Abstract:
An interferometer for Fourier spectroscopy, wherein the interferometer comprises a beamsplitter ( 14 ) and two retroreflectors ( 20, 26 ), characterized in that the beamsplitter ( 14 ) is mounted movable, e.g., mounted pivotally or displacably, while both retroreflectors ( 20, 26 ) are arranged as fixed retroreflectors. The proposed structure is simple to produce, can be made substantially insensible to environmental vibrations, and it is well suited for routine measurements for the determination of quantities of predefined components in a medium. The interferometer is particularly intended for measurements in the mid- or near-infrared range for determination of the quantities of specified components in a medium, and more specifically in a food product, e.g., a liquid such as milk.

Description:
This application is the national phase under 35 U.S.C. §371 of prior PCT International Application No. PCT/DK97/00299 which has an International filing date of Jul. 4, 1997 which designated the United States of America. 
    
    
     TECHNICAL FIELD 
     The present invention concerns an interferometer for Fourier spectroscopy, in which the interferometer comprises a beamsplitter and two retroreflectors which substantially are arranged to form a Fourier transform spectrometer or interferometer. 
     The interferometer is particularly intended for measurements in the mid-infrared range and/or near-infrared range for determination of the quantities of specified (predetermined) components in a medium or fluid, specifically a liquid, such as a solution, e.g. an aqueous solution, and more specifically in a food product, e.g. a liquid food product such as milk. The interferometer is preferably constructed for use in the infrared range and especially in the mid infrared and/or near infrared range. 
     BACKGROUND ART 
     Typically used, known interferometers have two reflecting mirrors, in some interferometers in the form of retroreflectors. Generally one of the reflecting mirrors is mounted movably in a longitudinal direction, i.e. along the path of the radiation beam. The theory and use of interferometers are thoroughly described in “Fourier transform infrared spectrometry”, by Peter R. Griffiths and James A de Haseth, Vol. 83 in “Chemical Analysis”. In this specification the term “retroreflector” means an optical device, such as a comer reflector or cat eye, which will reflect an incident light ray in a direction which is parallel to the incident ray, as explained in the above-mentioned reference, e.g. FIG. 4.12 on p.144. 
     When constructing the arrangement for the movement or longitudinal displacement of the movable mirror, or reflector, great efforts have to be made to ensure controlled displacement. A reliable interferometer shall be substantially insensible to environmental vibrations. A good interferometer will primarily be sensible to vibrations in the same direction as the controlled movement of the movable part. 
     U.S. Pat. No. 4,383,762 discloses a two-beam interferometer for Fourier spectroscopy designed to be housed in a cryostat aboard a spacecraft, with rigid pendulum structure mounting at least one of the movable retroreflectors in a fully compensated optical system immune to tilt and lateral movement distortions. By this structure the linear displacement is replaced by a pivotal mounting of one or both retroreflectors. Similarly, EP 369 054 describes a pendulum reflector system for a Michelson interferometer. In FIG. 1 of EP 369 054 two retroreflectors are arranged on two perpendicular arms bearing on a common pivot. These interferometers are favourable in that sense that they can be made in such a way that they are only sensible to vibrations in the direction of rotation. Further the weight of the movable parts may be compensated by a balance weight. The driving mechanism must be dimensioned to suppress movements caused by environmental vibrations. 
     The purpose of the present invention is to provide a simple arrangement for an interferometer said arrangement being a structure which is cheaper to manufacture than the hereto known interferometers, and which simultaneously still has the properties necessary to obtain useful measurements, which are sufficiently accurate for the purpose of the actual measurement. 
     SUMMARY OF THE INVENTION 
     According to the invention the beamsplitter is arranged in a movable beamsplitter arrangement, e.g. mounted pivotally or rotatably, and both retroreflectors are fixed. Such mounting can be realised in a very simple way, by which a cheaper manufacturing of the interferometer can be obtained. 
     The new interferometer is specific advantageous in that it can be made substantially insensible to vibrations. This is due to the fact that the mass of the movable, rotatable or pivotable beamsplitter can be made small, i.e. it can be considerably smaller than the mass of retroreflectors. Therefore the moment of inertia of the rotatable beamsplitter arrangement can be small compared to the moment of inertia of the movable retroreflector arrangement of prior art Accordingly, by the new interferometer, it is easier to arrange a drive mechanism which will be able to suppress vibrations. Typically, vibrations tend to be a problem in interferometers, and especially in interferometers for milk analysis, due to the fact that such apparatus often includes a homogenizer and high pressure pumps generating vibrations. 
     As it will be explained in more details below, the movement of the beamsplitter creates a retardation and thereby a variable interference as in conventional interferometers. 
     Preferably, the beamsplitter is located in a plane which is at least nearly substantially coincident with a plane of symmetry of the two retroreflectors. 
     In a preferred embodiment the interferometer is specifically dedicated to the use for determination of the quantities of specified components in a medium or liquid, and more specifically in a liquid food product such as milk. 
     Further, the new interferometer has proved good spectral resolution in the mid-infrared range and even by short wavelengths i.e. in the near infrared range. 
     In the preferred embodiment of the new interferometer the structure is simple, simple to produce, and mechanically sturdy and stable. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic view of a first embodiment of an interferometer according to the invention, 
     FIG. 2 same as FIG. 1 with the beamsplitter displaced in the opposite direction, 
     FIG. 3 shows a schematic view of a second embodiment of an interferometer according to the invention, 
     FIG. 4 shows a schematic enlarged, sectional view of the first embodiment of FIG.  1 . 
     FIG. 5 shows a preferred embodiment of the new interferometer. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In FIG. 1 a light source  10  emits a bundle of light beams of which—for the sake of clarity—only a centre beam  11  and a side beam  12  are shown on the drawings. The light beam  12  hits a beamsplitter  14 , splitting the beam  12  into a transmitted beam  16  and a reflected beam  18 . The beam  16  hits a retroreflector  20  and is reflected as the beam  17  towards the beamsplitter  14 , and part of the beam is reflected in the beamsplitter  14 . The retroreflectors  20 ,  26  are—for the sake of clarity—only shown as two plane mirrors forming a right angle. However, each of the retroreflectors  20 ,  26  comprises three reflecting, plane surfaces, which are mutually perpendicular. 
     The reflected beam  22  is directed towards a measurement cuvette  24 , containing the medium, e.g. a liquid food product which is to be analysed. A detector located on the back-side of the measurement cuvette receives the transmitted light, typically via a focusing system, such as a reflecting concave mirror. Focusing systems are well-known and will therefore neither be described in this description nor shown in the drawings. 
     The beam  18  which was reflected in the beamsplitter  14 , hits a second retroreflector  26 , which reflects the beam  19  back towards the beamsplitter  14 . Part of the beam is transmitted through the beamsplitter and is now substantially coincident with and interfering with the above-mentioned beam  22 , hitting the measurement cuvette  24 . 
     The beamsplitter  14  is mounted pivotally around the point O. The mount can be any kind of rotary joint, e.g. a hinge or similar articulation. Instead of a moving mirror—as it is done in the usually known kind of Michelson interferometers—the beamsplitter of the present invention is moved forward and backwards e.g. to a position as indicated by the dotted line  14   a  in FIG. 1 and 14 b  in FIG.  2 . 
     The preferred location of the pivot O is shown in FIGS. 1-2. The preferred location is characterized by O being a corner in a square including the two retroreflector tops A, B and the centre C. However other locations of O can be used, too. It is preferred that O is located in the plane of symmetry of the two retroreflectors. 
     For the sake of clarity the angle of beamsplitter displacement is shown in the drawings to be about 1.5°. In practical use the actual maximum displacement will depend upon the actual purpose of the measurement. 
     In a preferred embodiment, which is intended for determination of quantities of specific predetermined (known) components in a liquid food product, the contemplated angle is about 0.3-0.5°. 
     In the beamsplitter position  14   a  the path length of the beam  16 ,  17 , is shortened by a length Δ, compared to the original beam length in the neutral position  14 . 
     In similar way the path length of the beam  18   a ,  19   a , is lengthened by a distance Δ, compared to the original beam length in the neutral position  14 . 
     The total result of the beamsplitter displacement from position  14  into position  14   a  is a shortening of the first beam  16 ,  17  and a lengthening of the second beam  18 ,  19 . When the beamsplitter  14  is moved in the opposite direction, the beam  16 ,  17  is lengthened and the beam  18 ,  19  is shortened. The result of the indicated beamsplitter displacement is therefore comparable to a displacement of one of the retroreflectors. Thereby, generally, the arrangement shown in FIG. 1 will function as an interferometer, i.e. the beam  22  impinging the cuvette  24  will be a superposition of two interfering beams, and the interference will vary with the movement of the beamsplitter, forming an interferometer suited for Fourier Transform spectroscopy. However it is much simpler to realize than the hereto known interferometers. 
     As it appears from the simplified drawings of FIGS. 1 and 4 the simple construction has the disadvantages that the beam  22   a  is displaced a distance and diverted relative to the beam  22 . Calculations performed using the actually preferred dimensions show that this displacement is quite small, e.g. less than 0.038 mm, when the maximum divergence of the collimated light from the source is 3.6°. 
     Calculation of displacements when the beamsplitter is rotated a small angle θ=0.3°. If l is the distance from the beamsplitter window to the pivot point O, and the distance between the origin E of the beam  18  and the origin F of the beam  18   a  is called Δ1, the distance between the origin G of the beam  22  and the origin H of the beam  22   a  is called Δ2, and the corresponding distance along the beam  11  is called Δ, simple triangular calculations give that 
     
       
         Δ=l sin θ=½(Δ1+Δ2) 
       
     
     The total displacement of the beam at the cuvette in the position shown in FIG. 1, i.e. without a focusing system, is 
     
       
         2Δ=22l sin θ=1.1 mm for l=75 mm and θ=0.3°. 
       
     
     Such displacement can however be compensated by use of a focusing system. Preferably, if the cuvette is small, it will be arranged close to the point of focus of the focusing system. 
     An alternative arrangement is shown is FIG. 3, wherein the pivot O is located between the light source  10  and the cuvette+detector  24 . As it appears from the drawing, the resulting beams  22   c ,  22   c ′ are mutually spaced, the distance between the beams being bigger the more the source light beam is displaced from the centre beam  11 . Such displacement can be compensated by a focusing system, focusing both of the beams on the detector surface. 
     The retroreflectors are preferably cubic comer reflectors, i.e. having three mutually orthogonal reflecting inner surfaces. Due to the polarisation characteristics of such comer reflectors, the two comer reflectors are preferably arranged to have the same orientation of their polarisation. The mountings for the retroreflectors are preferably adjustable to allow alignment of the optical instrument to ensure the two split beams meet again, having the same polarisation at the beamsplitter. Such adjustable mountings are well-known and will therefore neither be described in this description nor shown in the drawings. 
     In a third embodiment the pivot O is located infinitely far away. The beamsplitter may be mounted e.g. in a slide bearing or between a couple of parallel springs, in order to give the beamsplitter a translatory movement. 
     The vital parts of a presently preferred embodiment of the new interferometer is shown in FIG.  5 . The interferometer should be enclosed in a hermetically sealed box. In order to show the interior parts the interferometer is shown without the sealed box. Further, for the sake of clarity, the light source ( 10  in FIG. 1) and the detector ( 24  in FIG. 1) are not shown in FIG.  5 . The components which are not shown can be of conventional known art, and they can be arranged in known manner. A base plate  30  is used for mounting the components of the interferometer. A pivotable arm  31  bearing in a hinge  34  carries the beamsplitter  32  which can be a circular disc mounted by use of springs  33  in a recess surrounding a circular opening. A balance weight  38  is applied for balancing the beamsplitter arm  31 . A motor  39  is arranged to move the beamsplitter arm  31 , i.e. rotate the beamsplitter arm a small angle, typically less than 1°, e.g. 0.3°. In the preferred embodiment the motor is acting on the extension of the beamsplitter arm  31 , i.e. on the balance weight  38 . Typically the travel path of the beamsplitter will be a few millimetres. The motor may be an electrodynamical actuator, combined with a velocity sensor  40  for the feed-back loop. The velocity sensor can either be of an electrodynamical type or be based on a phased locked loop on the laser interferogram. 
     Cylinders  36  accommodate the comer reflectors. Each of the comer reflectors is arranged inside the cylinder in such a way that the axis of symmetry of the comer reflector is coincident with the centre axis of the cylinder. The cylinder is mounted rotatably. By rotating the cylinder the polarisation axis of the corner reflector can be adjusted. 
     The cylinders  36  are adjustable mounted on mounting plates or holders  37 , which are adjustable mounted on the base plate  30 . The mounting is very compact. i.e. components are mounted close to each other. Preferably recesses or openings  41  are provided in the beamsplitter arm  31  to allow for the positioning of the cylinders  36  in close relation to the beamsplitter arm. The compact arrangement is favourable for the performance of the optics, by keeping the light path as short as possible. 
     As it will be obvious to people in the art the new interferometer as shown in the drawings and described in the preceding specification may be modified in several ways within the scope of the invention as defined in the following claims.