Abstract:
The invention concerns a system for achieving an optical image of an object ( 136 ), and/or for achieving optical collimation ( 137 ) of light from a light source ( 130 ), comprising a concave mirror ( 131, 134 ), the surface normal of which forms an angle with the incident light in a beam. A negative lens ( 132, 133 ) that co-operates with the concave mirror is arranged in the incident and/or exit beam paths such that an image with eliminated or reduced imaging aberrations is achieved in a focal plane ( 135 ), and/or that the exit light from a light source ( 130 ) is collimated ( 137 ).

Description:
TECHNICAL AREA  
         [0001]    The present invention concerns a system including a concave mirror or equivalent means for achieving an optical image of an object, and/or for achieving optical collimation of light from a light source.  
         PRIOR ART  
         [0002]    Optical imaging is usually achieved by exploiting the diffraction, or refraction, of light in lenses, or by exploiting the reflection of light in mirrors.  
           [0003]    A lens provides an image that varies with the wavelength of the light, a phenomenon known as chromatic aberration. The reason for this is that the said refraction depends on the wavelength of the light, that is, an object is imaged in different wavelengths onto different focal planes. This is remedied by combining two or more lenses of different optical materials that have refractive properties such that the said object is imaged onto approximately the same focal plane at all wavelengths. This method of obtaining what are known as achromatic lens systems works well over a limited range of wavelengths, such as, for example, the visible region of wavelengths that stretches from a wavelength of 400 nm to a wavelength of 700 nm. Most lens systems, such as for example, the objective lens of a camera, are optimised for this range of wavelengths. It is desired in many applications to produce imaging systems that can image objects over a significantly greater range of wavelengths. The imaging lens systems in these applications become very complex. Furthermore, if it is desired to include what is known as the ultraviolet range of wavelengths for the imaging, the number of available optical materials is considerably reduced since many materials are not transparent to ultraviolet light. The achievement of a lens system for imaging without chromatic aberration in the wavelength region from 200 nm to 1,200 nm is extremely difficult, if not impossible. Images in the said range of wavelengths, however, have considerable significance for applications including, among others, optical spectroscopy.  
           [0004]    On the other hand, a concave mirror provides imaging without chromatic aberration since the reflection of light does not depend on wavelength. However, this image arises in the incident light beam path. This has been remedied by introducing the registration of the image in the said beam path or by obtaining the focal plane outside of the same by the use of mirrors placed into the beam path. Both methods mean that the central part of the beam is blocked. This is acceptable in many cases, such as, for example, in astronomical telescopes. In other cases, such as, for example, the objective lens of a camera, it leads to a complication that among other things limits the depth of focus of the objective.  
           [0005]    One method of alleviating the said disadvantages is to tilt the mirror towards the incident beam such such that the image in the focal plane arises outside of the said beam. A general name for this type of optics is “non-axial”, which means that the optical components that are part of the system do not have cylindrical symmetry with respect to a central optical axis. This introduces a significantly higher degree of difficulty in achieving an image with the aid of spherical surfaces without imaging aberrations. In the said case using a tilted concave mirror, this must have a non-spherical or spherical surface shape, which involves a considerable increase in cost since these surfaces are very difficult to produce.  
         SUMMARY OF THE INVENTION  
         [0006]    The said disadvantages namely, the difficulty of obtaining achromatic images over a wide range of wavelengths with lens systems, the difficulties of obtaining a complete beam with axial mirror systems, and, finally, the difficulties of reducing imaging aberrations with non-axial optics, are eliminated with the present invention, which is principally characterised in that at least one lens with preferably a low negative power is placed in the incident beam path and/or in the exit beam path. The lens is placed in the said beam path such that the normals to both surfaces of the said lens form an angle with a central ray in the said beam path, whereby the said lens and the said mirror co-operate not only in achieving an image of the said object in a focal plane outside of the said incident beam, but also in eliminating or reducing imaging aberrations in the said image. Alternatively, the said mirror and lens co-operate in order to collimate the exit light from a light source placed beside the said image.  
           [0007]    Other characteristics of the invention are made clear by the accompanying claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The invention will be described in more detail below with the aid of embodiments shown in the accompanying drawings, in which:  
         [0009]    [0009]FIGS. 1A and 1B show a known system for non-axial imaging of an object comprising a tilted concave mirror;  
         [0010]    [0010]FIGS. 2A, 2B aud  2 C show a system for imaging according to the invention comprising a tilted concave mirror and a negative lens;  
         [0011]    [0011]FIGS. 3A, 3B and  3 C show a system for imaging according to the invention comprising a tilted concave mirror and two negative lenses;  
         [0012]    [0012]FIG. 4 shows a system according to the invention for collimation and imaging. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0013]    [0013]FIG. 1A shows an uncorrected system for non-axial imaging of an object onto a focal plane by means of a spherical concave mirror  100 . In the example in the drawing, the concave mirror has a radius of curvature of 100 mm. This receives light in the form or an incident beam  104  from a distant point object, not shown in the drawing. The concave mirror is mounted non-axially, which means that the normal  101  to the mirror surface  107  at the point  102  that constitutes the contact point on the said surface  107  of the central ray  103  in the beam  104  forms an angle  97  with the said central ray  103 . The said angle  97  has been selected to be sufficiently large that the focal plane  105 , in which the rays  106  converge after reflection in the surface  107  to an approximate point, lies outside of the incident beam  104 . The angle of tilt  97  has a value of 13.0° in the example shown. A further focal plane  108  is also specified in FIG. 1A. The focal planes  105  and  108  limit that part of the beam  106  in which the rays are at their most dense.  
         [0014]    In FIG. 1B, 109 shows an enlarged view of that part of the beam path that is limited by the focal planes  108  and  105 . The drawing shows the beam path  106  between the said focal planes  105  and  108 . The circles  110  and  111  that are contained in the said planes each have, in the example shown, a diameter of 2 mm. The drawing makes it clear that no correct image of the point-shaped object is obtained, since the rays in the beam  106  do not at any point converge to anything that approaches a well-defined point. The contact points  98  of the said rays in the plane  105  have an extent that is close to vertical, while corresponding contact points  99  in the plane  108  have an extent that is close to horizontal. The cause of the shape of these distributions is a geometric imaging aberration known as astigmatism.  
         [0015]    Furthermore, the rays  106  are not symmetrically distributed, but demonstrate a centre of gravity that lies to one side. This is caused by another similar imaging aberration known as coma. Both astigmatism and coma arise when using rays that form an angle with the optical axis.  
         [0016]    Furthermore, a further such imaging aberration that broadens the said image is the aberration known as spherical aberration, that is, the central rays are brought to a focus that lies at another point than the focus to which the peripheral rays are brought.  
         [0017]    [0017]FIG. 2A shows an embodiment of the present invention with the concave mirror  100  shown in FIG. 1. Thus, the mirror  100  is found in FIG. 2A in the same non-axial arrangement as in FIG. 1A, which means that the normal  101  to the mirror surface  107  forms the same angle  97  with the central ray  103  in the incident beam  104  as the angle in FIG. 1. Furthermore, the exit beam  106  after reflection in the said surface is also found in FIG. 2A. A convex-concave lens  112  has been placed in the incident beam path  104 . The convex surface  113  of the lens faces the object. This means that its concave surface faces the mirror  100 . The said lens is oriented such that the normals  115  and  118  to the two surfaces  113  and  114 , respectively, that pass through the central points of the said lens surfaces, each forms an angle  117  with the central incident ray  103 .  
         [0018]    In the example shown in FIG. 2A, the two normals  115  and  118  to the lens coincide with one common normal. The use of generally available software for the optimisation of optical systems will allow the angle of tilt  117  of the lens and the radii of curvature of the lens surfaces  113  and  114  to be calculated. Optical aberrations of the image in the focal plane  116  can be minimised with the said software with respect to input free parameters, the values of which will thus be determined when the image has optimal quality, that is, when the contributions from all imaging aberrations are at a minimum. A plurality of wavelengths in the incident light in the beam  104  can be simultaneously included in the said optimisation. If in the example shown, quartz glass is chosen as material for the lens, and if the radii of curvature of the lens surfaces  113  and  114 , the angle of tilt  117  of the lens and the distance of the focal plane  116  from the point  102  on the surface of the mirror are allowed to constitute parameters in such an optimisation, carried out simultaneously for the wavelengths 200 nm, 250 nm, 300 nm, 500 nm and 1,200 nm for the light in the beam  104 , the following values are obtained: the radius of curvature for surface  113  98.7 mm, the radius of curvature for the surface  111  90.2 mm, the angle of tilt  117  45.6° and the distance between the focal plane  116  and the central point  102  52.6 mm.  
         [0019]    In FIG. 2B, 122 specifies an enlarged view of the focal plane  116 . A circle  119  has been drawn in this plane such that this circle has the same diameter, 2 mm, as the equivalent circles  110  and  111  in FIG. 1B. The points  121  in the centre of the surface constitute the meeting points of the rays in the beam  106 . The distribution of the said meeting points compared with the equivalent meeting points  99  and  98  in the focal planes  110  and  111  in FIG. 1B demonstrates the advantage that the present invention provides, namely a major reduction in the previously mentioned imaging aberrations.  
         [0020]    The image according to the present invention can be further improved by allowing the said lens  112  to be prismatic, that is, a lens in which the normals  115  and  118  to the two lens surfaces do not coincide; instead, the two normals not only form an angle with the cental ray  103  but also form an angle mutually with each other. In this way, this angle also can participate as a parameter in the above-mentioned optimisation.  
         [0021]    In FIG. 2C  123  denotes another enlarged view of the focal plane  116 . The circle  119  is again present in this view, as is the distribution of the meeting points  120  of the rays in the beam  106  on the said plane, following an optimisation with the prismatic angle of the lens as a further variable parameter. The improvement achieved according to the invention becomes clear when the distributions  120  and  121  are compared. However, in many practical applications, the image that a non-prismatic lens  112  according to the invention gives is fully sufficient.  
         [0022]    The influence of the tilted lens  112  according to the invention can be understood in that the said lens essentially eliminates the abovementioned imaging aberrations astigmatism and coma, caused by the tilted mirror  100 . The said lens contributes, furthermore, to a reduction of imaging aberrations, such as spherical aberration. If the focal length obtained during the optimisation is calculated, a numerical value of −2,370 mm is obtained at a wavelength of 500 nm. Thus, the strength of the lens is low, that is, only −0.42 dioptres, which can be compared to the strength of the mirror at 20 dioptres. Thus, the mirror is responsible for the dominating focussing, which consequently is the same for all wavelengths of the light, due to the fact it arises from reflection. The combination according to the invention with the lens  112  corrects with a low refractive index the imaging aberrations of the mirror whereby the effects that depend on wavelength that are caused by the lens are small, and thus do not significantly worsen the imaging.  
         [0023]    The image obtained according to the invention can be further improved by, according to the invention, introducing a further tilted correcting lens between the mirror  100  and the focal plane  116 . FIG. 3A shows this embodiment. The incident beam  104  with its central ray  103  can again be found in FIG. 3A, as can the lens  112  with its normals  115  and  118 , the concave mirror  100  with the reflecting surface  107 , and the convergent exit beam  106  following reflection. A lens  124  has been placed between the focal plane  116  and the mirror  100 , but outside of the beam  106 . This serves the purpose of further improving the quality of the image on the focal plane  116 . Both surfaces  125  and  126  of the lens have a cylindrical surface form in the example shown. The cylindrical surface  125 , which faces the mirror, is oriented such that the axis of the cylinder of the said surface lies in the plane that FIG. 3A defines. The other surface  126  of the lens  124 , on the other hand, is oriented such that the axis of the cylinder is perpendicular to the plane of FIG. 3A. Thus, the axes of the cylinders of the two lens surfaces  126  and  125  form a right angle to each other. In a manner that is well known to one skilled in the arts, the radii of curvature of the surfaces of the lens can be determined with the aid of the previously mentioned software for optimisation of optical systems. Parameters that can be included in this optimisation include not only the radii of curvature of the said surfaces  126  and  125 , but also the orientation of the lens  124  and its location in the beam  106 , the distance between the focal plane  106  and the mirror  100 , the radii of curvature of the surfaces  112  and  113  of the lens  112 , and, finally, the angle of tilt  117  of the said lens.  
         [0024]    In the same way as in FIGS. 2D and 2C,  127  in FIG. 3D specifies a separate view of the focal plane  116 . The circle  119  with a diameter of 2 mm is again found in FIG. 3B. The points  128  constitute again the meeting points for the rays in the beam  106  with the focal plane  116 . The distribution of the said meeting points shows, when compared with the distribution of meeting points  120  and  121  of equivalent rays shown in FIGS. 2B and 2C, the further improvement in the quality of the image that is obtained through the embodiment in FIG. 3A of the present invention.  
         [0025]    The distribution of the meeting points  128  is shown enlarged in a separate view  129  in FIG. 3C. This is composed of the meeting points for the previously mentioned five different wavelengths between 200 nm and 1,200 nm. The total extent of the meeting points is 0.05 mm and the radial values of what is known as the R.M.S. value of all rays is 0.02 mm. This illustrates the advantage obtained according to the invention, that is, that through the invention it is possible to achieve an image with reduced or eliminated imaging aberrations, operative over a very wide range of wavelengths, which in the example shown is from 200 nm in the UV region, through the visible region, and up to 1,200 nm in the near infra-red region, NIR.  
         [0026]    [0026]FIG. 4 shows a device according to the invention both for collimation and for imaging. A concave mirror  131  is again present in FIG. 4 similar to the concave mirror  100  in FIG. 2A. Analogously, the lens  132  is similar to the lens  112  in FIG. 2A. The lens  132  and the mirror  131  are arranged such that they achieve, according to the inventor, an image in the plane  130  of a putative distant light source. However, a small object, such as, for example, a light source has been placed in the said plane  130 . Thus it follows that the beam path through the device according to the invention consisting of the light source  130 , the mirror  131  and the lens  132 , is reversed. The incident beam  104  shown in FIG. 2A becomes in FIG. 4 an exit beam  137 . The exit beam  137  now becomes, according to the invention, unidirectional, or collimated, which means that all rays in the beam  137  are essentially mutually parallel. The collimated rays pass the lens  133 , the concave mirror  134  and are finally collected by the focal plane  135 , which three latter named objects together form a device according to the invention. The collimated rays are received by the lens  133  and the mirror  134  as if they arise in a distant point object, whereby the image of this object, according to the invention, in the focal plane  135  will be free of imaging aberrations. The light source  130  and the focal plane  135  have been deliberately positioned in FIG. 4 such that they are placed on opposite sides of the common collimated beam  137 . This way of applying the invention may prove to be the most advantageous.  
         [0027]    The light source  130  can comprise an optical fibre or a round, rectangular or slit-shaped opening.  
         [0028]    In FIG. 4, 136 generally denotes one or several devices through which the collimated light passes. Such a device according to the invention in FIG. 4 may be obtained through allowing  136  to denote what is known as a “cuvette”, of known execution, containing a fluid or gaseous specimen that transmits light, whereby the light after passing the focal plane  135  is led onwards to a known device for the measurement of transmission.  
         [0029]    Another device according to the invention in FIG. 4 is obtained through allowing  136  to denote one or several optical means of dispersing wavelengths, from the group of prisms or diffraction gratings. The object  130  then comprises an input opening, through which light from a light source, not shown in FIG. 4, passes. The device according to FIG. 4 then demonstrates an optical spectrometer, also known as “spectrograph”. A spectrum is obtained in the focal plane  135  that can be recorded using known methods.  
         [0030]    The invention is not limited to the embodiments of the same that have been described, but it can be varied in a manner that is obvious to one skilled in the arts within the scope of the subsequent claims.