WO2009130062A1

WO2009130062A1 - A method and a device for optical viewing of objects

Info

Publication number: WO2009130062A1
Application number: PCT/EP2009/050713
Authority: WO
Inventors: Claudio Sedazzari
Original assignee: Claudio Sedazzari
Priority date: 2008-04-24
Filing date: 2009-01-22
Publication date: 2009-10-29
Also published as: ITRE20080038A1

Abstract

A method and a device for the optical viewing of objects, the method comprising the stages of illuminating an object with ultraviolet radiation, and acquiring an image of the object thereby illuminated using a lens (3) comprising at least a forward optical group (30) and an aperture diaphragm (31) exhibiting a transparent window (35) located at a focal point of the forward optical group (30) defined for the ultraviolet radiation.

Description

A Method and a Device for Optical Viewing of Objects

Technical Field

The present invention is a method and a device for the optical viewing of objects.

In particular, the invention is a method and a device applicable in optical metrology, i.e. including all applications designed to obtain the dimensional measurements of an object, for example of a mechanical component, using an image of the object captured by a telecamera or any other optical or optoelectronic device.

Background Art

As is known, images are generated by the combined action of a source of radiation, generically understood to be a source of electromagnetic waves, and the reflection of the radiation by contextual objects.

In optical metrology and in the majority of industrial applications involving artificial viewing, images are acquired in the visible spectrum, such that the objects to be viewed are illuminated with visible light, the wavelength of which is normally taken to be approximately between 400 and 700 nm.

The view is captured with special optical lenses that convey the light towards the image plane of a camera or telecamera, thereby forming an image of the observed objects.

The image plane is normally a photosensitive film, or more recently an electronic sensor system (CCD, CMOS), capable of fixing the image for subsequent processing.

An optical lens unit generally comprises a combination of lenses forming one or more optical groups through which the light from the view passes, and an aperture diaphragm with a transparent window, generally called the diaphragm aperture, normally positioned on the optical axis such as to regulate the quantity of light striking the image plane.

The projection of the diaphragm aperture over the space of the object, or the image of the transparent window formed by the optical group interposed between the diaphragm and the object, is normally referred to as the inlet pupil of the optical system and determines the angle of acceptance of the light rays that enter the optical system.

An optical lens commonly used in optical metrology is the telecentric lens, exhibiting a diaphragm aperture located on the focal plane of a forward optical group positioned between the observed object and the aperture diaphragm. In this way, the diaphragm aperture permits the transit of only the light rays that reach the forward optical group substantially parallel to the axis of the forward optical group.

More specifically, if the diaphragm aperture were infinitely small, only rays of light perfectly parallel with the optical axis of the forward group would pass through. Since instead the aperture of the diaphragm always has a certain diameter, light rays slightly inclined relative to the optical axis of the forward optical group also pass through the telecentric lens, up to a maximum a inclination expressed in radians, which for small angles is defined by the equation: D a = ^■

2 - Fl

where F1 is the focal length of the forward optical group and D is the diameter of aperture of the diaphragm.

Consequently it is commonly stated that a telecentric lens only accepts ray cones exhibiting a barycenthc axis parallel to the optical axis of the forward group, when the ray cones exhibit a relatively small vertex angle, equal to twice the angle a.

As a result of this characteristic, while traditional optical lenses display the surrounding space within a certain viewing angle, causing an error of parallax such that distant objects seem smaller than nearby objects, telecentric lenses, which only accept ray cones with parallel barycenthc axes, view an object with a viewing angle close to zero.

Consequently, an image of an object photographed through a telecentric lens has a substantially constant size as the relative distance between the object and the lens varies, such that a telecentric lens provides a substantially constant enlargement regardless of the distance from the object.

The optical characteristics described above enable telecentric lenses to acquire images practically free of errors of perspective.

In other terms, they are capable of registering only the surfaces of an object perpendicular to the optical axis of the forward group, eliminating all surfaces parallel to the optical axis.

For example, observing a cylindrical tube oriented parallel to the optical axis of the forward group, a telecentric lens will acquire only the image of the transverse cross-section of the tube (seen as a ring), without any trace of the internal or external lateral surfaces.

Optical telecentric lenses are therefore ideal for optical metrology, both because they ensure accurate measurement even when the relative distance between the object and the lens is not known in advance or when the object must be measured at a plurality of points located at different distances from the lens, and also because the absence of errors of perspective makes telecentric lenses particularly suited to the observation and measurement of cylindrical holes and cavities in general.

The telecentric lenses currently available on the market also guarantee low geometric distortion of the image, such that the image is as faithful as possible to the observed object, or such that the image of the object is deformed as little as possible.

A sub-category of telecentric lenses successfully used in optical metrology are the so-called bi-telecentric lenses, also known as object and image-sided telecentric lenses, or bilateral telecentric lenses.

The aperture diaphragm of an optical bi-telecentric lens is located not only on the forward focal plane of the system, but also the rear focal plane, this being the focal plane of a rear optical group between the aperture diaphragm and the image plane.

In addition to the advantages of traditional telecentric lenses, bi-telecentric lenses have the characteristic of directing towards the image plane only light cones with barycentric axes parallel to the optical axis of the rear optical group, which generally coincides with the optical axis of the forward optical group.

Consequently there is a further reduction of the enlargement error caused by variation in the distance of the observed object, the depth of field of the lens is increased, and more uniform illumination of the image plane is achieved.

Nevertheless, the use of telecentric and bi-telecentric lenses sets an important limit to the maximum resolution of the acquirable images.

In particular, telecentric and bi-telecentric lenses, like the majority of photographic and industrial lenses, exhibit a resolution inferior to that of the electronic sensors of modern digital cameras, and consequently the electronic sensors cannot be exploited to maximum potential.

Otherwise stated, the intrinsic resolution of optical lenses is not capable of ensuring sufficient image contrast at the spatial frequency defined by the dimensions of the pixels of the electronic sensors, i.e. optical lenses used according to known methods are not capable of fully exploiting the potential of the new sensors due to their incapacity of resolving the pixels with the necessary contrast, and consequently they are unable to create images from which dimensional information can be extrapolated that is accurate to the extent that the pixel dimensions would theoretically permit.

For example, the contrast provided by an optical lens can be expressed with the contrast transfer function (CTF), which is obtained by photographing a pattern comprising a sequence of black and white lines of equal thickness, and measuring the intensity of illumination of each line on the image plane, this being the distribution of the radiation that the lines generate in the image of the pattern created on the image plane.

It derives from this function that the limit thickness, expressed in millimetres, at which the contrast reduces to a null value and thus the optical lens stops generating a distinguishable image of the pattern, can be expressed by the equation: t = -λ - N

2 where λ is the wavelength, expressed in millimetres, of the radiation conveyed by the optical lens, this being the radiation with which the pattern is illuminated, and N is the so called F-number, or the parameter that expresses the luminosity of the lens calculated from the equation: 1

N = '

2 - A

where A is the numeric aperture of the image-side optical system calculated as the sine of the semi-angle of aperture of the ray cones that reach the image plane.

In optical metrology, telecentric and bi-telecentric lenses operate in the visible light spectrum and with an F-number close to 8, this being with the diaphragm substantially "closed", in order to photograph very deep objects with adequate depth of field and focus.

Consequently, assuming N = 8 and λ = 587 nm, the limit thickness of resolvable lines with telecentric and bi-telecentric lenses is approximately 2.4 microns.

It follows that operating in these conditions according to currently known methods, it is impossible to resolve pixels of dimensions of 2 microns or less.

A considerable fraction of electronic sensors currently used exhibit pixels of active area less than 2 microns per side, and consequently they are never fully exploited to achieve the theoretical maximum possible level of detail.

An aim of the present invention is to provide a method and a device for the optical viewing of objects, applicable in optical metrology, and enabling the capture of more highly defined and detailed images of observed objects, obviating the problems of the above-described prior art.

A further aim of the invention is to achieve the above mentioned aim by way of a simple, rational and relatively inexpensive solution.

These aims are achieved by the characteristics of the invention described in the independent claims. The dependent claims describe preferred and/or particularly advantageous embodiments of the invention. Disclosure of Invention

In particular, the invention provides a method for the optical viewing of objects, comprising stages of illuminating the objects with ultraviolet radiation, preferably in the near ultraviolet range exhibiting wavelengths included approximately between 300 and 400 nm, and then acquiring an image of the objects thereby illuminated using a telecenthc or bi-telecentric optical lens, configured for the ultraviolet radiation.

In this way, assuming that the wavelength λ of the ultraviolet radiation is 375 nm and the optical lens again operates at N = 8, the minimum thickness of resolvable lines using the method of the invention is approximately 1.5 microns, permitting full exploitation of sensors exhibiting pixels of active area of 2 microns per side.

The method of the invention is particularly advantageous in optical metrology and in general in all artificial viewing applications, because it enables attainment of more highly defined images with telecentric or bi-telecentric lenses, making it possible to extract more accurate dimensional information regarding the observed objects.

It is important to note that a telecentric lens is considered to be configured for ultraviolet radiation if the forward optical group is suitable for transmitting radiation in the ultraviolet wavelength range, and if the diaphragm is on the focal plane, defined for the ultraviolet range, of the forward optical group. Similarly, a bi-telecentric lens is considered to be configured for ultraviolet radiation if the rear optical group is also suitable for transmitting radiation in the ultraviolet range, and if the diaphragm is on the focal plane, defined for the ultraviolet range, of the rear optical group. For example, a telecentric lens configured for visible light cannot be considered suitable for ultraviolet radiation since the diaphragm is not positioned at the focal plane, defined for ultraviolet radiation, of the forward optical group.

The invention also provides a device for the optical viewing of objects comprising an ultraviolet light source for illuminating objects preferably in the near ultraviolet range, and a telecentric or bi-telecentric optical lens configured for ultraviolet radiation, suitable for acquiring images of objects formed from the radiation emitted by the ultraviolet light source.

In a preferred embodiment of the device the ultraviolet light source is an LED source, and more preferably a high power LED source, capable, for example, of generating more than 100 optical mW for each emitter element.

In this way, the lenses of the telecentric lens can be made with common optical glass of crown type and/or flint type despite the low ultraviolet radiation transmission characteristics thereof, advantageously reducing the lens production costs and thus the cost of the overall device.

Alternatively the lenses could be made of special optical glass types containing, for example, calcium fluoride or melted quartz, these offering a spectral transmittance close to 100% in the ultraviolet range but being generally rather expensive and difficult to make.

In a second preferred embodiment of the device, the source emits collimated ultraviolet radiation, thereby generating a band of ultraviolet radiation with the rays all substantially parallel to one another.

The collimated ultraviolet radiation source can comprise, for example, an

LED ultraviolet emitter and an optical group with focal point located on the

LED, thereby aligning and rendering parallel the rays emitted by the LED.

This type of illumination with collimated radiation is the most advantageous for use with telecentric and bi-telecentric lenses.

By arranging the source such that the collimated rays strike the telecentric lens parallel to the optical axis of the forward group, the telecentric lens is capable of receiving almost all the radiation emitted, consequently the degree of illumination of the image plane located behind the lens (for example photographic film or the electronic sensor of a telecamera), is greater than with any other type of illumination.

This configuration therefore achieves a high level of energy efficiency, which further enables the use of lenses made in common optical glass types without renouncing excellent image quality.

The viewing device described above can be used in combination with an optical or optoelectronic device for the acquisition of images, preferably a high resolution telecamera, this being a telecamera exhibiting a sensor of small dimension pixels.

This solution permits acquisition of highly detailed images that can be subsequently processed, for example in order to conduct extremely accurate measurements of the dimensions of mechanical components like screws, springs, tubes, transmission shafts, valves, and electronic components including connectors, cables, printed circuits, and integrated circuits.

In this way the invention introduces the use of ultraviolet radiation into optical metrology, this being the dimensional measurement of objects without contact using telecentric or bi-telecenthc lenses, thereby achieving significant improvements in accuracy of measurement compared to traditional optical metrology.

The invention further provides an optical viewing device that provides optical viewing, without any computer processing, of an image depicting only the outlines of observed objects.

This viewing device can operate with radiation of the entire electromagnetic spectrum, including for example visible light. However, it is preferable that it operates with ultraviolet light, thereby increasing the definition of the acquirable images as described herein above.

The viewing device comprises a collimated radiation source for illuminating objects in a given context and a telecentric or bi-telecentric optical lens configured for the ultraviolet radiation, the radiation source being configured such as to emit collimated radiation in a direction substantially parallel to the optical axis of the telecentric or bi-telecentric lens, the lens comprising an aperture diaphragm exhibiting a transparent annular-shaped window

(diaphragm aperture), in particular a circular transparent ring and a central zone which is opaque to incoming radiation, being centred on the optical axis of the lens.

In this way all the light rays emitted by the radiant source striking the forward optical group of the telecentric or bi-telecentric lens are normally focused on the opaque zone in the centre of the diaphragm aperture, preventing them from reaching the image plane where no image is formed.

When an object is interposed between the radiant source and the lens, the collimated light rays from the source that reach the object from behind are reflected back or absorbed by the opaque central portion of the diaphragm, while those that interact with the edges of the object are partly scattered in all directions in the surrounding space, and consequently the telecentric lens receives light rays from various directions.

The aperture of the annular diaphragm permits the passage only of those of the incoming rays that strike the forward optical group with an inclination relative to the optical axis which is sufficiently small to pass through the maximum diameter of the aperture, and at the same time which is sufficiently large not to be focused on the opaque central zone.

The rays that can pass through reach the image plane, forming an image of only the outline of the observed object.

Brief description of the Drawings

Further characteristics and advantages of the invention will better emerge from the detailed description made herein, provided by way of non-limiting example with reference to the accompanying figures of the drawings.

Figure 1 illustrates the layout of a viewing device of the present invention.

Figure 2 is the layout of figure 1 showing the optical components of the device in greater detail.

Figure 3 illustrates the layout of a viewing device in a preferred embodiment of the invention.

Figure 4 illustrates the layout of a further viewing device.

Figure 4A is an enlarged detail of figure 4.

Figure 5 illustrates the viewing device of figure 4 in use.

Figure 5A is an enlarged detail of figure 5.

Best Mode for Carrying Out the Invention

The viewing device 1 of figure 1 schematically comprises a radiant source 2 for illuminating an object 100 to be imaged, and an optical lens 3 for receiving the radiation from the scene and guiding it towards an image plane 40 of an acquisition device 4.

The acquisition device 4 can be any optical or optoelectronic device for the acquisition of images, including, for example, a camera or telecamera, exhibiting an image plane 40 which can be for example a photographic film or an electronic sensor(CCD, CMOS).

Preferably, the acquisition device 4 is a high resolution digital telecamera, exhibiting an electronic sensor 40 with individual pixels of lateral dimensions equal to or less than 2 microns.

The radiant source 2 comprises a LED emitter serving to emit radiation in the ultraviolet range, preferably in the near ultraviolet range, for example of wavelengths approximately from 300 to 400 nm.

The LED emitter is preferably high power, capable of generating more than 100 optical mW for each emitting element.

The lens 3 schematically comprises a forward optical group 30 proximal to the object 100, a rear optical group 32 proximal to the acquisition device 4, and an aperture diaphragm 31 interposed between the forward optical group 30 and rear optical group 32.

As illustrated in figure 2, the forward optical group 30 comprises only two positive lenses, respectively 33 and 34, which can exhibit a flat-convex, biconvex, or meniscus shape. The positive lenses 33 and 34 can both be made in common optical glass, for example they can both be made in low chromatic dispersion crown glass, including for example Schott glass varieties classified with codes N-SK16, N-BK7, or B270. The aperture diaphragm 31 comprises a circular window 35, transparent to ultraviolet radiation, which herein below will be referred to as the diaphragm aperture.

For example, the aperture diaphragm 31 can comprise an opaque plate preferably of thickness of a few tenths of a millimetre, and the diaphragm aperture can be defined a simple hole in the plate.

The diaphragm aperture 35 is coaxial to the optical axis A of the forward optical group 30, and positioned on the focal plane of the forward optical group 30, defined for the wavelength range of ultraviolet radiation emitted by the radiant source 2.

As is known, the position of the focal plane of a lens group depends on the refraction index of the material from which the lenses are made, which in turn depends on the wavelength of the electromagnetic radiation passing through the lenses.

The lens 3 only accepts ultraviolet ray cones B exhibiting a main(barycenthc) axis that is parallel to the optical axis A of the forward optical group 30, thereby being a telecenthc lens configured for ultraviolet radiation.

The rear optical group 32 serves to compensate and correct the residual chromatic dispersion generated by the forward optical group 30 in the ultraviolet wavelength in question.

As illustrated in figure 2, the rear optical group 32 comprises four lenses respectively numbered from 36 to 39.

Th elens 36 which is proximal to the diaphragm 31 can be a negative lens serving to partially or completely correct the chromatic aberrations generated by the forward optical group 30.

The negative lens 36 can be bi-concave, flat-concave, or meniscus shaped, and can be made of common optical glass, for example it can be made of high chromatic dispersion flint glass, for example Schott optical glass types classified with codes N-F2, LLF1 , or N-SF1.

The rear lenses 37, 38, and 39 are instead positive lenses that can all be made in common optical glass, for example in low chromatic dispersion crown glass, including the herein above cited Schott optical glass types classified with codes N-SK16, N-BK7, or B270.

The optical axis of the rear optical group 32 coincides with the optical axis A of the forward optical group 30, and the focal plane of the rear optical group

32, defined for the ultraviolet wavelength cited above, coincides with the plane on which the aperture diaphragm 31 is located.

In this way, the rays of ultraviolet radiation C conveyed by the rear optical group 32 towards the image plane 40, form light cones the main (barycentric) axis of which is parallel to the optical axis A of the lens 3.

The lens 3 is therefore both telecentric on the object side and telecentric on the image side, and overall the lens 3 is a bi-telecentric lens configured for ultraviolet light.

In particular, it is preferable that the lens 3 is optimized for operation with radiation in the ultraviolet range, such that the choice of materials from which the lenses are composed, and the characteristics of the lenses, including for example the curvature radius, thickness and spatial position, permit the lens 3 to operate in the above indicated wavelength range exhibiting very high contrast and with performance close to the diffraction limit. Purely by way of example, excellent results have been achieved using the following configuration.

Forward optical group 30 as follows: the lens 33 is a flat-convex positive lens of N-SkI 6, diameter 54 mm, thickness 5 mm, and curvature radius 124 mm; and the lens 34 is a meniscus shaped positive lens of N-SkI 6, diameter 54 mm, thickness 7.5 mm, and curvature radius 71 mm and -147 mm, positioned at 0.4 mm from lens 33.

Rear optical group 32 as follows: thee lens 36 is a bi-concave negative lens in N-F2, diameter 5 mm, thickness 1 mm, and curvature radius -9.5 mm and -24.5 mm; the lens 37 is a flat-convex positive lens in N-SkI 6, diameter 6 mm, thickness 7.5 mm, and curvature radius 11 mm, located at 0.3 mm from lens 36; the lens 38 is a flat-convex negative lens in N-SkI 6, diameter 7 mm, thickness 4 mm, and curvature radius 62.3 mm, located at 0.2 mm from lens 37; and lens 39 is a flat-convex positive lens in N-SkI 6, diameter 10 mm, thickness 11 mm, and curvature radius 26.2 mm, located at 13.5 mm from the lens 38.

The aperture diaphragm 31 of aperture 35 diameter is approximately 3 mm. In use, the object 100 is positioned in front of the bi-telecentric lens 3, where it is illuminated with ultraviolet radiation emitted by the radiant source 2. The ultraviolet radiation reflected by the object 100 passes through the bi- telecentric lens 3 and an image is formed on the sensor 40 of the telecamera 4.

The image obtained with the bi-telecentric lens 3 is an image substantially without errors of perspective and wherein the image size of the observed object is practically independent of the distance from the object. The use of the bi-telecentric lens 3 with radiation in the ultraviolet range also provides a high resolution image, exhibiting a level of detail of less than 2 microns, compatible with the maximum resolution of the electronic sensor 40 of the telecamera 4.

The lens 3 used in the ultraviolet range is therefore particularly suited for use with devices for high resolution acquisition 4, wherein the individual image point (pixel) is very small, and wherein the density of these pixels is very high, thereby enabling acquisition of highly detailed images.

An image acquired in this way will comprise a high number of pixels, each of which contains a significant geometric datum thanks to the high performance of the lens 3 operating in the ultraviolet range, thereby being particularly useful for assessing the dimensions of the object viewed by the lens.

The high level of detail provided by the individual pixels of the acquisition device 4 enables, after suitable processing of the image, an accurate determination of the outline of the object to be made, improving the efficiency of "edge detection" algorithms, these being algorithms of calculation normally used in the artificial viewing sector in order to select, from a set of pixels making up an image, those pixels that define the border of the objects depicted, and thereby to establish the spatial positioning and size of the objects.

Consequently, the viewing device 1 of the present invention offers a significant improvement in the accuracy of images in any type of application based on artificial viewing, in particular in the field of optical metrology, this being dimensional measuring, without contact, of objects, for example mechanical components including screws, springs, tubes, transmission shafts, valves and electronic components including connectors, wires, printed circuits, and integrated circuits.

Figure 3 illustrates a preferred embodiment of the viewing device 1 , which differs from the previously described device only as regards the radiant source.

In this case, the viewing device comprises a radiant source 2A for radiating collimated rays of ultraviolet radiation.

In practice, the radiant source 2A comprises an LED emitter 20, of the type described above, and an optical group 21 that collimates the radiation emitted by the LED emitter 20, serving to deviate the incident rays, such that the transmitted rays are almost completely parallel.

The radiant source 2A is located facing the bi-telecenthc lens 3, oriented such that the collimated rays leaving the optical group 21 are parallel to the optical axis A of the lens 3.

In this way, all the rays emitted by the LED emitter 20 and collimated by the optical group 21 are collected by the bi-telecentric optical lens 3, and projected onto the image plane 40 of the acquisition device 4, thereby supplying the acquisition device 4 high levels of radiation per surface unit. This characteristic makes the viewing device 1 extremely efficient from the point of view of consumption of energetic, as it provides an image with a very high signal/noise ratio, notwithstanding the low spectral transmittance of ultraviolet light provided by common optical glass, from which the lenses of the forward optical group 30 and rear optical group 32 of the bi-telecentric lens 3 are made.

Figure 4 illustrates, purely by way of non-limiting example, a viewing device 10 that enables acquisition by optical means of an image only of the outline of the object 100 viewed.

The viewing device 10 comprises a radiant source 2A of collimated radiation, and thus entirely similar to the device illustrated in figure 3 and described above, differing only as regards the aperture diaphragm 31 of the bi- telecentric lens 3.

In this case, the, aperture diaphragm 31 comprises a circular window 31 ' transparent to the emitted radiation, the window 31 ' being coaxial with the optical axis A of the forward optical group 30 and rear optical group 32, and being located on the focal plane of both.

In the centre of the circular window 31 ', the diaphragm 31 comprises a disk- shaped opaque zone 31 " exhibiting a diameter inferior to the diameter of the circular window 31 ', thereby leaving an annular transparent opening centred on the optical axis A of the forward optical group 30 and rear optical group 32, and defining the diaphragm aperture. For example, the diaphragm 31 can comprise a thin plate of transparent glass, which can be covered, using screen printing techniques, with a layer of chrome or other material opaque to radiation, thereby leaving only an annular portion of the glass plate exposed and thus forming the diaphragm aperture.

In this way, since the annular aperture of the diaphragm 31 lies on the focal plane of the forward optical group 30, with the opaque zone 31 " intercepting the optical axis A, it follows that the collimated rays from the radiant source

2A are focused by the forward optical group 30 at a point in proximity to the centre of the opaque zone 31 ".

As illustrated in the detail of figure 4A, the opaque zone 31 " prevents the passage of radiation, and consequently none of the collimated rays generated by the radiant source 2, can reach the image plane 40 which remains completely masked.

It should be noted that if the rays emitted by the radiant source 2 were perfectly collimated and thus completely parallel to each other, the opaque central zone 31 " could theoretically be punctiform. In reality the rays emitted by the radiant source 2 are never perfectly parallel to each other.

Consequently, the minimum diameter of the opaque zone 31 " is expressed by the equation:

d = 2 - Fl - α where σ is the maximum angle of divergence of the luminous rays emitted by the radiant source 2, expressed in radians, and F1 is the focal length of the forward optical group 30 of the bi-telecentric lens 3.

In use, it happens that when there are no objects interposed between the radiant source 2 and the bi-telecentric lens 3, no luminous rays B reach the image plane 40 of the acquisition device 4, because they are intercepted by the opaque central part of the 31 " of the diaphragm 31. When an object 100 is interposed between the radiant source 2 and the bi- telecentric lens 3 (see figure 5), the collimated rays B from the radiant source 2 that reach the object 100 from behind are reflected back or absorbed, while those that interact with the edges of the object 100 are partly scattered in all directions in the surrounding space, resulting in the forward group 30 of the bi-telecentric lens 3 receiving light rays from various directions.

As explained in the introduction, a telecentric lens 3 in general only accepts light rays that are substantially parallel to the optical axis A, when the inclination σof the rays relative to the optical axis A does not exceed a limit value defined as a function of the diameter D of the transparent window 31 ' of the diaphragm 31.

Of these almost-parallel rays, those that are perfectly parallel with the optical axis A or that have an inclination very close to parallel to the optical axis A, are blocked by the opaque zone 31 " of the diaphragm 31.

Consequently the annular transparent zone of the aperture diaphragm 31 permits passage only of the rays that reach the forward group 30 with an inclination relative to the optical axis A, sufficiently small to pass through the transparent window 31 ', and simultaneously sufficiently large not to be focused on the opaque central zone 31 ".

In practice, the annular transparent zone of the aperture diaphragm 31 permits the passage only of light rays for which the following relations are simultaneously valid: a < ^D and α > d

2 ^■ Fl 2 ^■ Fl

where a is the inclination of the luminous ray relative to the optical axis A of the lens 3, F1 is the focal length of the forward optical group 30, D is the diameter of the circular window 31 ' and d is the diameter of the opaque central zone 31 ".

In these conditions the only light rays capable of reaching the image plane 40 and consequently forming an image are those originating by scattering from the edge of the object 100 illuminated by the radiant source 2 of collimated radiation.

Therefore, the viewing device 10 makes it possible to optically obtain an image of only the outline of an object without any computer processing.

From the above explanation it is obvious that the same result could be obtained even if the viewing device 10 comprised a simple object-side telecentric lens, on the condition that the diaphragm 31 exhibits a diaphragm aperture in the form of an annular-shaped transparent window as described above.

The viewing device 10 described here is configured to operate with ultraviolet radiation, this being a source 2A of collimated ultraviolet radiation, in order to improve the obtainable resolution of the images.

However, the viewing device 10 could be configured to operate effectively with radiation of any wavelengths of the electromagnetic spectrum.

For example, an image of only the edge of the object 100 could also be obtained if the radiant source 2A were designed to emit collimated light in the visible spectrum, on the condition that the diaphragm 31 of the lens 3 is located on the focal plane of the forward optical group 30, defined for visible light.

Obviously, a technical expert in the sector might introduce numerous modifications of a practical-technical nature to the viewing devices described above, without going outside of the range of the invention as claimed below.

Claims

1 ). A method for the optical viewing of objects, characterized in that it comprises stages of illuminating an object with ultraviolet radiation, and of acquiring an image of the object thus illuminated by means of a lens (3) comprising at least a forward optical group (30) and an aperture diaphragm (31 ), the diaphragm (31 ) being provided with a transparent window (35) located at a focal point of the forward optical group (30) defined for the ultraviolet radiation.

2). The method of claim 1 , characterized in that the ultraviolet radiation is in the near ultraviolet range. 3). The method of claim 1 , characterized in that the object is illuminated with collimated radiation.

4). The method of claim 3, characterized in that the collimated radiation is directed substantially parallel to an optical axis (A) of the forward optical group (30). 5). The method of claim 1 , characterized in that the lens (3) also comprises a rear optical group (32), the aperture diaphragm (31 ) being interposed between the forward optical group (30) and the rear optical group (32), with the transparent window (35) being located at a focal point of both. 6). The method of claim 1 , characterized in that the acquisition phase comprises the lens (3) projecting an image of the object onto an image plane (40) of an optical or optoelectronic device (4) for the acquisition of images. 7). A method for dimensional measurement of objects, comprising stages of obtaining at least an image of an object to be measured, and processing the image such as to establish a measurement of the depicted object, characterized in that the stage of obtaining the image comprises an optical viewing method of any of claims from 1 to 6. 8). A device for optical viewing of objects characterized in that it comprises an ultraviolet radiation source (2, 2A) for illuminating of an object, and a lens (3) serving to view the object and comprising at least a forward optical group (30) and an aperture diaphragm (31 ) exhibiting a transparent window (35) located at a focal point of the forward optical group (30) defined for the ultraviolet radiation.

9). The device of claim 8, characterized in that the lens (3) further comprises a rear optical group (32), the aperture diaphragm (31 ) being interposed between the forward optical group (30) and the rear optical group (32), with the transparent window (35) being located at the focal point of both. 10). The device of claim 8, characterized in that the source of ultraviolet radiation (2, 2A) comprises an LED emitter (20).

11 ). The device of claim 10, characterized in that the LED emitter (20) generates more than 100 optical mW for each emitter element. 12). The device of claim 8, characterized in that the source of ultraviolet radiation (2A) is a source of collimated radiation.

13). The device of claim 12, characterized in that the source of collimated radiation (2A) comprises an LED emitter (20) and an optical group (21 ) serving to collimate the rays of radiation emitted by the LED emitter (20). 14). The device of claim 12, characterized in that the source of collimated radiation (2A) and the lens (3) are configured such that the forward optical group (30) of the lens (3) receives collimated rays running parallel to the optical axis (A) of the forward optical group (30).

15). The device of claim 8 or 9, characterized in that the forward optical group (30) and/or the rear optical group (32) comprise lenses made of common optical glass.

16). The device of claim 8 or 9, characterized in that the forward optical group (30) comprises lenses of crown glass.

17). The device of claim 9, characterized in that the rear optical group (32) comprises lenses of flint glass and/or lenses of crown glass. 18). A device for optical viewing of objects, comprising a source of collimated radiation (2A) serving to illuminate an object, and a lens (3) serving to view the object, the lens (3) comprising at least a forward optical group (30) and an aperture diaphragm (31 ) provided with a transparent window (31 ') located at a focal point, defined for the radiation emitted by the source (2A), of the forward optical group (30), characterized in that the aperture diaphragm (31 ) comprises an opaque zone (31 ") located at a centre of the transparent window (31 '), such as to leave a transparent window of annular shape substantially centred on an optical axis (A) of the forward group (30). 19). The device of claim 18, characterized in that the source of collimated radiation (2A) and the lens (3) are configured such that the forward optical group (30) of the lens (3) receives collimated rays directed parallel to the optical axis (A) of the forward optical group (30).

20). The device of claim 18, characterized in that the lens (3) also comprises a rear optical group (32), the aperture diaphragm (31 ) being interposed between the forward optical group (30) and the rear optical group (32), with the annular-shaped transparent window positioned at the focal point of both. 21 ). The device of claim 18, characterized in that the source of collimated radiation (2A) emits ultraviolet radiation.

22). The device of claim 18, characterized in that the source of collimated radiation (2A) comprises an LED emitter (20) and an optical group (21 ) serving to collimate the rays of radiation emitted by the LED emitter (20). 23). The use of a telecentric or bi-telecentric lens (3) for acquisition of images in ultraviolet radiation.