Abstract:
An apparatus for performing optical measurements comprising a first prism having a first sample surface, the first sample surface operatively arranged to receive a first sample, the first sample surface operatively arranged to reflect incident light, a first detector operatively arranged to measure intensity of light reflected from the first sample surface of said first prism, a second prism having a second sample surface, the second sample surface operatively arranged to receive a second sample, the second sample surface operatively arranged to reflect incident light, a second detector operatively arranged to measure intensity of light reflected from the second sample surface of the second prism, and, means to determine an optical characteristic based on the intensities of light measured by said first and the second detectors. The invention also includes a method for performing optical measurements.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to optical instruments for measuring refractive index of a substance, and more particularly to an optical configuration and method for measuring a refractive index of a test sample and a reference sample with one device.  
         BACKGROUND OF THE INVENTION  
         [0002]    Refractometers measure the critical angle of total reflection by directing an obliquely incident non-collimated beam of light at a surface-to-surface boundary between a high refractive index prism and a sample to allow a portion of the light to be observed after interaction at the boundary. In transmitted light refractometers, light that is transmitted through the sample and prism is observed, while in reflected light refractometers, the light that is reflected due to total reflection at the surface-to-surface boundary is observed. In either case, an illuminated region is produced over a portion of a detection field of view, and the location of the shadowline between the illuminated region and an adjacent dark region in the detection field of view allows the sample refractive index to be deduced geometrically. Differential refractometers, such as that disclosed in U.S. Pat. No. 5,157,454, have been developed for measuring a difference in refractive index between a test sample and a known reference sample, whereby variable test conditions affecting the measurement result, such as sample temperature, illumination level, etc., can be “subtracted out” to yield a more accurate and precise measurement result.  
           [0003]    However, existing differential refractometers generally are very bulky and expensive. There are a number of applications where a portable device is needed to measure an index of refraction of an unknown substance to a lesser degree of accuracy than an expensive differential refractometer.  
           [0004]    Clearly, then, there is a longfelt need for a portable apparatus that can inexpensively measure the refractive index of a test sample in comparison to a reference sample.  
         SUMMARY OF THE INVENTION  
         [0005]    The invention broadly comprises a method and an apparatus for performing optical measurements. The apparatus comprises a first prism having a first sample surface, the first sample surface operatively arranged to receive a first sample, the first sample surface operatively arranged to reflect incident light, a first detector operatively arranged to measure intensity of light reflected from the first sample surface of said first prism, a second prism having a second sample surface, the second sample surface operatively arranged to receive a second sample, the second sample surface operatively arranged to reflect incident light, a second detector operatively arranged to measure intensity of light reflected from the second sample surface of the second prism, and, means to determine an optical characteristic based on the intensities of light measured by said first and the second detectors.  
           [0006]    The method of the invention comprises the steps of measuring an intensity of light reflected from a first sample surface of a first prism, the first sample surface of the first prism receiving a first sample, measuring an intensity of light reflected from a second sample surface of a second prism, the second sample surface of the second prism receiving a second sample, and, determining an optical characteristic based on the measured intensities of light.  
           [0007]    A general object of the present invention is to provide a method and apparatus for determining the index of refraction of a sample in comparison to another sample with a single apparatus.  
           [0008]    Another object of the present invention is to provide a method and apparatus for determining the index of refraction of a sample that is portable and low in cost. 
       
    
    
       [0009]    These and other objects, features and advantages of the present invention will become readily apparent to those having ordinary skill in the art upon a reading of the following detailed description of the invention in view of the drawings and claims.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The nature and mode of operation of the present invention will now be more fully described in the following detailed description of the invention taken with the accompanying drawing figures, in which:  
         [0011]    [0011]FIG. 1 is a schematic of an embodiment of the present invention;  
         [0012]    [0012]FIG. 2 is a front view of a detector of the present invention with a region of light reflected from a sample space incident thereon;  
         [0013]    [0013]FIG. 3 is perspective view of an embodiment of the present invention wherein the samples are inserted in wells in the top of the apparatus housing;  
         [0014]    [0014]FIG. 4 is a perspective view of an embodiment of the present invention wherein the samples are inserted in open well cuvettes inserted into the housing of the apparatus;  
         [0015]    [0015]FIG. 5 is a perspective view of an embodiment of the present invention wherein one sample is inserted in an open well cuvette and one sample flows into a flow cell cuvette, wherein each cuvette is inserted into the housing of the apparatus;  
         [0016]    [0016]FIG. 6 is a perspective view of an embodiment of the present invention wherein the samples flow into flow cell cuvettes inserted into the housing of the apparatus;  
         [0017]    [0017]FIG. 7 is a perspective view of an open well cuvette of a preferred embodiment of the present invention;  
         [0018]    [0018]FIG. 8 is a front view of an open well cuvette of a preferred embodiment of the present invention;  
         [0019]    [0019]FIG. 9 is a side view of an open well cuvette of a preferred embodiment of the present invention;  
         [0020]    [0020]FIG. 10 is a perspective view of a flow cell cuvette of a preferred embodiment of the present invention;  
         [0021]    [0021]FIG. 11 is a front view of a flow cell cuvette of a preferred embodiment of the present invention;  
         [0022]    [0022]FIG. 12 is a side view of a flow cell cuvette of a preferred embodiment of the present invention;  
         [0023]    [0023]FIG. 13 is a view of a preferred embodiment of the present invention with the hinge open, to allow the prism surfaces to be cleaned;  
         [0024]    [0024]FIG. 14 is a view of a preferred embodiment of the present invention with the hinge closed, to allow operation of the apparatus;  
         [0025]    [0025]FIG. 15 is a schematic view of a preferred embodiment of the present invention, configured with flow cell cuvettes engaging gaskets on the sample surfaces of the prisms;  
         [0026]    [0026]FIG. 16 is a view of a preferred embodiment of the present invention wherein the light to each prism originates from a single light source and traverses fiber optic lines that guide the light towards the sample surfaces of the prisms; and,  
         [0027]    [0027]FIG. 17 is a view of an embodiment of the present invention wherein the user enters a tolerance and a reference sample into a closed well cuvette, a test sample flows through a flow cell cuvette, and a light indicates whether or not the test sample is within the specified tolerance of the reference sample. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0028]    It should be appreciated that, in the detailed description of the invention that follows, like reference numbers on different drawing views are intended to identify identical structural elements of the invention in the respective views.  
         [0029]    Adverting now to the drawings FIG. 1 illustrates refractometer  10  of the present invention. Refractometer  10  comprises first prism  40  having a first sample surface  47  for receiving a first sample  50  to be tested, second prism  140  having a second sample surface  147  for receiving a second sample  150  to be tested, a first detector  70 , a second detector  170 , and detection electronics  75 . Non-parallel light is obliquely directed along beam path  15  to the surface boundary between first sample  50  and first prism  40  after traversing a diffuser  30  and a first lens  42  cemented to a light entry surface  46  of prism  40  for focusing the light at a point F 1  slightly in front of first sample surface  47 . The divergent light from point F 1  includes rays having an angle of incidence greater than the critical angle of total internal reflection and rays having an angle of incidence not greater than the critical angle of total internal reflection, such that the former rays will be internally reflected at the sample/prism boundary to continue along a path to detector  70 , while the latter rays will be refracted by sample  50  and pass out of the system. The internally reflected light passes successively through an exit surface  48  of prism  40  and a second lens  44  cemented to the exit surface. The light then strikes a reflective surface of a mirror  60  and is thereby redirected toward detector  70 . Those skilled in the art realize that the index of refraction of sample  50  is a variable that determines the critical angle of total internal reflection at the prism-sample boundary, and thus the location of a shadowline  72  between illuminated and dark regions on detector  70 , as shown in FIG. 2. In a preferred embodiment, detector  70  is a linear scanned array. In the present application, “angle of incidence” is intended to mean the angle between the sample surface of the prism and the light beam as it approaches the sample surface.  
         [0030]    In a similar manner, non-parallel light is obliquely directed along beam path  115  to the surface boundary between second sample  150  and second prism  140  after traversing a diffuser  130  and a first lens  142  cemented to a light entry surface  146  of prism  140  for focusing the light at a point F 2  slightly in front of second sample surface  147 . The divergent light from point F 2  includes rays having an angle of incidence greater than the critical angle of total internal reflection and rays having an angle of incidence not greater than the critical angle of total internal reflection, such that the former rays will be internally reflected at the sample/prism boundary to continue along a path to detector  170 , while the latter rays will be refracted by sample  150  and pass out of the system. The internally reflected light passes successively through an exit surface  148  of prism  140  and a second lens  144  cemented to the exit surface. The light then strikes a reflective surface of a mirror  160  and is thereby redirected toward detector  170 . A shadowline is formed on detector  170  in a similar manner as shadowline  72  is formed on detector  70 , shown in FIG. 2. In a preferred embodiment, detector  170  is a linear scanned array.  
         [0031]    As shown in FIG. 1, in a preferred embodiment, the first sample surface  47  of first prism  40  and second sample surface  147  of second prism  140  are substantially vertically oriented. In the most preferred embodiment, first sample surface  47  of first prism  40  and second sample surface  147  of second prism  140  are parallel to each other.  
         [0032]    In a preferred embodiment, detector  70  comprises a plurality of photodiodes  71 . The photodiodes  71  generate a current proportional the intensity of light incident thereon. A substantial intensity of light is incident on photodiodes  71  in the illuminated region  73  corresponding to angles of incidence greater than the critical angle. As described above, the light incident at angles less than the critical angle is refracted by the respective sample and passes out of the system. Therefore, little or no light is incident on photodiodes in the dark region  74  corresponding to angles of incidence less than the critical angle. Thus, the current levels measured by detector  70  change substantially at the location of shadowline  72 .  
         [0033]    As is known in the art of automatic refractometers, the current levels generated by photodiodes  71  of detectors  70  and  170  are digitized and processing electronics  75  perform an algorithm designed to determine the location of the shadowline transition  72  between the illuminated region  73  of each detector and the dark region  74  of each detector. The cell crossing number of the shadowline on each detector is then used to geometrically calculate the index of refraction of each of the samples. Various algorithms are available, as taught for example by U.S. Pat. Nos. 4,640,616; 5,617,201; and 6,172,746; and by commonly-owned U.S. patent application Ser. No. 09/794,991 filed Feb. 27, 2001, each of these documents being hereby incorporated by reference in the present specification. In a preferred embodiment, processing electronics  75  comprise a microprocessor programmed to perform one of the above-referenced algorithms. However, it should be readily apparent to one skilled in the art that other processing means are possible, and these modifications are within the scope of the invention as claimed.  
         [0034]    After determining the indices of refraction, the processing electronics can determine the relationship between the two samples. Depending on the desired output, the present invention can be used, for example, to provide an index of refraction of one of the samples, an index of refraction of both of the samples, the difference in index of refraction between the samples, or an indication that the indices of refraction of the samples are within a specified tolerance of each other. It should be readily apparent to one skilled in the art that other configurations are possible, and these configurations are within the scope of the invention as claimed.  
         [0035]    [0035]FIG. 3 shows an embodiment of the present invention wherein the samples are received by wells  430  and  440  located in a top face of housing  420  of apparatus  410 . This configuration has the disadvantage that it is difficult to clean the samples out of the wells before a new sample is inserted.  
         [0036]    In a preferred embodiment, the samples are received by cuvettes, as shown in FIGS.  4 - 6 . FIG. 4 shows an embodiment of the present invention wherein the samples are received in open well cuvettes  190 . The structure of the open well cuvettes is shown in FIGS.  7 - 9 . Open well cuvettes receive a sample through hole  195  and hold the sample against the respective prism sample surface. A cover  198 , shown in FIG. 16, can be inserted in the open well to seal the sample in the cuvette. This is used, for example, to protect a reference sample from being contaminated by other samples.  
         [0037]    [0037]FIG. 5 shows an embodiment of the present invention wherein one sample is received in flow cell cuvette  290  and one sample is received in open well cuvette  190 . The structure of flow cell cuvettes is shown in FIGS.  10 - 12 . Flow cell cuvettes receive into chamber  292  a sample flow through a first tube  295  connected to first flow port  294 . Cuvette  290  allows sample fluid to leave chamber  292  through a second tube  297  connected to second flow port  296 . Thus, a continuous flow of fluid to be tested is held in contact with the sample surface of a prism. FIG. 6 shows an embodiment of the present invention wherein the samples are received in flow cell cuvettes  290 . The present invention can be configured to use any combination of open well cuvettes, sealed open well cuvettes, or flow cell cuvettes.  
         [0038]    The preferred embodiment also comprises a temperature control device, designated  80  in FIGS. 13 and 14. The temperature control device  80  maintains both samples at the same temperature, to remove any errors due to temperature fluctuations. Typically, the temperature control device is a Peltier temperature control device, as is well known in the art. For example, U.S. Pat. Nos. 5,841,064 (Maekawa et al.) and 6,067,802 (Alonso), which are incorporated by reference herein, disclose Peltier temperature control devices. It should be readily apparent to one skilled in the art that other temperature control devices may be used, and these modifications are within the scope of the invention as claimed.  
         [0039]    The preferred embodiment further comprises a means to open the apparatus to allow access to the sample surfaces of the prisms for cleaning. FIG. 13 shows apparatus  10  opening around hinge  90 . Cuvettes  190  are removed from recesses  85 , allowing direct access to the sample surfaces of prisms  40  and  140 . When the prisms are clean, the hinge is closed, as shown in FIG. 14. Cuvettes  190  are inserted into recesses  85 , and the apparatus is ready to be operated.  
         [0040]    [0040]FIG. 15 shows an embodiment of the present invention configured to receive two samples in two flow cell cuvettes. This figure also shows gaskets  298 . The gaskets, included in a preferred embodiment, form a fluid tight seal between the flow cell cuvette and the sample surface of the prism.  
         [0041]    [0041]FIG. 16 shows a first light source  20  emitting light along beam path  15  and a second light source  120  emitting light along beam path  115 . In a preferred embodiment, shown in FIG. 16, light source  220  emits light into first fiber optic member  222  and second fiber optic member  224 . Fiber optic member  222  is arranged to emit light from source  20  along beam path  15  and second fiber optic member  224  is arranged to emit light from source  20  along beam path  115 . Using light from the same source is preferred, as index of refraction is dependent on the wavelength of the light used in the measurement. Thus, any difference in wavelength between light sources  20  and  120  in FIG. 15 will introduce a source of error when the indices of refraction measured for the two samples are compared. However, it should be readily apparent to one skilled in the art that other configurations for providing light to the prisms are possible, and these modifications are within the scope of the invention as claimed.  
         [0042]    [0042]FIG. 17 shows an embodiment of the present invention wherein a tolerance is entered by a user. A reference sample is placed in open well cuvette  190  and the well is sealed with cover  198 . A test sample is pumped through flow cuvette  290 . Indicator light  182  stays lit as long as the index of refraction of the test sample stays within the entered tolerance of the index of refraction of the reference sample. If the index of refraction of the test sample differs from the index of refraction of the reference sample by more than the entered tolerance, indicator light  184  is lit.  
         [0043]    The cuvettes of the present invention comprise glass, metal, plastic, or any other substance known in the art. In one embodiment, the cuvettes comprise injection molded plastic. In another embodiment, the cuvettes comprise copper.  
         [0044]    Thus, it is seen that the objects of the present invention are efficiently obtained, although modifications and changes to the invention should be readily apparent to those having ordinary skill in the art, and these modifications are intended to be within the scope of the invention as claimed.