SAMPLE SIGNAL FOR INTERFEROGRAM GENERATION AND METHOD FOR OBTAINING SAME

Abstract: A method and means for generating a sampling control signal for use with an infrared spectrophotometer utilizing a Michelson interferometer to obtain light wavelength dispersion and a two frequency laser to control the interferometer operation.

Description: SAMPLE SIGNAL FOR INTERFEROGRAM GENERATION AND METHOD FOR OBTAINING SAME Field of the Invention The present invention pertains to spectrophotometric instrumentation utilizing light to measure spectral absorbance characteristics of a sample material, and in particular to Fourier transform analysis of infrared absorbance characteristics utilizing an interferometer and a laser to obtain spectral data. Background of the Invention The Fourier transform infrared (FT-IR) spectrophotometer consists of two basic parts: (1) an optical system which includes an interferometer through which an infrared light beam is directed before passing the beam through a sample, and (2) a dedicated computer which is used to analyze the spectral information contained in the light issuing from the sample. The advantage in improved performance of the FT-IR spectrophotometer results from the use of the interferometer, rather than a grating or prism, to obtain variance in wavelength of the infrared beam applied to the sample to measure spectral characteristics. An interferometer permits measurement of the entire spectral profile of the sample, increasing accuracy of analysis, in a fraction of the time previously required. The operation of a Michelson interferometer as applied to FT-IR spectrophotometry to vary and analyzing infrared light wavelength passing through a sample is well known. The interferometer consists of two perpendicularly arranged optical paths, each having a reflector or mirror positioned at its end to reflect light traversing the path. The mirror of one path is fixed. The mirror of the other is longitudinally movable to increase or decrease the length of the light path. An infrared light beam entering the interferometer is optically split into two components by a beam splitter so that a separate component of the beam will traverse each optical path. After reflection of each light beam component and redirection along its respective path, the components are recombined through the beam splitter to constructively and destructively interfere. The reconstructed beam is directed through a sample and focused onto a photodetector for measurement of intensity and intensity variance of the range of frequencies in the issuing beam. The intensity characteristic of any selected frequency of the reconstructed light beam depends in part on the difference in length of the optical paths over which the beam components travel. Generally, when the movable mirror is axially moved or scanned at a constant velocity, the intensity of an emerging light beam will modulate in a regular sinusoidal manner for any selected wavelength of light passing through the interferometer. A typical infrared light beam emerging from the interferometer is a complex mixture of modulated frequencies due to its polychromatic nature. After the infrared light beam has passed through a sample material, it can be detected to determine specific wavelengths of light which have been absorbed by the sample. This is accomplished by measuring change in the regular sinusoidal intensity pattern expected when the light beam leaves the interferometer. The measurement of the differences in the characteristics of sinusoidal patterns for each light wavelength composing the emerging beam indicates those wavelengths of light which are absorbed by the sample. Infrared light absorbance characteristics measured provide spectral data from which the matter comprising the sample can be determined. The output signal of the detector which measures the intensity modulation of the emerging light beam can be recorded at very precise intervals during scanning of the movable mirror, to produce a plot known as an interferogram. The inter erogram is a record of data points indicating the output signal produced by the infrared photodetector as a function of the difference in length of the optical paths traversed by. the components of the infrared beam passing through the inter¬ ferometer. Successive scans of the sample are obtained and co-added to obtain an average interferogram having improved signal-to-noise characteristics. The average interferogram provides information and data relating to the spectral characteristics of the sample material. After mathematical preparation, a Fourier transform calculation is performed on the interferogram to obtain a spectral fingerprint of the sample composition. The results are compared with known reference data to determine the composition of the sample. Most Fourier transform techniques require averaging of a large number of interferograms in order to obtain accurate results. As many as 32 to 50 scans of the movable mirror during which measurements are taken may be averaged. It is important that an interferogram be precisely reproducible in order to maintain accuracy in its averaging with other co-related interferograms. Since an interferogram is generated as a function of mirror position, more accuracy in the interferogram and the applied Fourier transformation will be obtained if more accuracy is obtained in the determination of mirror position at the time when the data points are measured to define the interferogram. To accomplish accuracy and reproducibility in generating an interferogram, both the timing of the data point measurement (sampling control) and mirror position change must be very precisely controlled. In other words, the exact position of the mirror along a scan must be determinable when the data point is measured. Modern systems accomplish sampling control and mirror velocity control and/or mirror position measurement by passing a laser beam through the interferometer, concurrently with the infrared light beam. The laser beam is used to directly measure the movement and/or position of the movable mirror to accurately determine change in path length of the interferometer. Since the laser beam undergoes the same optical splitting by the beam splitter and traverses the same change in optical path as the infrared light beam, the recombined laser beam exhibits a measurable monochromatic light wavelength displaying an intensity interference pattern containing information about the scan velocity of the movable mirror. The intensity interference pattern may also serve to indicate position of the mirror during a scan and to determine and correlate the collection of data points at uniform intervals of mirror displacement. In a conventional system, when the movable mirror is moving at a constant velocity, a Doppler shift in light wave frequency is generated in the component of the laser beam traversing the changing length optical path. When the Doppler shifted component is recombined with the component traversing the fixed length path, a modulated frequency beam exhibiting a measurable amplitude modulation or beat signal is produced, yielding a varying intensity or fringe pattern which may be analyzed to determine the mirror position and/or velocity. The beat signal is useful because the frequency of the laser beam produced by most lasers is much too high for measurement by common detectors. Conventional systems generally drive the movable mirror at a velocity which provides a 5 KHz amplitude modulation or beat signal frequency in the exiting beam. The beat signal frequency is equal to the magnitude of the Doppler shift in frequency because it equals the difference in frequency between the recombined light beam components after traversing their respective optical paths. At increased mirror velocities, the beat signal frequency will increase providing increased signal resolution while at slower mirror velocities the beat signal frequency will decrease to a point at which it is not distinguishable. Precision with this technique can be maintained within approximately one cycle in 5,000 to provide very accurate velocity and position information. In a conventional system, however, the movable mirror must be in motion to obtain a Doppler shift in frequency of the light beam traversing its path. Thus motion of the movable mirror is necessary to obtain a measurable beat signal frequency in the recombined light beam. To explain, when the movable mirror is stationary, component, light beams traveling along adjacent paths of the interfereometer are recombined to form an identical frequency light beam since no Doppler frequency shift has been introduced in either component. The emerging recombined beam exhibits no intensity modulation and no beat signal. Thus when the mirror is not moving, there is no information contained in the emerging laser beam which can be used to determine mirror position or mirror velocity. This occurs at every instance that the movable mirror is stopped, such as when it reaches the end of its scan and stops to proceed in the other direction. Furthermore, in a conventional interferometric system the Doppler shift in frequency generated by a scanning mirror produces the same intensity modulation effect in the recombined light beam independent of the direction of mirror travel. For instance, a 5 KHz amplitude modulation or beat frequency can be obtained for travel of the mirror in either a forward or backward direction. Thus it is impossible to determine the direction of mirror travel from the emerging laser beam, even though the difference between optical paths may be increasing or decreasing. This shortcoming generally requires additional circuitry to obtain an indication of direction of mirror travel so that the exact position of the mirror may be determined at any given time. Moreover, in conventional systems the amplitude modulation or beat frequency of the emerging light beam becomes very difficult to measure as the velocity of a scanning mirror becomes very slow. For instance, for a 0.3 centimeter per second scan velocity, a beat frequency of 5 KHz is generated in the emerging light beam. However, if the mirror is driven at a scan velocity of 0.03 centimeters per second, the beat frequency is reduced to .5 KHz, which becomes very difficult to measure. Thus, as the scan velocity is decreased, the modulation frequency in the recombined light beam is decreased to a level which is difficult to measure with modern electronic detectors, reducing accuracy and resolution. An FT-IR spectrophotometer has limited resolution in sample identication determined by its ability to produce and reproduce accurate interferograms. The only dynamic part fundamental to the optical system is the movable mirror of the interferometer. This part greatly determines the accuracy with which a spectrophotometer can generate interferograms. The accuracy with which the spectrophotometer can analyze a sample is directly related to the accuracy and reproducibility of the interferogram and thus the ability of the instrument to control and determine the velocity and position of the movable mirror. The conventional use of a laser reference to control and determine the velocity and position of the movable mirror continues to suffer limited precision in determining sample test and mirror scan control. Improvements in the precision with which mirror position can be measured and mirror velocity controlled will necessarily produce significant improvement in the accuracy with which an infrared spectrophotometer can analyze a sample substance. Summary of the Invention The present invention comprises a method and means for generating a sampling control signal which is useful with an improved mirror scan control for driving a movable mirror of an interferometer in an infrared (IR) spectrophotometer, described in copending Patent Application Serial No. 586,525. The present method and means permits more accurate determination of the timing of sample measurement with respect to position of the movable mirror throughout the scan range. In an IR spectrophotometer having a laser which generates a laser beam having two frequency components and a standard Michelson interferometer, a secondary beat signal has been discovered which possesses information of the exact change in movable mirror position throughout the mirror scan range. The secondary beat signal exhibits a frequency which equals the frequency change introduced to the light beam traversing the movable mirror path, by movement of the mirror. Each cycle of the secondary beat signal is indicative of a precisely determinable (calculatable) change in mirror position. The secondary beat signal may thus be advantageously utilized to generate a sampling control signal which directs sample measurements to be made at precise equal increments throughout a mirror scan to substantially increase accuracy and reproducibility of an interferogram. A laser beam having two frequency components of slightly different frequency is obtained by applying a magnetic field to a helium-neon laser. This phenomenon is well known and described as the Zeeman-splitting effect. The difference in frequencies between the components of the laser beam is stabilized at a selected difference by the laser servo control. This causes a continuous summary beat signal of constant frequency to be displayed by the intensity variation of the light beam due to the heterodyne mixing of the differing frequency components. Stabilization is accomplished by locking the phase of a signal generated from intensity detection of the laser beam which exhibits the beat signal, to a reference signal generated by the reference signal source, having a frequency equal to the selected frequency difference between the components of the laser beam. The laser beam, having differing frequency components selectively separated, is directed through the interferometer. Each frequency component of the beam is combined with its opposing frequency component after traversing their respective optical paths in the interferometer, through optical polarization techniques. The resultant light beam exhibits an intensity modulation or primary beat signal having a frequency equal to the frequency difference of the components, plus or minus a Doppler affected frequency change caused by the scanning of the movable mirror. The continually displayed modulation or primary beat signal frequency of the resultant light beam permits a continuous information signal to be generated indicating the scan velocity and position of the movable mirror in the interferometer. Additionally, the resultant light beam exhibits a latent intensity modulation or secondary reference signal having a frequency equal to the Doppler affected frequency change caused by the scanning of the movable mirror. Since the change in frequency resulting from the Doppler effect is proportional to the speed at which the movable mirror scans, each cycle of a signal having a frequency equalling this change represents a precise displacement of the movable mirror along the scanning path. This secondary beat signal is recognized, processed and used in the present invention to control sample measurements throughout a mirror scan which results in substantially increased accuracy and reproducibility of measurements of absorbtion characteristics of a sample during multiple scans of the movable mirror. The increased resolution in obtaining interferogram data points using this control servo system yields increased accuracy in analysis of the sample material by the spectrophotometer. Description of the Drawings Figure 1 is a schematic representation of an IR spectrophotometer depicting the optical paths of infrared and laser light beams, respectively. Figure 2 is a schematic representation of the interferometric portion of the spectrophotometer depicting the polarization relationship of the individual component frequency modes of the two-frequency laser beam as the beam passes through the interferometer; Figure 3 depicts the wave representations of a heterodyned signal comprising components having frequencies fi and f + Δf, and of a secondary beat signal measurable therefrom. Best Mode of the Invention The interferometer of a Fourier transform infrared (FT-IR) spectrophotometer is described with reference to Figure 1. A Michelson interferometer is depicted which comprises a beam splitter 10 positioned to distribute portions of an incident light beam along each of two perpendicular optical paths 11 and 13. The beam splitter 10 receives a laser beam 16 from a magnetically influenced laser 18, and an infrared light beam, whose boundary is indicated by lines 20, generated by an infrared light source 22. Generally the infrared beam 20 is reflected and collimated by a non-planar mirror 24 for entry into the interferometer, while the laser beam 16 is directly applied to the beam splitter 10 through an opening 26 centrally located in the non-planar mirror 24. The beam splitter 10 directs a first component of each light beam 16 and 20, along a first fixed length optical path 11, which is bounded by a mirror 12. The light beams 16 and 20 are reflected by the mirror 12 to return along the optical path 11 to the beam splitter 10. A second component of each of the light beams 16 and 20 is directed by the beam splitter 10 along a second optical path 13 which is bounded by a movable mirror 14. The movable mirror 14 is longitudinally movable with respect to the optical path 13, to change the length of the optical path within the selected scan range, as indicated by arrow 15. The movable mirror 14 is driven by mirror drive electronics 28 directing a linear motor which is a commercially available element manufactured by Systems Magnetic Co. and available under Part No. ES-11269. The second components of each of the light beams 16 and 20 are reflected from the movable mirror 14 to return along optical path 13 to the beam splitter 10, where they are recombined with the first components of the light beams 16 and 20, respectively, returning along the first optical path 11. The recombination of the first and second components of the laser beam 16 form a heterodyne beam 30 containing information of the velocity and position of the movable mirror 14 through intensity modulation caused by interference of the differing frequencies exhibited by the first and second components. The differing frequencies of the first and second components are due in part to the Doppler effected change in frequency caused in the light component traversing the changing length optical path. The recombined components of infrared beams 20 form a heterodyned beam 32 in which frequencies are modulated at a characteristic rate to provide a detectable frequency range of infrared light which can be applied to the sample material for analysis. In Figures 1 and 2, the directed and reflected beams 16 and 20 are illustrated with a skewed or angular relationship for descriptive purposes and it should be realized that such relationship may not in fact exist in an operable interferometer. The recombined laser and infrared light beams, 30 and 32, respectively, are directed along an exit path 33 of the interferometer in which a reflector 34 similar to reflector 24, is positioned. The reflector 34 receives the collimated infrared, beam 32 and reflects and focuses the beam on a sample chamber 36. The infrared beam 32 passes through the sample chamber 36 to a third mirror 38 and reflects therefrom to focus on an infrared photodetector 40. The photodetector 40 receives the intensity and frequency modulated infrared beam which has been modified by the absorbtion characteristics of the sample material through which it passes. The modulation in the infrared beam is detected to produce an electrical information signal proportional to the beam modulation, which is used to generate an interferogram. The recombined laser beam 30 passes from the interferometer through an opening 42 in the mirror 34. Laser beam 30 is directed to a detector 44. An electrical signal produced by detector 44 is used to obtain a measure of the intensity modulation, i.e., the beat signal, frequency which the light beam 30 exhibits. The detector 44 may simply comprise a single photodetector of appropriate specification centrally positioned in the emerging laser beam 30 to detect intensity modulation therein. The produced signal is applied to a Mirror Scan Servo Control to generate a drive signal which is applied to the Mirror Drive Electronics 28 to control the velocity and direction of movement of the movable mirror 14. The He-Ne laser 10 is magnetically influenced to produce a laser beam 16 having two component frequency modes 16a and 16b separated by a measurable frequency difference, each having opposing circular polarization. The differing frequencies and polarizations of the components 16a and 16b are used to obtain a continuous information signal in the heterodyne laser beams entering and leaving the interferometer. Referring to Figure 2, the laser beam 16 possessing the two component frequency modes is passed through a quarter wave plate 15 before entering the interferometer. The quarter wave plate 15 converts each of the circularly polarized component frequency modes into linearly polarized components. For illustration one of the linearly polarized components which exists in a plane parallel with the drawing, is shown by the bars 17, and has a frequency fj_ . The second linearly polarized component which exists in a plane perpendicular to the drawing, is shown by the dots 19, and has a frequency f2- Thus, since the laser beam directed into the interferometer consists of two individual components each having an identifiable frequency and circular polarization, it is modifiable by polarization techniques to provide two independent optical signals. The first component 21 of the laser beam 16 reflected along the fixed length optical path 11 by beam splitter 10, passes through a second quarter wave plate 23. The beam 16 then reflects from the mirror 12, and passes again through the quarter wave plate 23 to return to the beam splitter 10. Passing this first component 21 of the beam 16 twice through the quarter wave plate 23 rotates the polarization of each component frequency mode through an angle of 90° about the axis of the beam. For example, the first frequency component 17 having frequency f^ which was horizontally polarized upon entering the fixed optical path 11 shown by bars 17, returns to the beam splitter 10 from the optical path 11 with a vertical polarization as shown by dots 17'. Similarly, the second frequency component having frequency f 2 which was vertically polarized entering the fixed length optical path 11 shown by dots 19 returns to the beam splitter 10 from the optical path 11 with a horizontal polarization, shown by bars 19'. The second component 25 of the laser beam 16 which passes through the beam splitter 10 and along the optical path of the moving mirror 13, is reflected from the moving mirror 14 without change in polarization. Each of the reflected component frequency modes of the second components 25 beam may, however, be changed in frequency by a value Δf which is the result of a Doppler effect produced by movement of the movable mirror 14. Thus, the frequency component having frequency f]_ changes in frequency to f ] _ ± Δf, and the frequency component having frequency f 2 changes in frequency to f 2 ± Δf. Since only similarly polarized components of a light beam can combine through the beam splitter 10, the frequency component of the laser beam having frequency fj_ which has traversed the first optical path 11 and which has been changed in polarization by 90°, will recombine with the frequency component of the laser beam having frequency f 2 ± Δf which has traversed the second optical path 13, due to their like polarizations. A recombined component of a light beam 27 in one polarization plane will thus exhibit a frequency of f ι - (f 2 ± Δf). The recombined component of a light beam 29 in the perpendicular polarization plane by the same theory exhibits a frequency of (fj ± Δf) - f 2 « The light beam components 27 and 29, having orthogonal polarization, are directed from the interferometer through a polarizer 31 which removes one of the components 27 or 29. Thus, the detector 44 will receive a light beam having uniplanar polarization and having a frequency which is modulated through the combination of the differing frequency components of the laser beam having traversed different optical paths, respectively, where one may have a varying Doppler effected change in frequency Δf introduced. It should be noted that a Doppler shift in frequency Δf is introduced to the laser beam components only when the movable mirror 14 is in motion. When the ovable mirror 14 is stationary, there is no Doppler effect generated. When the mirror 14 is stationary the frequency component having frequency £ traversing the first optical path 11 will recombine through the beam splitter 10 with the same polarization frequency component having differing frequency f 2 traversing the second optical path 13, to yield a heterodyne frequency light beam which exhibits an amplitude (intensity) modulation, or beat signal, having a frequency equal to the difference between the component frequencies, i.e., f l ~ f 2 * I -^ frequency fj_ equals f 2 as in prior art systems, there is no beat signal generated. Where frequency f-_ differs from f 2 , as with the Zeeman split component frequency modes used in the present invention, a continuous beat signal is generated whose frequency, i.e. the beat signal frequency, is modulated by Doppler affected frequency changes Δf caused in one component. Thus, the photodetector 44 will receive a light beam having a measurable and continuous beat signal, even when the movable mirror 14 is stationary. It has been discovered that the emerging laser beam 30 also exhibits a latent intensity modulation, or secondary beat signal, during a scan of the movable mirror 14. This secondary beat signal has been found to contain information accurately indicating displacement of the movable mirror 14 along the scanning path. The secondary beat signal is measurable during a mirror scan as an amplitude modulation of either of the individual component frequency modes of the laser beam. This amplitude modulation may be defined pictorially as a countermoving sinusoidal envelope, as shown in Figure 3. Referring to this Figure 3, the envelope is identified as 46, and the laser beam component mode signal is identified as 48. The laser beam detected during a scan generates a signal 48 indicative of a component mode frequency with an amplitude that varies periodically with time. The frequency of the beat signal, or amplitude modulation of the envelope 46, is equal to the difference in frequencies of the light beam component which combine to form it, as is commonly known in interfering wave phenomenon. See Halliday and Resnick, Physics, Vol. 1. Thus the frequency of the secondary beat signal is equal to the difference in frequency between some portions of the light beam existing in the interferometer which combine. The frequency of the secondary beat signal has been measured in a laser beam 30 emerging from the interferometer during the scan of the movable mirror 14, and found to have a frequency of approximately 5 Khz while the mirror 14 maintains a constant velocity scan. The secondary beat signal expires as the movable mirror ceases motion. In viewing the information obtained from these measurements it is believed that- the secondary beat signal is a measure of the Dopper affected frequency change introduced to the laser beam component mode traversing the changing length optical path 13, due to the similarity in frequency exhibited (namely 5 Khz) to the theoretical frequency change of 5 Khz anticipated when the movable mirror is scanned at a constant velocity (.108 cm./second for the described instrument). Since the frequency of amplitude modulated signal 48 approximates the frequency of one or the other of the laser beam component frequency modes, it is believed that the secondary beat signal (amplitude modulation frequency) is generated through a heterodyne mixing of an individual component frequency mode of the laser beam having a portion traversing the fixed length optical path 11 and a counterpart traversing the changing length optical path 13 which has had a Doppler affected frequency change introduced by movement of the mirror 14. For example, a component mode having frequency £•_ is recombined with a modified counterpart of the component mode having frequency f ] _ + Δf, or a component frequency mode having a frequency f 2 is recombined with a modified counterpart of the component mode having a frequency f 2 + Δf. It is theorized that the component frequency mode can recombine with its modified like-frequency counterpart having a frequency shift due to Doppler principle in spite of their theoretical orthogonal planes of polarization which normally would prohibit combustion, through imperfect operative characteristics of the interferometer optics, namely quarter wave plates 15 and 23, and polarizer 31. Imperfect operative characteristics of the optics permit a component frequency mode, which is generally reflected or blocked passage due to its vibratory nature orthogonal to the plane of the polarizer or generally nonexistent if ideal light transmission characteristics of the optics prevail, to pass through the interferometer system with a low energy level. This results in an additional complex intensity modulation within the emerging laser beam in each of the polarized planes having a frequency f^ or f 2 which is theoretically to be removed from the emerging beam. For example, the component 27 of laser beam 30 which is shown having horizontal polarization may include low energy intensity modulation of frequency f 2 , in addition to the dominant frequency of f j _ - (f 2 ± Δ) exhibited, which results in a low energy signal of frequency f 2 - (f 2 ± detectable as the secondary beat signal when the movable mirror is scanning. Similarly, the component 29 of light beam 30 which is shown having a vertical polarization may contain a low energy intensity modulation of frequency f^ in addition to the dominant frequency of (f- j _ ± Δ) - f 2 which results in a low energy signal having a combination of frequencies £■_ and £__ ± Δ detectable as the secondary beat signal discussed. The emerging light beam 30 is detected to obtain an electrical signal representative of the secondary beat signal, which is passed through half-wave rectifier electronics to obtain a signal displaying an amplitude modulation unidirectionally from a zero point. This signal is then passed through band-pass filter electronics to remove frequencies other than those approximating the 5 Khz frequency of the secondary beat signal which is of interest. Prefeerably, the band pass filter permits a frequency range of 4 to 6 Khz in the processed signal. » The processed signal possesses a frequency which can be used to identify precise changes in mirror position along a scanning path, because the generated frequency is dependent on the Doppler effect, i.e., movement of the movable mirror. Thus, by maintaining a count of signal cycles in this processed signal, exact displacement of the movable mirror may be determined throughout the mirror scan path. Sample control signal can be easily generated based upon the number of cycles occurring in the processed signal, to obtain sample measurement at an exactly identical mirror position(s) during each scan of the moveable mirror 14. This makes sample data and thus interferogram generation, highly reproduceable. Cyclic count of the processed signal can easily be accomplished by electronic counting systems known in the art which produce an output signal upon reaching a selected count value. The secondary beat signal strength can be controlled and optimized by rotation of the polarizer 21 of the axis of the emerging light beam 30. Rotation of the polarizer 31 changes the plane of polarization of the light which is permitted to pass through the polarizer. By selectively adjusting the rotational position of the polarizer to 31, the strength of the secondary beat signal can easily be modulated and optimized in relation to the signal strengths received from the main components of the emerging beam 30. Thus, it is preferable to provide a means for selectively rotating the polarizer 31 about the optical axis of the emerging beam 30 to provide for optimized signal strength of the secondary beat signal. Detection and processing of the secondary beat signal provides a direct and accurate means for determining changes in mirror positions so that sample measurements can be directed through use of this signal at equal displacement positions of the movable mirror. This permits high accuracy and reproducibility of the data points which are being measured throughout the scan of the movable mirror 14, and results in improved interferogram production where multiple interferograms are measured and averaged to obtain analysis results.

Claims: What is claimed is: 1. In a spectrophotometer comprising a laser for generating a two-frequency laser beam, a light source for generating a light beam, an interferometer for modifying laser and light beam wave length by movement of a mirror, and a detector for detecting intensity of the laser and light beams, an improvement comprising: optical means for selectively polarizing said two-frequency laser beam in more than one plane, and means for measuring modulation of said lasar beam in the first polarization plane to generate a signal providing information indicating movement of said mirror, and for selectively measuring modulation of said lasar beam in a second polarization plane to generate the signal providing information indicating position of said mirror. 2. In a spectrophotometer utilizing an interferometer to modify characteristics of the light beam for identifying a sample material, said spectrophotometer comprising a lasar generating a two-frequency lasar beam, optical elements for selectively polarizing said lasar beam in more than one plane, and means for detecting one polarization of said two-frequency lasar beam to control operation of said interferometer, a method for generating a sample control signal used to control measurement of said light beam passing through said spectrophotometer comprising selectively measuring intensity modulation of a second polarization plane of said two-frequency lasar beam.

Inventors: ANDERSON PHILIP D, STACHELEK THOMAS M, ANDERSON, PHILIP, D., STACHELEK, THOMAS, M.
Applicants: BECKMAN INSTRUMENTS INC, BECKMAN INSTRUMENTS, INC.
Application year: 1985
Main classification: G01J3
IPC codes: G01J3