REFLECTIVE OBJECTIVE APPARATUS

Abstract: A reflecting beam splitting objective employing only three mirrors obtains a high reflective efficiency. In one embodiment, the objective contains a small intercepting mirror (10) between primary (22) and secondary (12) mirrors of a Cassegrain mirror arrangement. The position of the intercepting mirror is determined so that the mirror receives an input beam and directs that beam onto a first portion of the secondary mirror without vignetting the beam when it is reflected from the secondary mirror to a first portion of the primary mirror or by cutting off part of the image transmitted from the second portion of the secondary mirror to a focus (34) behind the primary mirror. The objective may use all energy that is input into the objective to image a sample. The objective has particular utility in the field of infrared spec­ troscopy of small samples.

Description: This invention relates to a reflective beam splitting objective optical system for redirecting an energy beam that approaches the system at an angle to its optical axis or along the optical axis from either direction. It is often desirable or necessary to input an energy beam at an angle to an optical axis of a focusing objective system. The reflected beam from the sample often needs to be collected. Since reflection occurs according to Snell's Law, the reflected energy must be collected at the same angle to the sample as the angle of incidence. In visual imaging systems, to accurately view detail of a sample plane without distortion, the viewing system must be situated near normal to the sample. These conditions dictate that the incident and the reflected beams co-exist in space such that they are co-axial or near co-axial. In order to separate these beams, a beam splitter is used. The most simple form of a beam splitter is a refractive beam splitter employing a partially reflective mirror that redirects the beam at substantially a right angle into a focusing system and then to the sample which then transmits part of a reflected beam back through the focusing system, beam splitter and onto a detector. A beam splitter of this type is theoretically limited to an efficiency of 25% because half of the input beam is lost on the initial reflection and half of the remaining beam is lost when the beam is transmitted through the beam splitter. This type of beam splitter also may introduce chromatic aberrations into the reflected beam. The refractive beam splitter, however, has the advantage of having a very simple construction. Another means for splitting an input energy beam involves off-axis paraboloids, ellipsoids or spherical mirrors. These off-axis systems have means for directing the beam to a focusing mirror which focuses the beam into a sample. Energy reflected from the sample is focused by a second mirror and directed to a detector through whatever optical arrangement is chosen. These systems have the disadvantage of introducing substantial distortions of the visual image, a phenomenon that is particularly troublesome at high magnifications. Off-axis systems are also not suitable for applications where it is desirable to mask part of the image being sent to the detector. Finally, complex mirror arrangements may be created that illuminate the sample and recreate an undistorted image of the sample that may be used by a detector. These complex mirror arrangements, while producing an adequate optical path, involve considerable mechanical complexity and expense in manufacture. Moreover, complex systems involved complex problems of alignment of the optical components. Finally, these systems are generally not compact, making for a system that is cumbersome, complex and difficult to maintain in optical alignment. Present beam splitters generally do not provide an economical and accurate method of redirecting an image beam at an angle to the optical axis of an infrared microscopic imaging system. This problem is particularly acute in connection with obtaining infrared spectra of extremely small size samples where it is necessary to mask the image received by the detector. EP-A-116321 discloses an optical system comprising a primary mirror intersecting an optical axis, and having a convex reflective surface positioned so as both to reflect an energy beam to the reflective surface of said primary mirror from where the beam is reflected to a focus behind the secondary mirror, and to reflect an energy beam received from said primary mirror. It also disclose a beam splitter which is arranged behind the primary mirror. Viewed from one aspect there is herein disclosed a reflective objective apparatus, for forming a real image of an object, including: a primary mirror of annular shape centered on the optical axis of the reflective objective apparatus, and having a generally concave reflective surface; a secondary mirror of smaller extension than that of the primary mirror, also centered on said optical axis and having a convex reflective surface positioned so as both to be able to reflect with a first portion thereof, an energy beam to a first portion of the reflective surface of said primary mirror from where the beam is reflected to an object position located behind the secondary mirror, and to be able to reflect with a second portion thereof a energy beam from the object reflected from a second portion of said primary mirror to an image plane located behind the primary mirror, characterised in that said apparatus further comprises an intercepting mirror having a planar reflecting surface and being positioned off the optical axis between the primary and secondary mirrors, so as to face said first portion of said secondary mirror, to redirect energy to or from said first portion of said secondary mirror, said intercepting mirror being positioned sufficiently far from the secondary mirror such that a ray between the centre of the secondary mirror reflective surface to the innermost point of the used first portion of the reflective surface of the primary mirror is not obstructed by said intercepting mirror, and sufficiently close to the secondary mirror such that the image formed by energy reflected off the second portion of the secondary mirror at said image plane located behind the primary mirror is not unevenly attenuated. Viewed from another aspect there is herein disclosed a method of operating an apparatus in accordance with the invention including directing an energy beam from the side to the intercepting mirror; from where the beam is reflected to said first portion of the secondary mirror; from where the beam is reflected to said first portion of the primary mirror; from where the beam is reflected to the object location intersecting the optical axis and located behind the secondary mirror; and collecting a portion of the beam reflected from the sample on said second portion of the primary mirror; from where the beam is reflected to said second portion of the secondary mirror, from where the beam is reflected and focused at the image plane located behind the primary mirror. Viewed from a further aspect there is herein disclosed a further method for operating apparatus in accordance with the invention including directing an input energy beam through a sample located at said object location towards said concave primary mirror; from where the energy is reflected onto said convex secondary mirror whereafter energy from said first portion of said secondary mirror is reflected onto said intercepting mirror from where the energy is brought to a first focus; and where the energy from said second portion of the secondary mirror is focused at said image plane located behind the primary mirror. Some embodiments of the present invention will now be described by way of example and with reference to the accompanying drawings in which:- Fig. 1 shows an embodiment of an objective optical system. Fig. 2 shows part of the mirror arrangement of the system of Fig. 1.Fig. 3 shows a known Cassegrain objective system and Fig. 4 shows a conventional beam splitter. Referring to Figure 1, the optical arrangement has an intercepting mirror 10 with a reflective surface 10a positioned above a secondary mirror 12 having a reflective surface 13. Positioned over a first portion 12a of secondary mirror 12 is an intercepting mirror 10 having a first edge 14 extending beyond an edge of secondary mirror 12 for example by a distance indicated by 15 in Figure 2. A second side edge 16 is of intercepting mirror 10 is aligned over secondary mirror 12 so as to form a great diameter on mirror 12 as shown in figure 2. The intercepting mirror 10 is positioned in between a primary mirror 22 having a reflective surface 22a and secondary mirror 12. A lower edge 18 of mirror 10 is separated from the uppermost surface of secondary mirror 12 by a distance 20. Mirror surfaces 13 and 22a are substantially concentric except for minor adjustments for aberrations. The position of the intercepting mirror is determined by the following considerations. If the intercepting mirror is close to focus plane 36, the mirror attenuates part of the image; therefore, to maintain an evenly illuminated image on plane 36, intercepting mirror 10 should be as far from the image plane as possible. In the optical arrangement shown in Figure 1, this design consideration, by itself, would ideally place the intercepting mirror as close as physically possible to secondary mirror 12. However, placing the intercepting mirror too close to the secondary mirror places first edge 14 of mirror 10 in the path of a portion of the energy reflected between first portion 12a of mirror 12 and the primary mirror 22 causing edge 14 to vignette the beam. Therefore, the distance 20 separating intercepting mirror 10 from secondary mirror 12 must permit a ray 105 to pass from the uppermost point on mirror 12 to the bore hole edge 24 of mirror 22 without being blocked by first edge 14 of mirror 10. Thus, mirror 10 should be positioned to neither unevenly attentuate the image on image plane 36 or vignette ray 105. The most preferred position for mirror 10 has the lower edge 18 of mirror 10 separated by a distance 20 that is the smallest distance necessary to prevent edge 14 from vignetting ray 105. As shown in Figure 3, in an optical arrangement employing a Cassegrain arrangement of a primary and secondary mirror without the intercepting mirror, one-half of a symmetric input beam 200 would be directed along the optical axis 250 of the system to secondary mirror 12. The input beam would be focused by the Cassegrain optics onto sample 30. the mirrors would image the sample back to the source at focus plane 36. However, one-half of the image must fall on the emitter 201 of beam 200. Therefore, employing a Cassegrain mirror arrangement without the intercepting mirror requires losing one-half of the image field. In contrast, by employing intercepting mirror 10 to rotate the source of input energy so that the input beam is roughly perpendicular to the optical axis 21, the intercepting mirror directs the input beam to the secondary mirror from where the beam is reflected to the primary mirror which focuses the light on the sample which is in turn focused to an image behind focus 34 at plane 36 as shown in Figure 1. This optical arrangement does not require source 201 to block out half of the image of the sample at focus plane 36 because source 201 is no longer positioned on the sample plane. Therefore, all points of the sample are imaged on plane 36. It is possible to obtain a 100% increase in efficiency over a conventional refractive beam splitter of the general type shown in Figure 4. In Figure 4, a symmetric input beam 120 falls on the refractive beam splitter 122. A part of the beam indicated by 121 passes through the refractive beam splitter and is lost from the system. Another part is reflected down to sample plane 130 through a reflecting objective 131. The energy reflected back from the sample through the reflecting objective encounters beam splitter 122 which reflects part of that energy from the sample plane to the input source. At most only one quarter of the input energy may reach focus 34 if the refractive beam splitter 122 transmits and reflects exactly one-half of the incident energy. The efficiency of the system decreases if refractive beam splitter 122 reflects or transmits more than one-half of the incident energy. In practice, it has been found that a conventional refractive beam splitter may obtain efficiencies of only approximately 20% or less. This is especially true for beam splitters used with infrared energy beams. This level of efficiency is unacceptable for many applications. With the optical arrangement disclosed herein only one-half of the energy contained in input beam 100 is lost. This energy is lost because secondary mirror 12 may reflect only one-half of a symmetrical beam; the other half of the beam being lost. The efficiency of the system also depends on the reflectivity of sample 30. If sample 30 is perfectly reflective, i.e., a mirror, the efficiency of the system approaches 50%, i.e., all the energy input into the objective and reflectively redirected by the intercepting mirror 10 reaches focus 34. If, however, sample 30 is diffusively reflective i.e., reflecting the incident energy in a hemispherical pattern, one-half of the collected reflected energy is reflected along the input path and does not reach focus 34. Thus, in the worst case, the reflective beam splitting objective obtains an efficiency of only 25% of the collectable energy, or an efficiency that is equal to the best possible efficiency obtainable with a conventional refractive beam splitter arrangement of the type shown in Figure 4. The reflective beam splitter apparatus in one embodiment may have an input beam 110 focused at sample plane 30 along the optical axis. Energy passing through the sample plane fills primary mirror 22. Substantially half of the energy from mirror 22 is reflected from secondary mirror 12 to intercepting mirror 10 and focused off the optical axis 21 of the system. Substantially half of the energy collected by the primary mirror reaches focus 34 on the optical axis of the system. Thus, operated in a transmission mode, the optical arrangement splits input beam 110 into two components and focuses each component. Each focus of the beam splitter contains information from the entire sample 30 so long as intercepting mirror 10 is not close to focus plane 36 and edge 14 does not vignette ray 105. The closer intercepting mirror 10 comes to focus plane 36 the greater the uneven attenuation of the image on the side of the optical axis of mirror 10 caused by mirror 10. If edge 14 vignettes ray 105, a portion of the image of the sample plane reaching focusing plane 36 is attenuated. Therefore, the same considerations apply for positioning mirror 10 as were described above. The input energy beam also could be sourced at the focus 36 and direct an image reflected from the sample at area 236, in reverse to that illustrated. It is also possible to place the intercepting mirror between the secondary and primary mirrors in such a way as to make the reflective beam splitting objective an integral unit. This arrangement has proven to have particular utility for obtaining a stable aligned optical configuration. It has been found that the alignment of the optical components is particularly critical to obtaining a high efficiency when the intercepting mirror is at its minimum distance from the secondary mirror. The secondary mirror may be made of glass while the intercepting mirror and the primary mirror are preferably made of metal, e.g., stainless steel, which may be coated with aluminum. The stainless steel enables holes to be placed into the base of the mirror so as to provide optical path and structural components. This mechanical assembly greatly simplifies initial optical alignment and serves to better maintain the optical alignment. Further, by placing the intercepting mirror between the primary and secondary mirrors, the reflecting beam splitting objective has a smaller size than can be obtained by placing the intercepting mirror elsewhere, such as behind the primary mirror. Also vignetting of ray 105 may be reduced by making first edge 14 as sharp as possible so that jagged edges do not protrude into the path of the ray. A Cassegrain arrangement of the mirrors in the system produces an optimum reflecting objective. However, other lens arrangements could be used. A pure mirror arrangement, such as disclosed above, has particular utility for use in infrared spectroscopy. Other optical arrangements, such a Maksutov optical arrangement, could be used. Thus it will be seen that, at least in preferred forms, there is provided a reflective beam splitting apparatus wherein an energy beam intercepting mirror is positioned to divide the optical axis of a Cassegrain or similar reflecting objective. The intercepting mirror is positioned a sufficient distance from a secondary mirror so as not to vignette an energy beam directed from the secondary mirror to a primary mirror and sufficiently close to the secondary mirror so as not to unevenly attenuate an image focused behind the primary mirror. Furthermore there is provided a procedure for directing an input energy beam at an angle substantially perpendicular to the optical axis of the system, wherein the beam is directed onto the axis of the system by an intercepting mirror which reflects the energy to a portion of the secondary mirror area then onto the primary mirror which directs the energy to the sample. Energy reflected off of a sample is then focused by the system behind the primary mirror. Alternatively it is possible to input an energy beam substantially on the axis of the system from the fast end and using the intercepting mirror to divide the resulting image into a first component that the Cassegrain optics focuses on-axis to the system and a second component that is focused at an angle to the optical axis. In all the embodiments described the apparatus focuses all points on a sample located at the respective image planes, thus enabling visual inspection of a sample to determine from which areas energy may be received by one or more detectors. The apparatus has proven to have particular utility in the field of infrared microscopy. Modifications to the specific embodiments and to any broad aspects thereof referred to or suggested herein may be apparent to those skilled in the art and the disclosure hereof is intended to encompass any such modifications.

Claims: A reflective objective apparatus for forming a real image of an object, including: a primary mirror (22) of annular shape centered on the optical axis of the reflective objective apparatus (21), and having a generally concave reflective surface (22a); a secondary mirror (12) of smaller extension than that of the primary mirror, also centered on said optical axis and having a convex reflective surface (13) positioned so as both to be able to reflect with a first portion thereof, an energy beam to a first portion of the reflective surface of said primary mirror from where the beam is reflected to an object position located behind the secondary mirror, and to be able to reflect with a second portion thereof a energy beam from the object reflected from a second portion of said primary mirror to an image plane located behind the primary mirror, characterised in that said apparatus further comprises an intercepting mirror (10) having a planar reflecting surface (10a) and being positioned off the optical axis between the primary and secondary mirrors, so as to face said first portion of said secondary mirror, to redirect energy to or from said first portion of said secondary mirror, said intercepting mirror being positioned sufficiently far from the secondary mirror such that a ray (105) between the centre of the secondary mirror reflective surface to the innermost point of the used first portion of the reflective surface of the primary mirror is not obstructed by said intercepting mirror, and sufficiently close to the secondary mirror such that the image formed by energy reflected off the second portion of the secondary mirror at said image plane located behind the primary mirror is not unevenly attenuated. An objective apparatus as claimed in claim 1, characterised in that said intercepting mirror includes at least one sharp edge (14) along its reflective surface terminating at a line extending from the centre of the reflective surface of the secondary mirror to an innermost point on the reflective surface of the primary mirror but not intersecting same, so as to avoid diffraction or vignetting of a beam extending between the secondary and primary mirrors along said line. An objective apparatus as claimed in any preceding claim, characterised in that the intercepting mirror is made from a metal that can be machined to high tolerance. An objective apparatus according to any preceding claim, characterised in that the primary mirror is made of a metal that can be machined to a high tolerance. An objective apparatus as claimed in any of claims 1 to 4 characterised in that the primary and secondary mirror reflective surfaces form a Cassegrain mirror arrangement. An objective apparatus as claimed in any preceding claim wherein said primary mirror is arranged to collect an input energy beam transmitted through a sample plane and to reflect said collected beam to said secondary mirror, said secondary mirror being arranged to reflect a part of said collected beam to a focus on the optical axis behind the reflective surface of the primary mirror and to reflect a second part of said collected beam to the intercepting mirror. An objective apparatus as claimed in any of claims 1 to 5 wherein said intercepting mirror is arranged to reflect an input energy beam to a first portion of the secondary mirror, said secondary mirror being arranged to reflect said input energy beam to the primary mirror, said primary mirror being arranged to reflect said energy beam to a sample plane and to collect an energy beam reflected from the sample plane and to reflect said collected energy beam to a second portion of said secondary mirror, and said secondary mirror being arranged so that said collected beam is focused on the optical axis behind the reflective surface of the primary mirror. A first method of operating the apparatus of claim 1 including directing an energy beam from the side to the intercepting mirror; from where the beam is reflected to said first portion of the secondary mirror; from where the beam is reflected to said first portion of the primary mirror; from where the beam is reflected to the object location intersecting the optical axis and located behind the secondary mirror; and collecting a portion of the beam reflected from the sample on said second portion of the primary mirror; from where the beam is reflected to said second portion of the secondary mirror, from where the beam is reflected and focused at the image plane located behind the primary mirror. A second method of operating the apparatus of claim 1 including directing an input energy beam through a sample located at said object location towards said concave primary mirror; from where the energy is reflected onto said convex secondary mirror whereafter energy from said first portion of said secondary mirror is reflected onto said intercepting mirror from where the energy is brought to a first focus; and where the energy from said second portion of the secondary mirror is focused at said image plane located behind the primary mirror.

Inventors: MESSERSCHMIDT, ROBERT G., STING, DONALD W.
Applicants: SPECTRA-TECH, INC.
Application year: 1986
Main classification: G02B17
IPC codes: G01J3, G02B21, G01N21, G02B17