Abstract: Fluorescence imaging of tissue is used as a diagnostic tool in which geometric effects and specular reflections are compensated for by normalizing a fluorescence image with a cross-polarized image.

Abstract: A disc serving as a spatial radiation modulator has dispersed radiation filters thereon. Each filter has a transmittance or reflectance modulation function of the form sin2(m&thgr;+p&pgr;/4), where m is a positive integer and p has one of the four values 0, 1, 2, 3. A radiation beam including selected wavelength components is diffracted into an elongated image dispersed according to wavelength. Different wavelength components are focused onto different filters on the modulator and are encoded by correspond filters. Since the modulation functions of the filters are orthogonal to one another, it is possible to extract the amplitude of each wavelength component after it has been encoded or modulated by corresponding filter from the total detected signal during one measurement.

Abstract: An optical analysis system includes an optical filter mechanism disposed to receive light from a light source and configured to optically compress data carried by the light into at least one orthogonal component of the light. A detector mechanism in operative communication with the optical filter mechanism measures a property of the at least one orthogonal component to measure the data.

Abstract: A device and process for measuring and analyzing spectral radiation within a desired wavelength range. A number of radiation sources are provided, in combination with a sensor for detecting radiation within the desired wavelength range. The radiation sources are selected to have spectral characteristics that are linearly independent from one another, but overlap so that, in combination, the radiation sources generate radiation over the entire desired wavelength range. Alternatively, a single radiation source generating radiation over the entire desired wavelength range is provided in combination with a plurality of sensors that have spectral sensing characteristics that are linearly independent from one another, but overlap so that, in combination, the sensors sense radiation over the entire desired wavelength range.

Abstract: A double-beam spectrophotometer of the invention is provided with a sector mirror used at a luminous flux separating portion or at a luminous flux combining portion, and a DC brushless motor is used for actuating the sector mirror to rotate. Since the DC brushless motor is used, the sector mirror can be rotated at high speed with few noise. Also, the double-beam spectrophotometer can measure the change in the short period of time.

Abstract: An apparatus, using two acousto-optical tuned filters for determining the wavelength and angle of approach of a light wave.

Abstract: A high resolution infrared densitometer system is described which comprises an intense thermal light source, an optical system for projecting first and second images of the light source along respective parallel first sample and second reference optical axes, adjustable optical attenuators for selectively balancing the light intensity of the images projected along the sample and reference axes at selected optical density, a spectrometer for filtering background radiation from the projected images except for a selected wavelength, and a detector for measuring intensities of the filtered projected images. A method for making optical density measurements is described.

Abstract: An optical transmittance apparatus has a pair of reference channels of glass rods and a pair of sample channels containing sample fluid. A beam splitter and mirror pass a first portion of an incident beam through an ingoing reference channel, and a second portion through an ingoing sample channel. The first portion is reflected back through an outgoing reference channel, and the second portion back through an outgoing sample channel. The beam splitter and mirror directs the outgoing beams into a common outlet path. A partial disk selectively passes either portion. For positioning the disk, poles of a magnet mounted on the disk axle face windings made about an axis perpendicular to the axle, the windings being on the opposite side of a barrier plate from the poles. Current of one polarity through the windings effects one orientation of the disk, and reversed olarity reorients it perpendicularly.

Abstract: An fixed grating, detector-array type analyzer rapidly multiplexes multiple channels. The analyzer comprises a light source, a detector array-type spectrograph, and an optical multiplexer. The multiplexer can select from one of a plurality of channels. Further, the multiplexer can convolute light passing through it with a reference spectrum for calibration and wavelength drift compensation. Rapid multiplexing is achieved using a fiber optic slit array which "perpendicularly multiplexes" the plurality of channels.

Abstract: A photomultiplier (10) with an elongated rectangular, cathode (20) tilted with respect to the inlet opening (14) is alternately illuminated by a measuring light beam (36) and a reference light beam (40). The measuring light beam (36) and the reference light beam (40) extend in a plane parallel to the longitudinal sides of the cathode (20) and impinge upon the cothode (20), such that they illuminate the same areas of the cathode (20).

Abstract: A spectroscopy characterization module having a particular configuration includes a light tight housing that accepts monochromatic radiation through a first optical port and optically chopped coherent radiation through a second optical port. A material sample is held on a sample mount within the housing according to the spectroscopy characterization technique to be used. A rotatable detector mount enclosed within the housing positions a detector in different positions depending on the measurement being performed. A first one-to-one image forming mirror focuses monochromatic radiation passing through the first optical port onto the material sample. A lens focuses optically chopped radiation passing through the second optical port onto the material sample. A second one-to-one image forming mirror focuses radiation transmitted through the sample onto the detector in a first position while a third one-to-one image forming mirror focuses radiation reflected off the sample onto the detector in a second position.

Abstract: An improved near infrared spectrophotometer is disclosed which reduces wave shift errors arising from hot spots in a light source. A ground quartz plate uniformly diffuses radiation from the light source to evenly illuminate the entrance slit for a diffraction grating. Radiation emerges from the entrance slit with a uniform angular intensity distribution and is dispersed into a spectrum by a reflecting diffraction grating towards an exit slit. A narrow wavelength band of radiation passes through the exit slit to illuminate a sample. Because the entrance slit is uniformly illuminated, without regard to variations in radiation intensity due to hot spots in the source, wave shift errors in the reflectivity measurements for the sample are reduced.

Abstract: A spectroscope device of the dispersion type receives light to be measured and emits it as dispersed light which corresponds to each of wavelengths to be measured. An optical-path switching device directs the dispersed light emitted from the spectroscope device to pass through first and second paths. A first photoelectric converter receives the light which is directed to pass through the first path by the optical-path switching device. A polarizing/separating device polarizes and separates the light, which is directed to pass through the second path by the optical-path switching device, into two polarized lights. Second and third photoelectric converters respectively receive the two polarized lights divided by the polarizing/separating device.

Abstract: In a color measuring instrument, an integrating sphere is used to illuminate the sample and fiber optics are used to carry light diffusely reflected from the sample and from an interior wall of the sphere to a spectrometer. The transmitting ends of the fiber optic bundles are fixed in the housing of the spectrometer as entrance slits for the spectrometer, which includes a fixed grating and one or two arrays of photodetectors to detect the spectra dispersed by the grating from light received from the two transmitting ends.

Abstract: The present invention is drawn to a spectrophotometer using a pulsed xenon flashtube as a light source. The energy to be applied to the light source of the spectrophotometer is set within a range of 0.03 joules to 1.0 joule. Therefore, the intensity of light emitted from the light source is not too great, which allows the spectrophotometer to measure a spectral reflectance under the presence of natural light or equivalents thereof. Further, the charging period of time required for the spectrophotometer to provide a desired light emission intensity is short, so that the light source is capable of emitting many flashes of light per second, wherein the amount of data produced within a predetermined measuring period is increased. Thus, accurate data can be generated.

Abstract: An apparatus and method for conducting optical spectroscopy in a hostile environment comprises a light source, e.g., a laser or an incandescent light connected to a multiplexer for supplying high intensity light to at least two optical fibers. One of the optical fibers extends to a material in the hostile environment to be analyzed. The second optical fiber is connected between the multiplexer and a standard sample for yielding known spectroscopic results. Either the same or additional optical fibers are used to return light from the material to be analyzed. Each optical fiber has an end portion covered by a sheath to shield the optical fiber from the hostile environment. The sheath has an open end covered by a transparent window which is preferably made of diamond, again, to seal and protect the optical fiber in the sheath.

Abstract: A probe, for use with a spectrophotometer, which senses the reflectance of a sample remote from the spectrophotometer. The probe includes a housing having a probe portion positionable proximate the sample, and an integrating chamber disposed within the probe housing and having a radiation input port, a sample port for passing diffused radiation to the sample and returning reflected radiation from the sample, a reference port, and an exit port to receive radiation reflected from the sample through the sample port. The probe further includes a guide for directing radiation to the radiation input port from a radiation source, and an element, responsive to the exit port and the reference port, for selectively conveying reflected radiation from the sample and the wall of the integrating chamber in the probe to the remote spectrophotometer.

Abstract: A spectrophotometer incorporating a pulsed light emitting diode as a source of radiation. The light emitting diode emits substantially monochromatic light thus negating the need for a separate wavelength control. The spectrophotometer may incorporate a pair of light emitting diodes for performing bichromatic spectrophotometric determinations. The light emitting diodes are pulsed with a duty cycle and pulse amplitude such that it is possible to obtain a higher amplitude pulse than the light emitting diode could sustain at a continuous voltage input level. Continuous voltages at this level would damage the light emitting diode.

Abstract: The invention can be used to determine the color to be given to a dental prosthesis, for example, on the basis of color measurements performed on adjacent teeth in the mouth of the patient. An optical fiber instrument picks up light reflected from a tooth and transmits it to the inlet of a spectrocolorimeter which associated with a microprocessor in order to determine the diffuse spectral reflectance of the tooth and to calculate the tristimulus values of its apparent color under various different types of illumination. The invention is particularly suitable for determining the color of dental prostheses.

Abstract: A method for the phase synchronization of step-drive-controlled equipment, in particular of optical modulation devices for multiple-beam spectrophotometers.Starting with the phase comparison of a synchronization signal formed from a sensor signal produced by a fixed phase difference from information signals of the device, the phase relationship of the commutator signals of the stepping motor is controlled with a predetermined reference signal.

Abstract: A method for the phase synchronization of equipment controlled by stepping drives, especially optical modulating systems for multiple-beam spectral photometers. Each time that the equipment is started up, a reference signal is made synchronous with a synchronizing signal which is derived from a sensor signal of the apparatus which is locked in phase with the intelligence signal. For a purely electrical correction the phasing between the intelligence signals and the synchronizing signal is set once. In the event of a phasing error between the synchronizing signal and the reference signal the reference signal is reset.

Abstract: Method and apparatus for measuring light by frequency-modulating a light signal from a sample being measured and demodulating the modulated signal in synchronism with the modulating operation, wherein there is provided a delay circuit which produces a series of pulses for synchronous rectification of the modulated signal in response to and with a predetermined time delay after the modulating operation, so that before measurement of a specific sample the delay time is changed so as to determine the phase of the synchronous rectification pulses most suitable for measurement of the specific sample.

Abstract: A spectrophotometer (10) is provided having the capability to accurately measure spectral reflectance at relatively long sample distances. A first illumination optics arrangement (14) assures uniform illumination to a portion of the sample and a second optical arrangement (20) focuses the reflected image of part of the illuminated sample onto a polychromator (22). Reference beam means are provided so that the polychromator sequentially measures the spectral characteristics of the reference beam and the sample. Continuous monitoring of the illumination at select wavelengths provides illumination normalization data so that a microprocessor (40) can normalize the illumination and compare the reference beam and sample measurements to accurately determine the spectral reflectance characteristics of the sample. Angular and raster scanning capability is also provided.

Abstract: The invention relates to a spectrometer/colorimeter apparatus comprising an opto-electronic card (12) with the essential components of the apparatus mounted thereon, and in particular a spectrometer (14) having an inlet slot (16) and a strip (18) of photodetectors (20), and two optical fiber measurement paths (28) and two calibration paths (26), with the ends of the optical fibers being superposed in the inlet slot (16) of the spectrometer. The apparatus is particularly intended for use in an industrial environment.

Abstract: A double polarized light beam spectrophotometer of a light-source modulation type. A modulated light beam emitted by a light source is conducted through specimen atom vapor generated by a graphite atomizer. Wavelength of light undergone atom absorption is selected and spatially separated into a pair of linearly polarized light beams perpendicular to each other. The pair of the linearly polarized light beams separated are alternately passed through a chopper and received by a photoelectric conversion device to be converted into electric signals which are utilized for determining atomic absorption of the specimen. The phase of modulation of light radiated from the light source is synchronized with phase of a current supplied to the graphite atomizer for heating thereof and the switching timing of the chopper.

Abstract: A timing system for a spectrophotometer having a line driven chopper including a rotating disc with alternate open and opaque segments, index apertures in the disc spaced at invariant angular relationship to one of the open segments for providing indexing pulses having fixed time relationship to the light segments, and a programmable microcomputer assembly for generating a string of timing pulses for timing spectrophotometer operations and adjusting the rate of the timing pulses to provide a fixed number between indexing pulses in fixed phase relationship thereto.

Abstract: The invention provides on a method for measuring the spectrum of a material, wherein a sample (10) of the material to be tested is irradiated with a radiation of required wavelengths and the spectrum signals produced by an intensity measuring unit (34) as a result of the radiation reflected or transmitted by the sample (10) are measured.

Abstract: A system for stabilizing the arc in a DC arc lamp. An AC signal is superimposed on the DC power source of the lamp to cause small regular fluctuations in the arc. The small fluctuations prevent large fluctuations while the small regular fluctuations may be averaged to obtain a constant average value. The average is taken over 5 to 10 periods of the AC signal to produce a constant average signal with a very small standard deviation.

Abstract: In a ratio-type double-beam infrared spectrophotometer, an output signal of the photodetector is frequency discriminated to derive therefrom a component having a fundamental frequency f corresponding to at least one of the sample and reference beam intensities and another component having a frequency 2f corresponding to at least the other beam intensity, and these frequency components are computed to determine the ratio of the sample beam intensity to the reference beam intensity. According to the invention, the component of fundamental frequency f is synchronously rectified into a rectified signal which is further differentiated into a differential value, the differential value is subtracted from the rectified signal to give a signal which is compensated for rapid changes of spectral absorption due to the presence of H.sub.2 O and CO.sub.2 in the beam paths during wavelength scanning and is used as the f-component signal.

Abstract: In a double-beam spectrophotometer intended for spectral analysis in the infrared region, inputs to a light detector are switched to dark, reference light, sample light, and reference light in this order, and those components in an output signal of the detector which have a fundamental frequency corresponding to one cycle of the switching and a frequency twice the fundamental frequency undergo arithmetic operation to determine the ratio of the sample to reference light intensity, which is representative of the transmittance of the sample.

Abstract: Spectrophotometer apparatus and method is disclosed which utilizes a totally unobstructed test light flux channel in combination with a periodically occluded reference light flux channel. The outputs of the test and reference channels are mixed and presented to a common light analyzing device from which successive timed-multiplexed measurements can be used to provide spectrophotometric data automatically corrected for drift in critical parameter values associated with the common source and/or light analyzing device. For example, by subtracting light flux measurements obtained during occluded periods from those obtained when the reference channel is not occluded, a measurement can be derived for the light flux passing through the reference channel alone. This derived measurement can then be used to drift-correct measurements made on the light flux coming through the test channel alone when the reference channel is occluded. Comparison of successive light flux measurements made under similar conditions (i.e.

Abstract: Disclosed is an optical system used in a spacecraft to observe a remote surface and provide a spatial and spectral image of this surface. The optical system includes aspheric and spherical mirrors aligned to focus at a first focal plane an image of the surface, and a mirror at this first focal plane which reflects light back on to the spherical mirror. This spherical mirror collimates the light and directs it through a prism which disperses it. The dispersed light is then focused on an array of light responsive elements disposed at a second focal plane. The prism is designed such that it disperses light into components of different wavelengths, with the components of shorter wavelengths being dispersed more than the components of longer wavelengths to present at the second focal plane a distribution pattern in which preselected groupings of the components are dispersed over essentially equal spacing intervals.

Abstract: In a reflectance measuring instrument, an integrating sphere is provided to illuminate the sample of which the reflectance is to be measured. A first fiber optic cable is provided to receive a light beam reflected by the sample and a second fiber optic cable is positioned to receive a light beam reflected from the wall of the integrating sphere. The transmitting ends of the fiber optic cables are mounted in a pivoted plate to selectively position the transmitting ends to emit light into the entrance slit of a monochromator.

Abstract: This invention relates to an optical system which will enable easy use in the field. Known systems either require sample and reference targets to be imaged within the same telescope field or do not enable accurate identification of the measuring field in a view finder.The present invention overcomes this problem by providing a system having a first branch (10, 12) and a second branch (14) which receive light from a sample and reference target respectively. A shutter 22 sequentially allows light from the two branches to pass to a mirror 20 having a slit 30, reflective portions 32 and transparent portions 34. Light from the first branch passes through portions 30 and 34 to a detecting branch (42, 46, 50-56) and some light is reflected through a lens 16 to eye piece 19. Similarly some light from the second branch passes through mirror 30 and 34 to eye piece 19 and some light is reflected to the detecting branch.

Abstract: Disclosed herein is a spectrophotometer which includes a source of radiation; an optical system for forming a beam of radiation from the source and directing it along a sample path and along a reference path, which paths come together at a location to form a combined path that impinges on a detector; a chopper mounted at said location for successively directing one or the other of the beams along the combined path or providing a blank chopper area for blocking radiation from the combined path; a monochromator having a grating and slits disposed in the combined path, the angular position of the grating being adjustable for passing selected successive wavelengths of radiant energy through the optical system; and controls are provided for controlling the movement of the grating and/or slits so that movement only occurs when the blank chopper area is blocking the combined path.

Abstract: An instrument is disclosed which performs measurements and calculates the average and the variance of the measurements. The particular instrument illustrating the invention is a spectrophotometer which measures sets of dark, reference and sample spectra for use in calculating an average absorbance spectrum and its variance. The measurements are performed in an order which enables the cancellation of measurement variation due to variation in instrument response. The order of measurements is determined by measurement subroutines which can be aggregated to produce a series of measurements performed over a user selected period of integration time.

Abstract: A spectrophotometer which can be used selectively as a single-beam and a double-beam type and comprises a light source, a monochromator, a chopper mirror for causing the monochromatic light beam from the monochromator to alternately advance along a first and a second path, in which a reference and a sample cell are disposed respectively, a beam combiner for causing the alternate beams to advance along a common path leading to a photoelectric detector, and a mirror movable for selective positioning in and out of the monochromatic light beam between the monochromator and the chopper mirror.

Abstract: The accuracy of the ratio between two time-varying quantities represented by electrical signals, such as the sample transmission (or absorption) signal and the reference signal in a double-beam, ratio-recording, infrared spectrophotometer, is impaired if the electrical signals are subject to inconstant phase shifts. The present invention counteracts the effect of phase shift variations, whatever their origin, by alternately reversing the order in which an elemental component of one quantity is made to occur with respect to an elemental component of the other quantity so that the said effect reverses in sign when the order is reversed and substantially cancels out when the respective elemental portions are combined to form the numerator and the denominator of the ratio. The invention is particularly described with reference to a spectrophotometer of the type referred to.

Abstract: Apparatus for monitoring gaseous pollutants (e.g. on a chemical plant) produces two infra-red laser beams having different wavelengths (one corresponding to an absorption line of the gas to be monitored, the other being a reference) which are modulated with different frequencies or phases, and then combined into a single scanning beam. The scanning beam is continuously displaced angularly so as to scan the area to be monitored. A portion of the beam is returned to the apparatus by scattering or reflection, and is measured. By correlating this with the direction of the beam, a plan of the monitored area superimposed with a continuously updated indication of the amount of gas can be displayed e.g. on a VDU.

Abstract: A phase synchronization apparatus useful for synchronizing the sample signal and the demodulation signal at a spectrometer includes a stepper motor the position of which is controlled so that the desired phase synchronization is ensured.

Abstract: An instrument is disclosed which performs measurements and calculates the average and the variance of the measurements. The particular instrument illustrating the invention is a spectrophotometer which measures sets of dark, reference and sample spectra for use in calculating an average absorbance spectrum and its variance. The measurements are performed in an order which enables the cancellation of measurement variation due to variation in instrument response. The order of measurements is determined by measurement subroutines which can be aggregated to produce a series of measurements performed over a user selected period of integration time.

Abstract: Spectrum analyzer for the light absorption of a solution comprising a small transparent measuring cell containing the solution; a light source; means for dividing the beam of light of the source into two beams, one being the reference beam and the other the measuring beam transmitted respectively to a reference absorber element and to the measuring cell; means for selecting the measuring wavelength; means for modulating the reference and measuring beams which have respectively traversed the absorber element and the measuring cell; means for detecting the energies of each of the modulating beams after they have traversed the reference absorber element and the sample; the modulation means comprising alternate transparent and bireflecting sectors in front of each of the beams which have passed through the reference absorber and the measuring cell, said sectors being associated with means for selecting the wavelength of the beams in such a way that, as a function of the successive positions of the sectors relativ

Abstract: An ultraviolet light beam is split into two beams which are directed toward a reference cell and a sample cell, respectively. A mechanical rotating light chopper permits the reference and sample cells to be alternately illuminated. Radiation from the reference and sample cells is directed toward and sensed by a single photomultiplier tube. An up/down counter counts the number of photons impinging on the photomultiplier tube from the reference and sample cells. Photodiodes in conjunction with the mechanical light chopper cause the up/down counter to count up when photons are arriving from the sample cell and cause the up/down counter to count down when photons are arriving from the reference cell. A source intensity monitor prohibits further measurements after a preselected amount of incident flux has been sensed. The intensity monitor includes a photodetector circuit which senses a portion of the incident light.

Abstract: In a spectrophotometer including a photomultiplier tube and an operational amplifier responsive to the current from the photomultiplier tube for generating a DC voltage proportional to light intensity, a method and means for simultaneously compensating for the dark current of the photomultiplier tube and the offset of the operational amplifier so that the output voltage of the operational amplifier is zero volts under dark conditions.

Abstract: A dual-beam ratio recording spectrophotometer has signal processing circuitry for determining the transmittance of a substrate. The signal processing circuitry has a bandwidth which is dependent on the magnitude of the radiation passing through a reference cell and includes control logic circuitry which produces a pulse whose width is proportional to the square of the magnitude of the radiation detected when the beam passes through the reference cell. A sample and hold circuit whose sample time is controlled by this pulse samples the output of an integrator which produces at regular intervals a signal representative of the transmittance of a substance. Thus, the response time of the sample and hold circuit and hence the bandwidth of the signal processing circuitry depends on the magnitude of the radiation passing through the reference cell.

Abstract: In a spectrophotometer of the single wavelength-double beam type or the dual wavelength type, light signals of predetermined periods having an interval during which reference light serving as a reference enters, an interval during which sample light to be measured enters and an interval during which entering light is intercepted are detected and converted into electric signals by means of a photomultiplier. The electric signals are converted into digital signals by means of an A/D converter. Using the digital signals, a digital processing unit executes operations for measuring the transmission factor or the absorption of a sample and operations for changing the supplied voltage of the photomultiplier so as to keep the reference signal constant at all times. On the basis of a voltage operated in the digital processing unit, the supplied voltage of the photomultiplier is changed in the interval during which the entering light is intercepted, whereby the stability of the feedback system is enhanced.

Abstract: A dual beam spectrophotometer includes a radiation chopper, a monochrometer, and a diffraction grating moved by a stepper motor. The chopper includes a gate pulse generator which produces gate pulses G.sub.S and G.sub.R corresponding to periods during which radiation from the source passes through a sample cell and a reference cell respectively. A stepper motor drive circuit produces pulses to step the stepper motor which are synchronized with the chopping cycle by means of the G.sub.R and G.sub.S pulses. The stepper motor is arranged to step an equal number of times in each half of a chopping cycle. A further condition which is preferably satisfied is that the first pulse of a sequence should start in the opposite half of the chopping cycle from that in which the first pulse started in a previous chopping cycle. At low stepping rates the motor will step twice in one chopping cycle and then pause for several cycles before again stepping twice.

Abstract: A dual beam spectrophotometer includes signal processing circuitry for producing sample-dark and reference-dark signals. The dark signals substracted from the sample and reference signals are the average of the dark signals produced on each side of the sample and reference pulses. The signal processing circuitry may comprise four sample and hold circuits which store the sample signal, reference signal and first and second dark signals, a resistive combining network for the two dark signals and first and second substractor circuits. The first substractor circuit provides the sample-dark signal at its output while the second substractor circuit provides the reference-dark signal at its output.

Abstract: The control apparatus is used with a dual beam spectrophotometer in which light of variable frequency from a light source is split into two beams which are directed along first and second optical paths to a photosensitive device. The control apparatus comprises electric control, processing and memory circuits coupled to the spectrophotometer for receiving and storing signals therefrom such as an output signal indicative of the particular wavelength of the light beams at any point in time and a log difference signal indicative of absorbance. The circuits include a first memory and a second memory for receiving and storing the log difference signals and control and processing circuitry for manipulating the data stored in the memory circuits. A wavelength digital display, an absorbance digital display and a CRT are provided. The control and processing circuitry causes a graph of wavelength v.

Abstract: Process and apparatus for measurement of brightness of milled products, especially flour, are disclosed. The process is of the general type in which the surface of a layer of flour is illuminated, and the returned quantity of light is taken to an optical electrical transducer. A light source is used for illumination, and the light beam is optically decomposed into at least two part-beams (reference beam and measurement beam) for transmission to the transducer. At least one of the part-beams is cyclicly interrupted or broken into two essentially time-shifted measurement phases in such a way that of the two part-beams, only one of the two-part beams will strike the transducer during a first measurement phase, and during a second measurement phase at least the other part-beam will be applied to the transducer.