A dual beam tunable spectrometer

Abstract: A dual beam tunable spectrometer (10) comprises a radiation source, generating means, a detector, and a shutter arrangement. The radiation source (12) generates an incident radiative beam. The generating means, which includes an acousto-optic tunable filter (14), receives the incident radiative beam and generates therefrom a reference beam and a sample beam. The detector (24) detects at least part of the reference beam, and detects at least part of the sample beam emitted from a sample following illumination of the sample with the sample beam. The shutter arrangement includes a first shutter (16) selectively permitting passage therethrough of the part of the reference beam and a second shutter (18) selectively permitting passage therethrough of the part of the sample beam. The shutter arrangement opens the first shutter (16) and closes the second shutter (18) to permit the detector to detect only the part of the reference beam. Similarly, the shutter arrangement closes the first shutter (16) and opens the second shutter (18) to permit the detector to detect only the part of the sample beam.

Description: Background of the Invention Optical spectrometers are utilized to quantitatively measure the concentration of a variety of analytes. Optical spectrometers utilize that portion of the spectrum commonly known as ultraviolet, visible, and infrared radiation. There are two types of commonly used spectrometers: reflectance spectrometers and transmittance spectrometers. Reflectance spectrometers measure the concentration of an analyte when a sample absorbs and scatters portions of a sample beam. The portion of a sample beam that is not absorbed or transmitted is reflected from the sample into a detector and measure. The difference between the reflected sample beam and a reference beam quantitatively indicates the concentration of the analyte in the sample. Similarly, transmittance spectrometers measure the concentration of an analyte when a sample absorbs a portion of a sample beam. However, the sample does not reflect the sample beam into a detector. Rather, a portion of the sample beam is absorbed as it travels through the sample. The difference between the transmitted sample beam and a reference beam quantitatively indicates the concentration of the analyte in the sample. Spectrometers typically employ a radiation source that produces radiation with a wide frequency distribution. Tungsten filament lamps or deuterium lamps are commonly used radiation sources that produce ultraviolet (UV), visible and some infrared radiation (IR). A desired wavelength is selected through the use of filters, diffraction gratings, prisms, acousto-optic tunable filters and other means. In JP 59-92318 a spectrometer is disclosed comprising a light source, an optical fiber to irradiate a reference body and a body to be measured with the light from the light source and a photo detecting element. The spectrometer comprises half mirror to couple part of the light from the light source to the detector and an auxiliary detector. Two shutters are arranged before the detector, one in the path of light from the reference body and the sample body and the other in the path of the light with the auxiliary detector. The two shutters are opened and closed depending if a photometric measurement with the auxiliary detector is to be performed or a measurement of the sample and reference body is to be performed. An acousto-optic tunable filter (AOTF) diffracts light through a sound-light interaction. This phenomenon is described by Professor Chieu D. Tan in Anal. Chem., 64:20, 971-981 (1992), incorporated herein by reference. Briefly, an AOTF is a transparent medium (e.g. quartz, tellurium dioxide (TeO 2 )) in which an acoustic wave can be propagated. The index of refraction of the transparent medium is perturbed by the acoustic wave as it propagates through the medium. The perturbation in the index of refraction arises from compression and rarefaction of the transparent material caused by the travelling acoustic wave. As an incident light beam passes through the transparent medium, the propagating acoustic wave produces a moving grating that diffracts potions of the incident light beam. An AOTF can be constructed such that only the first-order Bragg diffraction is observed. When only the first-order Bragg diffraction pattern is observed, two first-order beams with relative orthogonal polarizations are produced. Typically, the acoustic wave is transduced in the transparent medium by applying a radio frequency (RF) signal in the megahertz region to a piezoelectric transducer attached to the crystalline transparent medium. The use of an acousto-optic tunable filter (AOTF) to diffract a wide frequency light source to a desired frequency is advantageous in that the desired frequency can be obtained almost instantaneously, on the order of microseconds, by tuning the AOTF with a proper RF signal. Spectrometers which utilize an acousto-optic tunable filter are known. U.S. Patent No. 5,039,855 to Kemeny et al. describes a dual beam transmittance spectrometer which utilizes an acousto-optic tunable filter. Kemeny isolates two radiation beams from an AOTF and utilizes one beam as a reference beam and a second beam to analyze a sample (sample beam). Kemeny employs one detector to measure the reference beam and a second detector to measure the sample beam. The obtain accurate readings in spectrometers that use two detectors, the detectors must be matched because the difference between the sample beam and the reference beam measures the analyte of interest. There is a need for a dual beam transmittance spectrometer with one detector capable of measuring both a reference beam and a sample beam. U.S. Patent No. 4,771,629 to Carlson et al. describes a spectrometer comprising a radiation source which is in optical communication with an emerging specie from the chromatographic column by a first optical path, whereby radiation is modified by characteristic of the specie. A detector is in optical communication with the emerging specie by a second optical path whereby a first output signal reflective of the detected radiation is generated. An acousto-optic tunable filter system is disposed in one of either first or second optical paths. The radiation is passed through the crystal at a predetermined angle relative to the crystal's optic axis. A acoustic transducer is coupled to the crystal to launch acoustic waves into the crystal to interact with a selected narrow bandwidth portion of the radiation to make it distinguishable from the remaining radiation. A computer to which the detector output signal is applied determines the emerging specie by the identifiable characteristics. Reflectance spectrometers are known and are available commercially. Typically in a reflectance spectrometer, a sample is illuminated at one angle relative to the sample and the reflectance is detected at a second angle relative to the sample. This configuration is preferably designed to reject specular and surface reflections from the sample so as to minimize the amount of noise reaching the detector and enhance the sensitivity of the spectrometer. Reflectance spectrometers available today are not capable of illuminating a sample at a ninety degree angle relative to the sample and detecting the reflectance from the sample at the same ninety degree angle relative to the sample. Thus, there is a need for a reflectance spectrometer capable of illuminating the sample and detecting the reflectance from the sample at the same angle (ninety degrees) relative to the sample and, at the same time, capable of minimizing the amount of specular and surface reflections reaching the detector. In addition, there is a need for a reflectance spectrometer that utilizes one or both of the orthogonally-polarized first-order beams from an AOTF. Summary of the Invention In one particular embodiment, the present invention provides a dual beam tunable spectrometer comprising a radiation source, generating means, a detector, and a shutter arrangement. The radiation source generates an incident radiative beam. The generating means, which includes an acousto-optic tunable filter, receives the incident radiative beam and generates therefrom a reference beam and a sample beam. Said reference beam is a first polarized first-order beam and said sample beam is a second polarized first-order beam, whereby said reference beam is orthogonally polarized relative to said sample beam. The detector detects at least part of the reference beam, and detects at least part of the sample beam emitted from a sample following illumination of the sample with the sample beam. The shutter arrangement includes a first shutter selectively permitting passage therethrough of the part of the reference beam and a second shutter selectively permitting passage therethrough of the part of the sample beam. The shutter arrangement opens the first shutter and closes the second shutter to permit the detector to detect only the part of the reference beam. Similarly, the shutter arrangement closes the first shutter and opens the second shutter to permit the detector to detect only the part of the sample beam. A polarizing beam splitter is disposed between the sample and said detector, and conveying means direct part of said reference beam and said sample beam into opposite sides of said polarizing beam splitter. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of a dual beam tuneable spectrometer, in accordance with the present invention, capable of taking reflection measurements; and FIG. 2 is a perspective view of a polarizing beam splitter cube employed in the spectrometer in FIG. 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates a spectrometer 100, in accordance with the present invention, capable of taking reflectance readings. This reflectance spectrometer 100 includes an artificial radiation source 102, an acousto-optic tuneable filter (AOTF) 104, a shutter system having a pair of mechanical or electro-optic shutters 106, 108, a polarizing beam splitter cube 110, and a detector 112. The foregoing elements of the spectrometer 100 are described in detail below in conjunction with the description of the operation of the spectrometer 100. The radiation source 102 emits controlled radiation having a wide frequency distribution toward the AOTF 104. Suitable radiation sources include deuterium discharge tubes, incandescent lamps (e.g., tungsten filament), halogen lamps, fluorescent lamps and lasers. A collimating lens 114 or other suitable collimating device is situated between the radiation source 102 and the AOTF 104 so as to colimate the radiation prior to reaching the AOTF 104. To most effectively focus the radiation from the radiation source 102 into a parallel beam, the radiation source 102 is positioned near the principal focus (focal point) of the collimating lens 114. The collimating lens 114 is commercially available as part no. 01 LPX 017 from Melles Griot of Irvine, California. When the collimated radiative beam passes through the AOTF 104, the AOTF 104 process the radiative beam into a zero-order beam and a pair of orthogonally-polarized first-order beams. An RF tuning signal from the RF source 116 controls the wavelength of the first-order beams, and the frequency of the RF tuning signal is, in turn, controlled by the control unit 118. The RF tuning signal may be varied such that the pair of first-order beams from the AOTF 104 have a wavelength falling within the ultraviolet, visible, or infrared spectrum. The RF signal is selected so that the first-order beams from the AOTF 104 have a wavelength(s) encompassing the absorption wavelength(s) of the targeted analyte in the sample. The AOTF 104 is commercially available as part no. TEAF-.80-1.65 from Brimrose Corporation of Baltimore, Maryland. A plano-convex lens 119 directs the zero-order beam toward a beam stop 120, which absorbs the zero-order beam. Furthermore, the lens 119 directs one of the first-order beams (the "reference beam") toward the shutter 106 and directs the other first-order beam (the "sample beam") toward the shutter 108. By controlling the shutters 106, 108 so that only one of these shutters is open at any given time, the detector 112 separately takes reference and sample measurements. The lens 119 is commercially available as part no. LPX 084 from Melles Griot of Irvine, California, and the shutters 106, 108 are each commercially available as part no. VS143SITOK from Vincent Associates of Rochester, New York. To take reference measurements, the shutter system opens the shutter 106 and closes the shutter 108. The open shutter 106 allows the reference beam to pass therethrough, while the closed shutter 108 prevents the sample beam from passing therethrough. The reference beam is focused by a converging lens 122 onto the detector 112 via a conventional mirror 124 and the polarizing beam splitter cube 110. As the mirror 124 receives the reference beam from the direction of the lens 122, the mirror 124 reflects the reference beam in the direction of the polarizing beam splitter cube 110. Referring to FIG. 2, the polarization of the reference beam is parallel to the plane of incidence of the splitter cube 110. Referring to FIG. 2, the polarization of the reference beam is parallel to the plane of incidence of the splitter cube 110, where the plane of incidence is defined as a plane containing the direction of propagation of the reference beam striking the beam splitting face 126 and aline perpendicular to that beam splitting face 126. As defined by the beam splitting face 126, this polarization is commonly referred to as "P" polarization. As viewed in FIG. 1, the plane of incidence is the plane of the page and the "P" polarization of the reference beam is parallel to the plane of the page. The angled face 126 of the polarizing beam splitter 110 reflects approximately 5 to 10 percent of the incident reference beam upward (as viewed in FIG. 1) toward the detector 112, and transmits the remainder (90 to 95 percent) of the reference beam. The detector 112 detects the reflected portion of the reference beam. In one embodiment, the detector 112 is an InGaAs detector focusing on the infrared spectrum and having a built-in amplifier to minimise noise interference. Such a detector is commercially available as part no. SD 6085 from Advanced Photonics of Camarillo, California. In an alternative embodiment, the detector is commercially available as part no. SD 6085 from Advanced Photonics of Camarillo, California. In an alternative embodiment, the detector 112 does not include a built-in amplifier. Instead, since the detected radiation is relatively weak (as low as less than one percent of the first-order reference beam), a lock-in amplifier 128 coupled to the detector 112 is used to measure the detected radiation without noise interference. As is conventional, the amplifier 128 uses some form of automatic synchronisation with an external reference signal, such as the RF tuning signal from the FR source 116. The control unit 118, which is communicatively coupled to the lock-in amplifier 128, measures the amount of radiation detected by the detector 112. For typical reference readings, the shutter 108 remains closed for a few seconds or less. To take sample measurements, the shutter system simultaneously closes the shutter 106 and opens the shutter 108. In this case, the closed shutter 106 prevents any of the first-order reference beam with "P" polarization from reaching the detector 112. The first-order sample beam is focused by a converging lens 130 onto the sample via a conventional mirror 132 and the beam splitter cube 110. The converging lens 130, as well as the converging lens 122, are each commercially available as part no. LDX 069 from Melles Griot of Irvine, California. The mirror 132 reflects the sample beam in the direction of the polarizing beam splitter cube 110. Referring to FIG 2, the polarization of the sample beam is orthogonal to the "P" polarization of the reference beam. Also, the polarization of the sample beam is orthogonal to the plane of incidence of the sample beam, where the plane of incidence is defined as a plane containing the direction of propagation of the sample beam striking the beam splitting face 126 and a line perpendicular to that beam splitting face 126. As defined by the beam splitting face 126, this polarization is commonly referred to as an "S" polarization. As viewed in FIG. 1, the plane of incidence is the plane of the page and the "S" polarization of the sample beam is perpendicular to the plane of the page. The angled face 126 of the polarizing beam splitter cube 110 reflects most of the incident sample beam downward (as viewed in FIG. 1) toward the sample at an angle of 90 degrees relative to the sample. The sample specularly and diffusely reflects the incident portion of the sample beam back upward (as viewed in FIG. 1) toward the polarizing beam splitter cube 110 at the same 90 degree angle relative to the sample. The polarizing beam splitter cube 110 prevents the detector 112 from receiving the specular reflections. In particular, since any specular reflections from the sample retain reflections from the sample retain, by definition, the "S" polarization, the splitter cube 110 reflects these specular reflections back out of the cube 110 along the path that the sample beam entered the cube 110. On the other hand, the randomly polarized diffuse reflections are precipitated by the targeted analyte in the sample. Instead of completely discarding these diffuse reflections, approximately 50 percent of the randomly polarized diffuse reflections are transmitted through the cube 110 toward the detector 112. The detector 112 detects these reflections transmitted through the cube 110, and the control unit 118 measures the amount of radiation detected by the detector 112. The control unit 118 compares the detected radiation from the sample with the previously measured reference value to quantitatively determine the concentration of the targeted analyte in the sample. It should be apparent from the foregoing description that the reflectance spectrometer 100 employs both of the orthogonally-polarized first-order beams from the AOTF 104. One of these first-order beams is used as the reference beam, while the other of the first-order beams is used as the sample beam. The spectrometer 100 employs the single detector 112 for detecting portions of both the reference beam and the sample beam. Moreover, using the polarizing beam splitter cube 110, the spectrometer 100 is capable of illuminating the sample at a 90 degree angle relative to the sample and detecting the randomly-polarized diffuse reflectance from the sample at the same 90 degree angle relative to the sample. The polarizing beam splitter cube 110 also permits the spectrometer 100 to minimise the amount of specular and surface reflectance that reaches the detector 112. Although the polarizing beam splitter 110 in FIG. 2 is illustrated as cubic in shape, non-cubic beam splitters may also be employed.

Claims: A dual beam tunable spectrometer (100) comprising: a radiation source (102) generating an incident radiative beam; generating means, including an acousto-optic tunable filter (104), for receiving said incident radiative beam and processing said incident radiative beam into a zero-order beam, a first polarized first-order beam, and a second polarized first-order beam, said first first-order beam being orthogonally-polarized relative to said second first-order beam, wherein said first first-order beam is a reference beam and a second first-order beam is a sample beam, a detector (112) for detecting at least part of said reference beam, and for detecting at least part of said sample beam emitted from a sample following illumination of the sample with said sample beam; and a shutter arrangement including a first shutter (106) selectively permitting passage therethrough of said part of said reference beam and a second shutter (108) selectively permitting passage therethrough of said part of said sample beam, characterized in that in said shutter arrangement either said first shutter (106) being open and said second shutter (108) being closed to permit said detector (112) to detect only said part of said reference beam, or said first shutter (106) being closed and said second shutter (108) being open to permit said detector to detect only said part of said sample beam, a polarizing beam splitter (110) is disposed between the sample and said detector (112), and that conveying means direct said reference beam and said sample beam into opposite sides of said polarizing beam splitter (110). The spectrometer of claim 1, wherein said polarizing beam splitter (110) is oriented relative to the polarizations of said reference and sample beams such that said part of said reference beam is being conveyed toward said detector (112) when said first shutter (106) is open and such that a portion of said sample beam is being conveyed onto the sample when said second shutter is open and such that radiation reflected from said sample is being received by said polarizing beam splitter (110) and a randomly-polarized diffusely-reflected portion of said radiation is being conveyed to said detector (112) when said second shutter (108) is open. The spectrometer of claim 1 or 2, wherein said polarizing beam splitter (110) is oriented so, that the plane of incidence of said reference beam is parallel to the direction of polarization of said reference beam and that the plane of incidence of said sample beam is orthogonal to the direction of polarization of said sample beam. The spectrometer of one of the claims 1 to 3, wherein said conveying means includes a converging lens (122,130) and a mirror (124,132). The spectrometer of one of the claims 1 to 4, further including a radio frequency source (116) coupled to said acousto-optic tunable filter (104).

Inventors: JOHNSON RICK J, JOHNSON, RICK J.
Applicants: BAYER AG, BAYER CORPORATION
Application year: 1995
Main classification: G01J3
IPC codes: G01N21, G01J3