Abstract: The invention relates to a device for measuring a Doppler frequency shift, including an optical signal channel delivering a signal light beam illuminating a reference medium; and a reference channel delivering a reference light beam for detecting the Doppler frequency shift. The frequency shift is determined from the difference in frequency between the light signal illuminating the reference medium and the light beam returned by the reference medium. The device includes at least two light radiation sources, preferably lasers, with one light radiating source generating the light beam for the signal channel and the other source generating the light beam for the reference channel.

Abstract: Machine-implemented methods and apparatuses to automatically set-up a signal processing system are described. The signal processing system is set to a first bandwidth. A sampling frequency of the signal processing system is set to a first sampling frequency. Next, first samples of first signals are received at the first bandwidth and the first sampling frequency. First parameters of the first signals based on the first samples are determined. Next, a second sampling frequency is determined based on the first parameters to sample second samples. The first parameters of the first signals may be a mean transit time, a minimum transit time, a mean frequency of the signals, and a standard deviation of the frequency of the signals. Next, a mixer frequency is determined based on the first parameters. A low pass filter is set based on the mixer frequency.

Abstract: Preferred embodiments of the present invention are directed to systems for phase measurement which address the problem of phase noise using combinations of a number of strategies including, but not limited to, common-path interferometry, phase referencing, active stabilization and differential measurement. Embodiment are directed to optical devices for imaging small biological objects with light. These embodiments can be applied to the fields of, for example, cellular physiology and neuroscience. These preferred embodiments are based on principles of phase measurements and imaging technologies. The scientific motivation for using phase measurements and imaging technologies is derived from, for example, cellular biology at the sub-micron level which can include, without limitation, imaging origins of dysplasia, cellular communication, neuronal transmission and implementation of the genetic code.

Abstract: A partial coherence interferometer incorporates a focusing system for resolving measurement ambiguities. A focus-sensing beam is directed through a common objective with the measurement beam of the interferometer for conveying the beams to and from a test surface. An unambiguous measuring range is equated to a predetermined range of focusing errors.

Abstract: The aim of the invention is to provide a method for determining the frequency response of an electrooptical component, particularly, for example, of a light-generating or light-modulating component, which is easy to carry out. To this end, the invention provides a method during which optical pulses with a pulse frequency (fp) are generated. The electrooptical component (60) is controlled by an electrical measuring signal (Smess) with a measuring frequency (fmess) in such a manner that an optical output signal (Saus) is formed that is modulated with the measuring frequency (fmess). The measuring frequency (fmess) is equal to an integral multiple of the pulse frequency (fp) plus a predetermined frequency offset ($g(D)f). The pulses and the output signal (Saus) are mixed, and a mixed product (M) is detected whose modulation frequency corresponds to the predetermined frequency offset ($g(D)f).

Abstract: An instrument for use in a borehole, the instrument including a pressure tuned light source disposed in a housing adapted for insertion into the borehole.

Abstract: A method and system for performing two-dimensional laser Doppler vibrometry (LDV) are disclosed. A high speed fiber optic heterodyne imaging vibrometer can be used for the imaging of high speed surface deformation and/or vibration. Images provided by the high speed fiber optic heterodyne imaging vibrometer can be representative of movement, e.g., displacement or vibration, of the surface being imaged.

Abstract: A method for triggering a heterodyne interferometer having two acousto-optical modulators in separate light paths, a receiver generating an analog signal and a downstream A/D converter converting the analog signal into a digital signal is described. The one acousto-optical modulator is triggered at a modulation frequency f1 and the other acousto-optical modulator is triggered at another modulation frequency f2, the difference between modulation frequencies f1 and f2 forming a heterodyne frequency fHet and the analog signal being converted into the digital signal in the A/D converter at sampling frequency fa. In such a heterodyne interferometer, a fixed ratio of modulation frequencies is maintained, and they are prevented from shifting due to aging and drift by forming at least two of the frequencies of modulation frequencies f1, f2 and sampling frequency fa from a fundamental frequency fqartz of a common oscillator.

Abstract: An interferometer and method for interferometric analysis are provided. The methodology includes generating first and second light beams from a light source, interacting the first light beam with an object under inspection, forming, from light emanating from the object under inspection in response to the interacting, an image of the light source on an image sensor, projecting the second light beam on the image on the image sensor, the combination of the light emanating from the object under inspection and the second light beam forming a collective image on the image sensor, applying a Fourier transform to the collective image formed on the image sensor, thereby forming a phase image, and isolating a wavefront map of the object under inspection from within the phase image.

Abstract: An apparatus and method are provided for conducting heterodyne frequency-comb spectroscopy. The apparatus includes a first and second frequency-comb generators for generating corresponding first and second continuous wave laser beams, respectively. The first beam defines a spectrum of light having a plurality of modes spaced by a first frequency. The second beam defines a spectrum of light having a plurality of modes spaced by a second frequency that is greater than the first frequency. The first and second beams are combined and the optical power of the combined beam is monitored with a data acquisition system to record a time trace. The recorded time trace is Fourier transformed such that each of spectrums of the first and second beams will exhibit a low-frequency comb. By superimposing the two combs, a beat frequency in a low-frequency region is assigned to an optical frequency.

Abstract: In a tomograpy apparatus: low-coherence laser light is split into measurement light and reference light; the frequency of the reference light is slightly shifted from the frequency of reflected light generated by reflection of the measurement light by a sample; the reference light is optically combined with the reflected light; interference light generated by interference of the reference light with the reflected light when the reference light is combined with the reflected light is detected: fluorescence emitted by excitation of a fluorescent dye or a fluorescent pigment in the sample when the sample is irradiated with the measurement light is detected: a first tomographic image of the sample is formed by the detected interference light, and a second tomographic image of the sample is formed by the detected fluorescence.

Abstract: A system and method for superheterodyne detection in accordance with the invention. The system comprises a first conversion unit for performing a first heterodyne operation on an optical input signal to generate an electrical IF signal. A second conversion unit is electrically or optically coupled to the first conversion unit. The second conversion unit performs a second heterodyne operation to generate an electrical output signal suitable for signal processing.

Abstract: Exemplary embodiments of an apparatus are provided. For example, the exemplary apparatus can include at least one first arrangement providing at least one first electro-magnetic radiation to a sample, at least one second electro-magnetic radiation to a first reference and at least one third electro-magnetic radiation to a second reference. A frequency of radiation provided by the first arrangement generally varies over time. The exemplary apparatus may also include at least one second arrangement which is configured to detect a first interference between at least one fourth electro-magnetic radiation associated with the first electro-magnetic radiation and at least one fifth electro-magnetic radiation associated with the second radiation. The second arrangement is also configured to detect a second interference between at least one sixth electro-magnetic radiation associated with the first electro-magnetic radiation and at least one seventh electro-magnetic radiation associated with the third radiation.

Abstract: A system for probing a DUT is provided, the system comprising a tunable or CW laser source, a modulator for modulating the output of the laser source, a beam optics designed to point a probing beam at a designated location on the DUT, optical detector for detecting the reflected beam, and collection and signal processing electronics. The system deciphers perturbations in the reflected beam by detecting beat frequency between operation frequency of the DUT and frequency of the modulation. In an alternative embodiment, the laser is CW and the modulation is applied to the optical detector.

Abstract: A method for measuring the velocity of a multiphase fluid flowing in a pipe. The method comprises directing at least two collimated beams of light from an illuminator through the multiphase fluid by transparent portions of the pipe. The at least two collimated beams are spaced apart in a direction of flow of the multiphase fluid by a predetermined distance. The method also includes detecting scattered, deflected and attenuated light with at least two photodetectors to produce at least two signals. The at least two photodetectors are associated with the at least two collimated beams. The method also includes calculating a cross-correlation function between the at least two signals to determine a time delay between the signals and calculating the average velocity of the multiphase fluid by taking the ratio of the predetermined distance to the time delay.

Abstract: The present invention includes a system and method for coherent imaging. The system of the present invention includes a light source adapted to provide coherent light to illuminate a sample resulting in optically mixed coherent signals and a detector adapted to receive the optically mixed coherent signals and produce an output signal in response thereto. The optically mixed coherent signals will be a set of sum and difference frequency combinations of the frequencies in the coherent light. A processor is connected to the detector and adapted to ratio a selected two of the optically mixed coherent signals in response to the output signal received by the detector and to generate an image in response to the ratioed optically mixed coherent signals, which can be displayed for a user.

Abstract: An object of the invention is to provide a detection apparatus, an optical path length measuring apparatus, a method for evaluating an optical member, and the like, which can evaluate optical characteristics such as chromatic dispersion and an optical path length at a high speed. Another object is to provide a detection apparatus and a method for detecting a change in temperature, which can evaluate chemical and biological reactions and an effect of hyperthermia with high accuracy. In a detection apparatus 10A, zero-order light B0 undergoing chromatic dispersion of a measurement object S is superimposed on first-order light B1 shifted in frequency, whereby a low-frequency beat is produced, and wavelength dependence of a beat phase thereof is measured. Moreover, using an AC electric signal from a frequency shifter 12 as a phase reference signal, a relative position of a probe signal is detected.

Abstract: There is provided a wavelength determining device including a reference wavelength measuring section 42 that, based upon a number A of interference fringes generated by an optical path difference of first reference wavelength light (wavelength: ?1) and a number C of interference fringes generated by the optical path difference of second reference wavelength light (wavelength: ?2), measures the wavelength of the second reference wavelength light, an input light wavelength measuring section 44, based upon the number A of the interference fringes generated by the optical path difference of the first reference wavelength light (wavelength: ?1) and a number B of interference fringes generated by the optical path difference of input light (wavelength: ?x), measures the wavelength of the input light, a correction coefficient determining section 46 that determines a second correction coefficient k based upon the measured wavelength ?c of the second reference wavelength light and the measured wavelength ?m of the inpu

Abstract: An apparatus and to a method of monitoring an interferometer, comprising the steps of: coupling a first optical signal into the interferometer and into a wavelength reference element, detecting a first resulting interference signal being a result of interference of parts of the first optical signal in the interferometer, detecting a resulting reference signal of the wavelength reference element, the resulting reference signal being a result of interaction of the first optical signal with the wavelength reference element, and comparing the first resulting interference signal with the resulting reference signal to detect a drift of the interferometer, if any.

Abstract: Systems and methods are described for faster processing of multiple spatially-heterodyned direct to digital holograms.

Abstract: A swept wavelength imaging optical interrogation system and a method for using the same to interrogate one or more biosensors are described herein. The swept wavelength imaging optical interrogation system is built upon a swept wavelength optical interrogation technology where a 2-D label free image is extracted from a series of high speed spectral images of the biosensor(s) without the need of performing mechanical scanning.

Abstract: A system is disclosed for detecting a signal field from a sample volume. The system includes a source system and an optical fiber system. The source system provides a first electromagnetic field at a first frequency and a second electromagnetic field at a second frequency that is different from the first frequency. The optical fiber system includes at least one optical fiber for guiding the first and second electromagnetic fields to the sample volume to generate coherent radiation characteristic of molecular vibrations by non-linear interaction of the first and second fields with the sample volume. The optical fiber system also receives the signal field resulting from the coherent radiation, and guides the signal field to a detector.

Abstract: The present invention relates to a cascade type interferometric nonlinear optical imaging apparatus, and the imaging apparatus comprises a light source for generating Stokes beam having a desired frequency band and a pump beam for exciting medium molecules in a sample with the Stokes beam; a phase shifting unit for shifting a phase of the beams passing through a standard sample and a phase of an anti-Stokes beam generated from the standard sample on the same path, wherein the Stokes beam and the pump beam generated from the light source pass through the standard sample at the same time to generate the anti-Stokes beam; a scanning unit for scanning the beam phase-shifted by the phase shifting unit onto a space of a test sample; and a detecting unit for detecting optical interference due to a phase difference between a light signal generated by passing the test sample and a light signal generated by passing the standard sample.

Abstract: Time magnification and heterodyning are combined to allow the single-shot characterization of the electric field of optical waveforms. The electric field of the source under test is obtained by Fourier processing of the magnified temporal intensity of the source heterodyned with a monochromatic source. An experimental implementation of this technique is characterized and used to measure various optical signals.

Abstract: An interferometery system for making interferometric measurements of an object, the system including: a beam generation module which during operation delivers an output beam that includes a first beam at a first frequency and a second beam at a second frequency that is different from the first frequency, the first and second beams within the output beam being coextensive, the beam generation module including a beam conditioner which during operation introduces a sequence of different shifts in a selected parameter of each of the first and second beams, the selected parameter selected from a group consisting of phase and frequency; a detector assembly having a detector element; and an interferometer constructed to receive the output beam at least a part of which represents a first measurement beam at the first frequency and a second measurement beam at the second frequency, the interferometer further constructed to image both the first and second measurement beams onto a selected spot on the object to produce

Abstract: An imaging method and associated system for producing high-resolution images. The method includes illuminating an object or scene with coherent radiation such as beams from a laser and then, collecting scattered light with a plurality of subapertures rather than a single large aperture. The method continues with coherently detecting, such as with heterodyne detection, the scattered light to measure the complex amplitude incident on each subaperture and digitally reconstructing images from the coherently detected light for the subapertures. Then digital co-phasing is performed on the subapertures using an image sharpness or quality metric to form an image having the resolution of the total subaperture area. The method may also include determining an aimpoint in the formed image, calculating a phase screen, directing laser beams through the subapertures towards the aimpoint, and co-phasing the laser beams by applying the phase screen to form a single beam.

Abstract: An optical-phase monitoring apparatus includes an optical switch, an optical-phase detecting circuit, and a display circuit. The optical switch multiplexes signal light and a control optical pulse, and outputs a portion of the signal light that overlaps with the control optical pulse in time as output light. The optical-phase detecting circuit detects an optical phase of the output light. The display circuit displays information concerning the optical phase.

Abstract: A coherent differential absorption lidar (DIAL) device 2 comprises a transmit portion 4 for directing a combined light beam to a remote target and a receive portion 5 for receiving light returned from the remote target and for coherently mixing the received light with its associated local oscillator beam. The combined beam comprises at least two component light beams of discrete wavelengths. The device further comprises a signal correction means. The signal correction means comprising a means for extracting a portion of each component light beam from the transmit portion, a means for introducing a frequency difference 62 between each extracted component light beam and its associated local oscillator beam and a means for directing the extracted beam into the receive portion. This provides an additional correction signal thereby improving device performance.

Abstract: A novel technique using an acousto-optic modulator (AOM) as part of a heterodyne interferometer which measures optical path differences between a test signal and a reference signal is disclosed. An array of distinct frequencies are used to drive the AOM, yielding a spatially dispersed array of frequency-shifted subaperture beams of the test signal which are interfered with the wavefront to be measured and then combined with the dispersed reference signal. The frequency shifting of the AOM allows a single detector to collect the beams for signal processing to determine a measurement of the wavefront.

Abstract: An improved technique for acoustic sensing involves, in one embodiment, launching into a medium, a plurality of groups of pulse-modulated electromagnetic-waves. The frequency of electromagnetic waves in a pulse within a group differs from the frequency of the electromagnetic waves in another pulse within the group. The energy scattered by the medium is detected and, in one embodiment, the beat signal may be used to determine a characteristic of the environment of the medium.

Abstract: A heterodyne optical signal analyzer (HOSA) permits accurate reconstruction of an optical input signal (Es) in the time domain. In one embodiment, a vector representation of the light is used to account for two polarization states of the optical signal. The components of a heterodyne optical signal analyzer (10), including optical couplers (12), all have errors and offsets. For example, optical power detectors (16) are very sensitive to changes in polarization of the optical signal (Es) and of the reference signal (Er). Several HOSA calibration procedures including detector calibration, vector calibration, and reference signal calibration are described.

Abstract: An apparatus and method for generating a terahertz (THz) wave are provided. The apparatus comprises: an fiber optic probe injecting an optical wave transmitted through an optical fiber into a device under test (DUT); a driving oscillator generating and injecting an electrical wave into the DUT; and the device under test (DUT) generating a THz wave using the produced optical and electrical waves.

Abstract: The present photonic RF generation and distribution system provides a system and method for distributing an RF output signal. The photonic RF distribution system includes two optical sources for generating optical signals. A first optical source (42) is operable to generate a first optical signal having an operating frequency. A second optical source (44) is operable to generate a second optical signal having an operating frequency. A modulator (46) is operable to impress an RF modulation signal on a tapped portion of the first optical signal such that a modulated signal is generated. A first coupler (52) combines the modulated signal with a tapped portion of the second optical signal, thereby forming a combined signal having a difference frequency component. A control photodetector (50) is responsive to the combined signal to generate a tone signal.

Abstract: An optical tomograph is equipped with: a light source unit for emitting a plurality of light beams, the wavelengths of which are swept within different predetermined wavelength bands respectively with the same period; light dividing means, for dividing each light beam into a measuring light beam and a reference light beam; combining means, for combining reflected light beams, which are the measuring light beams reflected by a measurement target when the measuring light beams are irradiated thereon, with the reference light beams; interference light detecting means, for detecting an interference light beam, which is formed by the reflected light beam and the reference light beam being combined by the combining means, for each of the light beams as an interference signal; and tomographic image processing means, for generating a tomographic image of the measurement target employing the plurality of interference signals detected by the interference light detecting means.

Abstract: Disclosed herein is an apparatus for measuring the differential mode delay of multimode optical fiber. The apparatus includes a tunable laser source, an interferometer, a data collecting device, and a computer. The tunable laser source outputs light, frequencies of which vary linearly. The interferometer generates multimode light and single mode light by separating light, which is output from the tunable laser source, transmitting the multimode light and the single mode light to the multimode optical fiber, which is a measurement target, and a single mode path, which is a reference, and generating a beating signal by causing the multimode light and the single mode light to interfere with each other. The data collecting device collects the beating signal from the interferometer. The computer converts the frequency components of the beating signal into components in a time domain, and calculating the mode delay of the multimode light.

Abstract: A polarization control system includes a light source that generates two light beams with different polarization states and optical frequencies. A polarization state modulator changes the polarization states of the two light beams. A first detector path generates a first beat signal from the two light beams passing through a first polarizer. A second detector path generates a second beat signal from the two light beams passing through a second polarizer that is oriented substantially orthogonal to the first polarizer. An amplitude detector generates an amplitude beat signal from the first and the second beat signals. The system then uses the amplitude beat signal to determine how to adjust the polarization state modulator in order to generate the first and the second light beams with the desired polarization states.

Abstract: A method and apparatus for correcting an input beam (102) that uses an array of detectors (114) sensing a scanning fringe pattern to generate phase error information which can be corrected by a physically adjacent beam correction device such as an array of micro-electrical-mechanical-system (MEMS) mirrors (116).

Abstract: Systems and methods are described for recording multiple spatially-heterodyned direct to digital holograms in one digital image.

Abstract: A method is provided for predicting an installed performance parameter of an optical fiber cable. The method includes obtaining a measurement indicative of a value of the performance parameter at a first moment in time. A measurement indicative of a value of the performance parameter at a second moment in time may then be obtained. A first correlation may then be determined between the measurement at the first moment in time and the measurement at the second moment in time. A value of the performance parameter at the second moment in time may then be estimated based upon the measurement at the first moment in time in combination with the first correlation, the first correlation being based upon observations of a manner in which the performance parameter varies over time for at least a second optical fiber.

Abstract: Machine-implemented methods and apparatuses to automatically set-up a signal processing system are described. The signal processing system is set to a first bandwidth. A sampling frequency of the signal processing system is set to a first sampling frequency. Next, first samples of first signals are received at the first bandwidth and the first sampling frequency. First parameters of the first signals based on the first samples are determined. Next, a second sampling frequency is determined based on the first parameters to sample second samples. The first parameters of the first signals may be a mean transit time, a minimum transit time, a mean frequency of the signals, and a standard deviation of the frequency of the signals. Next, a mixer frequency is determined based on the first parameters. A low pass filter is set based on the mixer frequency.

Abstract: In a method for measuring an absorption coefficient and a reduced scattering coefficient of a multiple scattering medium, a source light beam is outputted, and is transformed into a transformed light beam that includes a mutually parallel circularly polarized photon pair. The transformed light beam is split into a signal beam, which is focused and projected into the multiple scattering medium to produce a diffused polarized photon pair density wave, and a reference beam, which is converted into a reference heterodyne interference signal. The diffused polarized photon pair density wave is converted into a test heterodyne interference signal. Amplitude attenuation and phase delay of the signal beam that has propagated through the multiple scattering medium is obtained based on the reference and test heterodyne interference signals, from which the absorption coefficient and the reduced scattering coefficient of the multiple scattering medium are inferred.

Abstract: An optical frequency measurement apparatus of the present invention includes: a coarse light frequency measurer that computes coarse light frequency (Fw) of measured input light based on a reference wavelength that is a wavelength of reference light; a pulsed light source that generates pulsed light including a plurality of optical frequency components with different repetition frequency (fs1) based on reference frequency (Fs) that is frequency of the reference light; a first beat signal generating section that generates a first beat signal having a first difference frequency (fc) that is difference frequency between optical frequency of the pulsed light closest to optical frequency of the measured input light among the plurality of optical frequency components and the optical frequency of the measured input light; a frequency measurer that measures the first difference frequency (fc) of the first beat signal; and an arithmetic section that computes the fine light frequency (Fx) based on the coarse light freq

Abstract: A microscope includes a photon source which sequentially generates sets of quantum-mechanically entangled photons including at least two photons, a lens which focuses a set of photons, an actuator which varies a relative distance between a focal position of the lens and a specimen with a minute displacement, a detector detecting photons transmitted through or scattered by the specimen, and a counter counting coincidence detections of n-numbers of photons with the detector during a gating time which is set so that a rate that a number of photons detected during thereof belong to a single set of quantum-mechanically entangled photons exceeds a predetermined rate depending on the varied relative distance.

Abstract: Systems and methods are described for obtaining two-wavelength differential-phase holograms. A method includes determining a difference between a filtered analyzed recorded first spatially heterodyne hologram phase and a filtered analyzed recorded second spatially-heterodyned hologram phase.

Abstract: Methods and related systems for determining properties of optical systems (e.g., interferometers) and/or optical elements (e.g., lenses and/or lens systems) are described. For example, information related to an optical thickness mismatch of an interferometer can be determined by providing scanning interferometry data. The data typically include obtaining one or more interference signals each corresponding to a different spatial location of a test object. A phase is determined for each of multiple frequencies of each interference signal. The information related to the optical thickness mismatch is determined based on the phase for each of the multiple frequencies of the interference signal(s).

Abstract: A method and apparatus for detecting stray particles upon a relatively smooth surface by measuring the amount of collimated light reflected from the surface from clear areas of the surface and from particles resting on the surface. Optical pick-up units similar to those used in DVD's and CD's can be used. A method for detecting defective depressions is also covered.

Abstract: A method is provided for predicting an installed performance parameter of an optical fiber cable. The method includes obtaining a measurement indicative of a value of the performance parameter at a first moment in time. A measurement indicative of a value of the performance parameter at a second moment in time may then be obtained. A first correlation may then be determined between the measurement at the first moment in time and the measurement at the second moment in time. A value of the performance parameter at the second moment in time may then be estimated based upon the measurement at the first moment in time in combination with the first correlation, the first correlation being based upon observations of a manner in which the performance parameter varies over time for at least a second optical fiber.

Abstract: A distributed fiber sensor based on spontaneous Brillouin scattering uses a single-frequency fiber laser as a source and a cw Brillouin fiber ring laser as an OLO to optically shift the frequency of the OLO to set the Brillouin/OLO beat frequency within the bandwidth of a conventional heterodyne receiver. The distributed fiber sensor is capable of real-time measurement of both temperature and strain.

Abstract: A method spectrally and temporally characterizing the fluorescence and/or phosphorescence of a material includes using a frequency-domain cross-correlation fluorometer-phosphorimeter employing an interferometer. A flux-modulated excitation signal is used to excite the material. An interferometer is used to generate an optical path difference (OPD) between portions of an emission signal produced by the excited material to obtain an optical interference signal. The optical interference signal is detected by a heterodyning detection system to determine phase shift and demodulation of the emission signal relative to the excitation signal. Phase shift and demodulation data are acquired at a series of OPD values generated by the interferometer and at a plurality of light source flux-modulation frequencies at each OPD to spectrally and temporally resolve the emission signal.

Abstract: In one embodiment, a method determines the spectral content of an optical signal. Specifically, the optical signal and an optical local oscillator (LO) signal are provided to inputs of an optical hybrid (e.g., an N×N optical coupler where N is greater than two). The phase-diverse components from the optical hybrid are photodetected allowing for mixing of the optical signal and the optical local oscillator. Bandpass filtering is performed to eliminate or reduce relative intensity noise (RIN). The filtered signals are mixed with an electrical LO signal. A quadrature representation of a phase-diverse heterodyne signal is generated from signals from the mixing. The negative image and the positive image from the quadrature representation are separated. The spectral content of the optical signal is determined from the images.