Patent Publication Number: US-2015062572-A1

Title: White cell for fluid detection

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Application claims the benefit of U.S. Provisional Application 61/874,048 filed on Sep. 5, 2013. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to optical absorption spectroscopy, and more particularly to use of a white cell to measure fluid properties. 
     2. Description of Related Art 
     The concentration of one or more fluid substances (i.e. gases or liquids) within a sample can be determined via optical absorption spectroscopy, by passing light through the sample and detecting the optical absorption characteristics of those substances. 
     The amount of light absorbed by the substance depends on the concentration of the substance and the path length of light through the substance. In gases, the concentration in terms of molecules per unit volume is generally much lower than in liquids or solids and therefore the path length of the light through the sample must be correspondingly higher. Large path length can be achieved either by placing the light source and the detector far apart or by reflecting the light backwards and forwards through a sample in a measurement cell so that it passes through the sample numerous times before reaching the detector. A multi-pass measurement cell provides a significant path length in an apparatus having a compact form. One example, the White cell named after J. U. White and initially published in “Long Optical Paths of Large Aperture”, Journal of the Optical Society of America, May 1942, which is incorporated by reference in its entirety, has been used for years for absorption spectroscopy. 
     A typical White cell consists of three concave mirrors of identical radius curvature. A field minor faces two object mirrors. A light source positioned adjacent the field minor transmits light towards the first object mirror which reflects the light back onto the field mirror. The field minor is oriented such that it reflects the light towards the second object minor, which refocuses the light back to the field minor. After a number of passes the light falls off one side of the field mirror and is collected by a detector. This light is then analyzed by a spectrograph to detect the optical absorption spectra of the substances through which the light has passed. 
     The use of the White cell allows for a relatively long path length in a relatively small volume of space. However, when the frequency of the fluid is greater and/or the path length is increased, the result is several diverging beams of light which create noise or disturbance in the optical absorption spectra. 
     There is still an ever present need in the art for optical absorption spectroscopy devices and methods that allow for an increased path length in a White cell. There also remains a need in the art for such a devices and methods that are easy to make and use. The present disclosure provides a solution for these problems. 
     SUMMARY OF THE INVENTION 
     An optical absorption spectroscopy apparatus includes a field mirror and at least one object mirror configured to reflect transmitted light multiple times between the field minor and the at least one object mirror through a sample volume. At least one fold mirror is configured to allow the transmitted light to double pass across the field mirror. The apparatus can further include a light source and a detector to detect an optical absorption spectrum of the light transmitted from the light source through the sample volume. 
     In certain embodiments, the fold mirror can include two minors positioned at approximately a ninety degree angle to each other. The fold mirror can be disposed on the field minor. The fold minor can be angled symmetrically about a normal line to the surface of the field minor. 
     It is also contemplated that in certain embodiments, the distance between the field minor and the at least one object mirror is equal to or greater than 200 millimeters. 
     A dual channel optical absorption spectroscopy apparatus includes a first field minor and a second field minor and first and second pairs of object mirrors configured to reflect transmitted light multiple times between the first and second minors through a sample volume. Two fold minors are cooperatively connected to the first and second field minors and configured to allow the transmitted light to double pass across the first and second field mirrors. The first field minor and the first pair of object mirrors are facing each other coaxially to an optical first axis and the second field mirror and the second pair of object mirrors are facing each other coaxially to an optical second axis. The apparatus can further include a light source and two detectors configured to detect an optical absorption spectrum of the light transmitted from the light source through the sample volume. The first and second field mirrors can be disposed perpendicular to each other. 
     In certain embodiments, the two fold minors can each include two mirrors positioned at a ninety degree angle to one another. The two fold mirrors can be disposed on the two field minors. The two fold mirrors can be angled symmetrically about a normal line to the surface of the respective field mirror. 
     It is also contemplated that in certain embodiments, the distance between the first field minor and the at first pair of object minors can be equal to or greater than 200 millimeters and the distance between the second field mirror and the second pair of object minors can be equal to or greater than 200 millimeters. 
     The method for measuring one or more components of a fluid using optical absorption spectroscopy includes reflecting light from a light source multiple times through the fluid in a sample volume using a field minor and at least one object minor. The method can further include double passing the reflected light across the field mirror through a fold minor disposed thereon and detecting the light transmitted to determine the concentration of one or more components of the gas. 
     It is also contemplated that in certain embodiments, the step of reflecting can further include reflecting light through a terahertz gas. 
     These and other features of the systems and methods of the subject disclosure will become more readily apparent to those skilled in the art from the following detailed description of the preferred embodiments taken in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that those skilled in the art to which the subject disclosure appertains will readily understand how to make and use the devices and methods of the subject disclosure without undue experimentation, preferred embodiments thereof will be described in detail herein below with reference to certain figures, wherein: 
         FIG. 1  is a schematic plan view showing optical arrangement of a traditional White cell; 
         FIG. 2  is a schematic plan view showing increased modulation as an effect of increasing the path length while using the optical arrangement of a traditional White cell; 
         FIG. 3  is a perspective view of exemplary embodiment of a measurement cell constructed in accordance with the present disclosure, showing the use of a fold minor disposed on a field minor; 
         FIG. 4  is a perspective view of the measurement cell of  FIG. 3 , schematically showing reduced optical modulation; and 
         FIG. 5  is a perspective view of another exemplary embodiment showing a dual channel arrangement in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made to the drawings wherein like reference numerals identify similar structural features or aspects of the subject disclosure. For purposes of explanation and illustration, and not limitation, a partial view of an exemplary embodiment of an optical absorption spectroscopy apparatus in accordance with the disclosure is shown in  FIG. 3  and is designated generally by reference character  100 . Other embodiments of methods and devices in accordance with the disclosure, or aspects thereof, are provided in  FIGS. 4 and 5 , as will be described. The systems and methods described herein can be used for measuring the concentration of one or more fluid substances and more particularly when the fluid is characterized by increased path length. 
     The optical arrangement of a standard White cell  10  is illustrated in  FIG. 1 . The White cell  10  includes a field minor  12  facing two adjacent object mirrors  14 ,  16 . Usually, the minors are mounted within a measuring chamber (not shown for sake of clarity) having inlet and outlet ports allowing a sample fluid to be introduced into and removed from the chamber. A light source  18  is located adjacent one edge of the field minor  12  and a detector  20  is located adjacent the opposite edge of the field mirror  12 . Typically with this optical arrangement, light from the source  18  is focused by the first object minor  16  onto the surface of the field mirror  12 . The field minor  12  is oriented such that it reflects the light towards the second object mirror  14 , which refocuses the light and reflects it onto the detector  20 . 
     However, when using this technique in the frequency range between a few hundred gigahertz and several terahertz, there is an increased number of diverging beams that overfill the object mirrors  14 ,  16  as shown in  FIG. 2 . This in turn causes beams  22  to diverge at the field minor  12  and overlap. This phenomenon causes increased modulation of the final signal. A modulated beam is less observable because of the lack of sufficient signal. This diminishes meaningful detection by the detector  20  because there is increased noise and/or distortion in the signal. 
     An apparatus  100  for optical absorption spectroscopy according to an exemplary embodiment is shown in  FIG. 3 . The apparatus, which in this example is designed for analyzing gas samples, includes a White cell housed within a measurement chamber  110 . A field mirror  112  is positioned facing two object mirrors  114 ,  116  at least 200 millimeters from the field minor. A light source  118  and detector  120  are placed on opposing adjacent edges of the field minor  112 . A fold mirror  130  is disposed on the field minor  112 . As shown in  FIG. 3 , the fold minor  130  is placed to one side of the field minor  112 , but it is contemplated that the fold minor  130  can be disposed on any location on the field minor  112 . The fold mirror  130  is defined by two mirrors  130   a ,  130   b  coupled at approximately a ninety degree angle from one another. The fold minor  130  is angled symmetrically about a normal line on the surface of the field mirror. 
     The addition of the fold mirror  130  causes the light beams  122  to make two passes or allows for a double pass across the field minor  112 . This technique allows the beams and/or spots on the object minors to spread out, reducing or eliminating overlap and provides a clean signal to detect as shown schematically by the beam lines in  FIG. 4 . In other words, the light beams  122  that reflect from a first fold mirror  130   a  will also reflect from the second fold mirror  130   b  as a true image while allowing the light beam reflections to spread out on the object minors  114 ,  116 . This in turn decreases the amount of beam overlap therefore decreasing modulation while the path length remains the same. Additionally, this embodiment does not require additional space or construction to the measurement chamber. 
       FIG. 5  illustrates an embodiment of an apparatus  200  having a dual channel configuration. This embodiment allows for placing multiple channels in a minimum volume. Each system can be stacked to achieve a large bandwidth while keeping the total volume of the measurement chamber to a minimum. In this embodiment a first field mirror  212   a  is positioned facing a pair of first object mirrors  214   a ,  216   a . The first field minor  212   a  and the pair of first object minors  214   a ,  216   a  are coaxially aligned along a first optical axis. Similarly, a second field minor  212   b  is positioned facing a pair of second object minors  214   b ,  216   a . The second field minor  212   b  and the pair of second object minors  214   b ,  216   a  are coaxially aligned along a second optical axis. Each of the first and second field mirrors  212   a ,  212   b  has a respective fold minor  230   a ,  230   b  disposed thereon. Additionally, aligned with each of the first and second filed minors  212   a ,  212   b  are respective light sources  218   a ,  218   b  and detectors  220   a ,  220   b . In this embodiment, the first and second field minors  212   a ,  212   b  may be oriented perpendicular to one another and may be stacked. 
     A methodology of measuring one or more components of a fluid using the optical absorption spectroscopy of the exemplary body is as follows. First, light is reflected from a light source multiple times through the fluid in a sample volume using a field mirror, e.g. field minor  112 , and at least one object mirror, e.g. object mirror  116 . Using a fold mirror, e.g. fold mirror  130 , disposed on the field mirror, the light beams are reflected and double passed across the field minor through a fold mirror. The light transmitted is detected to determine the concentration of one or more components of the fluid. The light is reflected through a gas sample and a good spectrograph signal can then be produced. 
     The methods and systems of the present disclosure, as described above and shown in the drawings, provide for an optical absorption spectroscopy apparatus with superior properties including measuring the concentration of fluid by reducing the overlap and modulation of the light beams to detect a cleaner signal. While the apparatus and methods of the subject disclosure have been shown and described with reference to preferred embodiments, those skilled in the art will readily appreciate that changes and/or modifications may be made thereto without departing from the spirit and scope of the subject disclosure.