One-shot phase-recovery using a cellphone RGB camera on a Jamin-Lebedeff microscope.

Chapter 11 - Differential Interference Contrast (DIC) Microscopy

Differential Interference Contrast (DIC) microscopy is a nondestructive imaging modality that has been widely used by biologists to capture microscopy images of live biological specimens. However, as a qualitative technique, DIC microscopy records specimen's physical properties in an indirect way by mapping the gradient of specimen's optical path length (OPL) into the image intensity. In this paper, we propose to restore DIC microscopy images by quantitatively estimating specimen's OPL from a collection of DIC images captured from multiple shear directions. We acquire the DIC images by rotating the specimen dish on the microscope stage and design an Iterative Closest Point algorithm to register the images. The shear directions of the image dataset are automatically estimated by our coarse-to-fine grid search algorithm. We develop a direct solver on a regularized quadratic cost function to restore DIC microscopy images. The restoration from multiple shear directions decreases the ambiguity among different individual restorations. The restored DIC images are directly proportional to specimen's physical measurements, which is very amenable for microscopy image analysis such as cell segmentation.

Optical path length, temperature, and wavelength effects simulation on ozone gas absorption cross sections towards green communications

Chapter 12 - Amplitude Modulation Contrast Microscopy

A new differential interference contrast microscopy (DICM), which uses Savart prisms as a shearing plate and a phase-shifting device, is proposed. The system consists of a phase-shifting module (PSM) and a DICM module (DICMM). The PSM has two Savart prisms: the first prism separates the incident beam into two parallel beams, and the second prism recombines these two beams. The optical path difference (OPD) of the two beams, which is represented by a biased OPD, can be adjusted by rotating the angle of the normal surface of the second prism. In the DICMM, the other Savart prism is used to replace the Nomarski prism (NP) in conventional DICM. It combines with an afocal microscopic system (AMS) to produce a Savart-DICM system, which is able to perform a phase-shifting technique by changing the biased OPD to produce a phase shift of π/2 for each step. This paper describes the configuration and measurement theory of the microscope. The experimental results confirm the validity and capability of the proposed microscope.

In this paper a novel method for determining refractive indices of a multi-layered samples using low coherence interferometry (LCI), developed at the National Physical Laboratory, UK, is introduced. Conventional Optical Coherence Tomography (OCT) utilises a lateral scanning optical probe beam to construct a depth resolved image of the sample under investigation. All interfaces are detected in optical path length, resulting in an image depending on the refractive index of all prior layers. This inherent ambiguity in optical and geometric path length reduces OCT images to purely qualitative ones. We have demonstrated that by optically probing the sample at multiple angles we can determine bulk refractive index of layers throughout plane parallel samples. This method improves upon current approaches of extracting refractive index parameters from multi-layered samples as no prior geometrical information is required of the sample and the phase index for each layer is obtained as opposed to the group index. Consequently the refractive index result for each layer is independent of the refractive index of surrounding layers. This technique also improves on conventional measurements, as it is less susceptible to error due to surface defects. This technique is easily implemented, and can easily be modified to obtain in situ measurements. Investigating a silica test piece and comparing the refractive index obtained by that of standard critical angle refractometry has validated the robustness of the technique.

We report a full-field, phase-shifting microscope with precise control of the optical path difference (OPD) and show topographic phase images of living cells. Our system is based on a Linnik interference microscope with Kohler illumination of a halogen lamp for the imaging lightsource. Phase-sensitive active-stabilization of the OPD is employed with an infrared laser whose optical path is the same as that of halogen light. We previously reported the results of cultured cell topography with this stabilization scheme; however the phase stability and the image quality were insufficient. We have improved the noise-cancellation system and achieved control of the OPD with stability down to 0.7 nm in the bandwidth of 500 Hz. Quarter wavelength phase-shifting was carried out with sub-nanometer accuracy, and clear topographic phase images of cultured single-layer cells were obtained. The Kohler illumination with the halogen lamp, whose coherence length is 2 mm, enables homogenous illumination and suppression of artifact signals arising from optical components not associated with the surface of interest.

Two different sinusoidal wavelength-scanning (SWS) interferometers with a SWS light source using a superluminescent laser diode are proposed for step-profile measurement and real-time distance measurement, respectively. An optical path difference (OPD) longer than a wavelength is measured from detection of sinusoidal phase- modulation amplitude Zb of the interference signal that is proportional to the OPD and the scanning width 2b. In step-profile measurement, if measurement error in the OPD obtained from Zb is smaller than a half wavelength, this measured value of the OPD is combined with a fractional value of the OPD obtained from the conventional phase of the interference signal. This combination enables us to measure the OPD longer than a wavelength with a high accuracy of a few nm. In real-time distance measurement, the amplitude Zb is kept at a specified constant value for changes of OPD by controlling the scanning width 2b of the wavelength with a feedback system. The amplitude Zb is detected by processing the interference signal with electric circuits in real-time. The value of b is easily controlled in the SWS light source, and an OPD longer than a wavelength is measured from the value of b with an accuracy of about a wavelength.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

This paper presents an all-optically controlled, all-fiber optical path-length modulator. The presented system takes advantage of the heating effect induced within vanadium-doped fiber through laser excitation. It can be applied in various applications, for example in white-light interferometry. The system consists of a Michelson interferometer with vanadium-doped fiber in one arm, a 980 nm excitation high-power laser diode, and 1310/1550 nm signal sources or channels. Due to the spectral-absorption properties of vanadium ions in silica, the absorbed optical power emitted by the 980 nm source is mostly converted via a non-radiative relaxation process into heat within the vanadium-doped fiber. A rise of fiber core temperature causes the fiber core refractive index to change and consequently, a change in the optical path difference of the interferometer. The extinction laser diode operates in pulse mode for continuous scanning of the white-light interferometer. The vanadium-doped fiber is, therefore, periodically heated and self-cooled. The optical path difference of the scanning interferometer is simultaneously measured using a high-coherence source that provides the needed reference trace. The achieved modulated optical path is over 150 μm, with a system time constant of below 1 s. This all-optical configuration of the scanning interferometer allows for the remote and electrically passive control of the optical path length differences in various fiber-optic systems. In particular, the proposed design would be suitable as an interrogation system for various sensors, where an absolute optical path length variation/measurement is required.

Guttae are a hallmark of Fuchs endothelial corneal dystrophy (FECD) and the disease’s progression. A posterior fibrillar layer (PFL) that covers central guttae has been previously described, but its formation and progression remain unclear. This study aims to further investigate the characteristics of guttae and the PFL in FECD. In a well-characterized prospective FECD patient cohort, a total of 43 DMEK (Descemet membrane endothelial keratoplasty) specimens were immunostained for ZO1 or COL1, flat-mounted, and analyzed using differential interference contrast (DIC), autofluorescence (excited at 480 nm), and polarized light microscopy. Guttae and PFL were quantified and correlated with clinical data. Guttae were visualized by DIC and autofluorescence imaging and classified into two types with gradual transition: peripheral, knob-like guttae and central, flat guttae. Guttae with low autofluorescence often were covered by corneal endothelial cells (CEnC). Central guttae were found to be covered by a PFL that contained COL1 and was visualized using polarized light microscopy. The PFL displayed a fibrous structure that blurred guttae in DIC illumination. Its presence correlated with clinical parameters, such as anterior scatter and post-operative corneal edema resolution providing insight into the relation of PFL and FECD severity. The combination of DIC, autofluorescence, and polarized light microscopy provides a robust method for investigating guttae and the PFL. This could facilitate preoperative assessment of corneal donor tissue and enhance postoperative clinical diagnostics by detailed detection of guttae and PFL in patients.

The digital micromirror device (DMD) which is one kind of Micro-Electro-Mechanical System has been widely adapted for use in lighting projection systems, and some new imaging systems such as high-dynamic range imaging, aperture-coded spectral imaging, or medical imaging. The micromirrors on the DMD surface are non-continuous distribution and capable of tilt relative to individual rotation axis, which is different from the plane mirror. Therefore, when the DMD is applied in a convergence imaging path, there is an optical path difference (OPD) between the off-axis chief ray and the on-axis chief ray, and moreover severe non-symmetric imaging on the focal plane. Based on the geometrical optics and aberration theory, this paper proposes a calculation formula that gives the OPD as a function of the image height, the incident angle, the DMD pixel size and the DMD tilt angle in the convergent imaging path. This formula can give the OPD of the system when the optical axis is vertical to the surface of the DMD in an ideal state, which has great significance to the design and adjustment of optical path using DMD devices.

Direct optical path measurement in fibre-optic gyroscopes: A potential method for compensating slow-drifting errors

White-light interferometry combined with phase-shift technology becomes a widely used 3-D shape measurement tool in precision engineering. In white-light interference 2&#960; phase ambiguity can be avoided in measurement of optical path difference by searching a modulation peak of light intensity change. This interference surely involves a change in modulation intensity against optical path difference, which is an envelope curve of sinusoidal variations. Then, the phase-shift algorithm by which phase is accurately measured even under the modulation change is required. There is often another requirement that phase shift between captured interferograms would be not restricted to &#960;/2 in the use of the algorithm. Computer simulation has been carried out to estimate phase errors which are retained by applying well-known algorithms to a white-light interference. They are Carre, 4-frame, 5-frame Hariharan, and 7-frame Groot algorithms. All the algorithms have non-negligible errors under the both requirements. Therefore, I extract individual terms (<i>I_i</i>±<i>I_j) </i>in an algorithm equation by considering symmetry of light intensity against phase, where <i>I_j</i> is light intensity just after the <i>j</i>-th shift. Using computer simulation again, I then search for appropriate coefficients by which the terms are multiplied in the equation. I finally have found an algorithm satisfying both the requirements.

The Spatial Infrared Imaging Telescope (SPIRIT III) is the primary sensor aboard the Midcourse Space Experiment (MSX), which was launched 24 April 1996. SPIRIT III included a Fourier transform spectrometer that collected terrestrial and celestial background phenomenology data for the Ballistic Missile Defense Organization (BMDO). This spectrometer used a helium-neon reference laser to measure the optical path difference (OPD) in the spectrometer and to command the analog-to-digital conversion of the infrared detector signals, thereby ensuring the data were sampled at precise increments of OPD. Spectrometer data must be sampled at accurate increments of OPD to optimize the spectral resolution and spectral position of the transformed spectra. Unfortunately, a failure in the power supply preregulator at the MSX spacecraft/SPIRIT III interface early in the mission forced the spectrometer to be operated without the reference laser until a failure investigation was completed. During this time data were collected in a backup mode that used an electronic clock to sample the data. These data were sampled evenly in time, and because the scan velocity varied, at nonuniform increments of OPD. The scan velocity profile depended on scan direction and scan length, and varied over time, greatly degrading the spectral resolution and spectral and radiometric accuracy of the measurements. The Convert software used to process the SPIRIT III data was modified to resample the clock-sampled data at even increments of OPD, using scan velocity profiles determined from ground and on-orbit data, greatly improving the quality of the clock-sampled data. This paper presents the resampling algorithm, the characterization of the scan velocity profiles, and the results of applying the resampling algorithm to on-orbit data.

A multipass trace gas cell with 24 m optical path length and 80 cc volume was developed and built. A direct ray tracing and genetic algorithm was used to determine optimal mirror arrangement. © 2019 The Author(s)