered. First, for each optical technique, the light source 109 spectral bandwidth and polarization must correspond to the optical characteristics of the corresponding optical component 103. When the selection of the light source 109 and corresponding optical component 103 are optimal, the corresponding optical component 103 completely reflects the portion of interest of reflected light 102 towards the imaging assembly 104, allowing the rest of the reflected light 102 to pass through freely.

Figure 2 schematically illustrates an optical system 200 for camera-based object 101 observations and inspection. In this particular embodiment, the optical system 200 further includes a camera 113. A photosensitive element 112 of camera 113 is positioned at a plane 111 of the camera where the object images are directed by the imaging assembly 104. The camera 113 transforms the object images of each optical technique into video signal. Area scan and line scan cameras may be used as camera 113. Relative motion of the object 101 and combined optical system 200 is required if a line scan camera 113 is used. The camera 113 is connected to a data acquisition / image processing system 114. The data acquisition / image processing system 114 acquires the object images and displays the latter if desired. The data acquisition/ image processing system 114 further processes the object images and calculates certain parameters and characteristics so as to determine the presence of a defect.

Figure 3 schematically illustrates an optical system 300 including an integrated optical structure and imaging assembly 115. This embodiment of the optical system is in some cases advantageous in terms of system cost and in some cases is the only technically possible embodiment. In the present embodiment, reflected light 102 passes initially through front optical elements 116 such as for example objective lens or field lens, and is then redirected by the plurality of optical components 103 towards terminal optical elements 117. The terminal optical elements 117 together with the front optical elements 116 and plurality of optical components 103 create a single optical channel for providing object images obtained by means of multiple optical techniques for an observer and a camera 113.

Those skilled in the art will recognize that the various embodiments of the present optical system have limited field of views. For observation and/or inspection requiring larger field of view, multiple optical systems with overlapped or stitched field of views could be used.

It will be apparent to those skilled in the art that various modifications and variations can be made to the combined optical system for observations and inspections of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.