KEYWORDS: Digital signal processing, Mirrors, Heterodyning, Signal processing, Cameras, Interferometry, Signal detection, Phase measurement, Imaging systems, Interferometers
We describe a heterodyne interferometry system based on a complementary metal-oxide semiconductor digital signal processor (CMOS-DSP) camera that is utilized for full-field optical phase measurement using a carrier-based phase retrieval algorithm, with no need for electro-mechanical scanning. Camera characterization test results support the adoption of a single-pixel approach to perform quasi-instantaneous differential phase measurements, which are immune to mechanical vibrations and thermal drifts. We developed an optical configuration based on a Mach-Zehnder heterodyne interferometer to perform a static test on a mirror surface. The profiles of the mirror surface set at two angular positions, the relative displacements in the range of nanometers, and the corresponding tilt angle were determined.
This paper describes two setups suitable for interference microscopy for high resolution morphological measurements on
living cells in culture medium. The first system incorporated a PZT actuator in the reference path of a Mach Zehnder
configuration to facilitate digital phase-stepping interferometry. The second system employed two phase-locked acoustooptic
modulators to generate a temporal optical carrier to allow a heterodyne approach to phase demodulation. This setup
incorporated a digital CMOS camera with full random pixel access which allowed the heterodyne approach to be
implemented as a full-field method without any need for electromechanical scanning. The heterodyne approach offers
benefits over the phase-stepping method in terms of measurement resolution and speed, typically offering the equivalent
of nanometer resolution for cell height measurements with a bandwidth in the order of 200-300 Hz for 1000 pixels.
Results for morphological measurements using both systems on red blood cells and keratinocytes are presented.
Fluctuations in the output intensity and wavelength of an external cavity diode laser can introduce significant error to wavelength-tuned interferometric measurement. However, a robust phase-retrieval algorithm can compensate for these nonlinearities. Employing an inexpensive phosphor-coated charge-coupled device camera sensitive to C-band infrared, full-field interferometric phase retrieval utilizing wavelength tuning of a 1555 nm external cavity diode laser is reported. Phase measurement of a tilted mirror is presented with an estimated accuracy within 7 nm.
Digital stepping is desirable in optical metrology--operation is simple, absolute position is known, and random regions of interest can be skipped to, rapidly and accurately. However, in white-light interferometry, analog scanning has traditionally been employed because, in one operation, it achieves depth scanning of a sample and an electronically detectable optical carrier through a Doppler shift. This is not obligatory nor efficient in functional machine vision, especially if approximate preknowledge of the sample exists. Two methods, utilizing digital depth stepping and a superluminescent diode, are presented to decouple optical carrier generation from depth scanning in full-field white-light interferometry. One technique employs a complementary metal-oxide semiconductor camera and acousto-optic modulation to generate a frequency difference between two arms of a Mach--Zehnder interferometer. The other technique uses a Michelson interferometer with a piezoelectric transducer integrated to the digital stepper motor to facilitate 2λ analog scanning and an optical carrier of 4 periods, sampled with a standard charge-coupled device camera. In the former case, random depth access measurement of an engineering gauge block calibration sample is presented, while the latter demonstrates the application of the random depth access full-field white-light interferometry to a small punch test. A further benefit of these techniques is the possibility of interferometric phase retrieval on condition of path length matching; this is proven by the implementation of a heterodyne phase retrieval algorithm in the gauge block measurement. Both techniques represent an advance in optical metrology, offering an inexpensive and functional solution to machine vision and industrial measurement applications.
KEYWORDS: Cameras, Optical coherence tomography, Digital signal processing, Signal processing, Signal to noise ratio, Semiconductors, Optical signal processing, Optical filters, Light sources, 3D image processing
Full-field optical coherence tomography (OCT) using a complementary metal-oxide semiconductor (CMOS) camera with an integrated a digital signal processor (DSP) is demonstrated. The CMOS-DSP camera employed is typically used in machine vision applications and is based on an array of 1024×1024 direct readout pixels that are randomly addressable in space and time. These characteristics enable the camera to be used as a fast full-field detector in carrier-based optical metrology systems. The integrated DSP facilitates basic signal processing including real-time filtering and undersampling. The optical setup used to implement this OCT method is composed of a free-space Michelson interferometer and a superluminescent diode (SLD) light source, with an electromechanical shaker for depth scanning. Unlike classical OCT approaches, however, the setup does not require any electromechanical device for lateral scanning. A 64×30 pixel region of interest was imaged at 235 frames/s and sampled in depth, corresponding to a volumetric measurement of 875×410×150 µm. Measurements carried out on a simple calibration specimen indicated lateral and axial resolutions of 14 and 22 µm, respectively. The presented approach offers an inexpensive and versatile alternative to traditional OCT systems and provides the basis for a functional machine vision system suitable for industrial applications.
KEYWORDS: Cameras, Optical coherence tomography, Digital signal processing, Machine vision, Signal to noise ratio, Signal processing, Sensors, CMOS sensors, Aluminum, 3D image processing
Presented is a comprehensive characterisation of a complementary metal-oxide semiconductor (CMOS) and digital signal processor (DSP) camera, and its implementation as an imaging tool in full-field optical coherence tomography (OCT). The camera operates as a stand-alone imaging device, with the CMOS sensor, analogue-to-digital converter, DSP, digital input/output and random access memory all integrated into one device, autonomous machine vision being its intended application. The 1024x1024 pixels of the CMOS sensor function as a two-dimensional photodiode array, being randomly addressable in space and time and producing a continuous logarithmic voltage proportional to light intensity. Combined with its 120dB logarithmic response range and fast frame rates on small regions of interest, these characteristics allow the camera to be used as a fast full-field detector in carrier based optical metrology. Utilising the camera in an OCT setup, three-dimensional imaging of a typical industrial sample is demonstrated with lateral and axial resolutions of 14μm and 22μm, respectively. By electronically sampling a 64x30 pixel two-dimensional region of interest on the sensor at 235 frames per second as the sample was scanned in depth a volumetric measurement of 875μm x 410μm x 150μm was achieved without electromechanical lateral scanning. The approach presented here offers an inexpensive and versatile alternative to traditional OCT systems and provides the basis for a functional machine vision system suitable for industrial applications.
Most full-field heterodyne interferometry systems are based on complex electro-mechanical scanning devices. In this
study, however, we present an alternative non-scanning approach based on a low frequency heterodyne interferometer
employing standard CCD and CMOS cameras. Two frequency locked acousto-optical devices were used to obtain two
laser beams with an optical frequency difference as low as 3 Hz. The interference of those beams generated a suitably
low frequency carrier signal that allowed the use of a common 25 frame/second CCD camera. Using a digital CMOS
camera and acquiring a limited number of randomly accessible pixels, measurements with much higher carrier
frequencies were also possible.
The advantages of the heterodyne technique with respect to common phase-stepping methods are the shorter response
time and lower sensitivity to sources of uncertainty such as drift, vibrations and random electronic noises. In order to
directly compare the heterodyne and phase-stepping techniques experimentally, the same interferometer was used for
both methods. The switching between operation modes was achieved by simply altering the electronic driving signals of
the acousto-optical devices where for the phase-stepping mode, the frequency difference of the driving signals was set
to zero. The phase steps were obtained by a piezo-driven mirror. Comparing the phase difference between two pixels in
an image, approximately 0.01 radian of standard deviation, corresponding to a resolution of λ/628, was achieved by
heterodyne technique, as compared to 0.06 radian by the phase-stepping method.
The interferometer with the CMOS camera was applied to monitor the refractive index variation across a micro-channel
where two liquid flows were mixed. Also, the capability for fast, time-resolved full-field optical refractive index
measurements was demonstrated. The examples presented show how the high sensitivity of the heterodyne technique
allows the study of a number of sources of uncertainty that were not otherwise easily quantifiable using standard full field
methods.
KEYWORDS: Cameras, Digital signal processing, Optical coherence tomography, Signal processing, Glasses, CMOS sensors, Signal to noise ratio, CMOS cameras, Coherence (optics), Sensors
A comprehensive characterisation of a complementary metal-oxide semiconductor (CMOS) and digital signal processor (DSP) camera, used typically in machine vision applications, is presented in this paper. The camera consists of a direct read-out CMOS sensor, each pixel giving a direct analogue voltage output related to light intensity, with an analogue-to-digital converter and digital signal processor on the back-end. The camera operates as a stand-alone device using a VGA display; code being pre-programmed to the onboard random access memory of the DSP. High detection rates (kHz) on multiple pixels were achieved, and the relationship between pixel response time and light intensity was quantified. The CMOS sensor, with 1024x1024 pixels randomly addressable in space and time, demonstrated a dynamic logarithmic light intensity sensitivity range of 120dB. Integrating the CMOS camera with a low coherence Michelson interferometer, optical coherence tomography (OCT) axial depth scans have been acquired. The intended application is an imaging device for simple yet functional full-field optical coherence tomography. The advantages of the CMOS sensor are the potential for carrier-based detection, through the very fast pixel response with under-sampling, and the elimination of the electromechanical lateral scanning of conventional OCT by replacing it with electronic pixel scanning.
The measurement of thermal diffusivity on bulk samples has been previously performed using techniques which require a modulated point heating of the surface and a movement of the temperature detector or of the heating source to determine the phase shift of the modulated surface temperature at an increasing distance from the heating source. Such a movement must be realized with great accuracy and can be the cause of many experimental difficulties. The method proposed herewith is based on the fact that the modulated heating of a bulk sample on a surface spot gives a phase shift, between modulated temperature and heating source, which only depends on the ratio between the spot linear dimension and the thermal diffusion length. A frequency sweep produces a variation of the thermal diffusion length and a variation of the measured phase shift between 0° and -45°, thus allowing for a static single sided measurement of the thermal diffusivity of the sample.
This investigation describes the implementation of a Single Pixel Carrier Based Demodulation (SPCBD) approach on a digital CMOS-DSP camera for full-field heterodyne interferometry. A full-field vibration measurement system is presented as an alternative to a classical scanning Laser Doppler Vibrometer (LDV). The Heterodyne set-up, CMOS-DSP camera and the signal demodulation techniques adopted are described. Characterisation tests that describe the basic performance of the CMOS-DSP camera, in terms of acquisition rates and time response are presented. A simple experiment was performed to demonstrate the novel laser vibrometry system that consisted of determining the displacement of a point on the surface of a vibrating mirror. The measured velocity and displacement data were compared to the output from a commercial LDV. The integration of a CMOS sensor, DSP and a laser-doppler interferometer has lead to the development of a fully digital “functional” machine vision system that provides a flexible, compact and inexpensive tool for automated high-precision optical measurements.
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