Digital in-line holographic microscopy

We first briefly review the state of the art of digital in-line holographic microscopy (DIHM) with numerical reconstruction and then discuss some technical issues, such as lateral and depth resolution, depth of field, twin image, four-dimensional tracking, and reconstruction algorithm. We then present a host of examples from microfluidics and biology of tracking the motion of spheres, algae, and bacteria. Finally, we introduce an underwater version of DIHM that is suitable for in situ studies in an ocean environment that show the motion of various plankton species.     

Coherence effects in digital in-line holographic microscopy

We analyze the effects of partial coherence in the image formation of a digital in-line holographic microscope (DIHM). The impulse response is described as a function of cross-spectral density of the light used in the space-frequency domain. Numerical simulation based on the applied model shows that a reduction in coherence of light leads to broadening of the impulse response. This is also validated by results from experiments wherein a DIHM is used to image latex beads using light with different spatial and temporal coherence.     

一种基于照明目的的有机白光发光二极管

研制了一种以照明为目标的有机白光发光二极管(WOLED),该二极管在8V时的色度坐标为(x=0．319,y=0．337),对应的显色指数(Ra)为85．4,色温(Tc)为6151K。该二极管是含NPB和CBP两个基质的多层掺杂型结构器件;此外,NPB{4,4‘-bis[N-(1-naphthyl)-N-phenylamino]bipheonyl}除了用作绿光和黄光基质外,还用作空穴传榆材料,CBP{4,4‘-N,N‘-dicarbazole-biplaenyl)用作红光磷光配合物的基质材料。3个掺杂层分别提供白光发射的红、黄和绿光成分,而蓝光成分则来自于空穴传榆层NPB本身的发射。该器件在直流电流密度为0．1mA／cm^2时最大白光发光效率可达5．6cd／A(3．9lm／W),在15V时达到的最大亮度为5100ccl／m^2。其性能参数达到了白光照明光源的要求。     

Automated optimization of an aspheric light-emitting diode lens for uniform illumination

In this paper, an automated optimization method in the sequential mode of ZEMAX is proposed in the design of an aspheric lens with uniform illuminance for an LED source. A feedback modification is introduced in the design for the LED extended source. The user-defined merit function is written out by using ZEMAX programming language macros language and, as an example, optimum parameters of an aspheric lens are obtained via running an optimization. The optical simulation results show that the illumination efficiency and uniformity can reach 83% and 90%, respectively, on a target surface of 40 mm diameter and at 60 mm away for a 1×1 mm LED source.     

Partially coherent lensfree tomographic microscopy [Invited

Optical sectioning of biological specimens provides detailed volumetric information regarding their internal structure. To provide a complementary approach to existing three-dimensional (3D) microscopy modalities, we have recently demonstrated lensfree optical tomography that offers high-throughput imaging within a compact and simple platform. In this approach, in-line holograms of objects at different angles of partially coherent illumination are recorded using a digital sensor-array, which enables computing pixel super-resolved tomographic images of the specimen. This imaging modality, which forms the focus of this review, offers micrometer-scale 3D resolution over large imaging volumes of, for example, 10-15 mm(3), and can be assembled in light weight and compact architectures. Therefore, lensfree optical tomography might be particularly useful for lab-on-a-chip applications as well as for microscopy needs in resource-limited settings.     

Quantitative phase and refractive index measurements with point-source digital in-line holographic microscopy

Point-source digital in-line holographic microscopy with numerical reconstruction is ideally suited for quantitative phase measurements to determine optical path lengths and to extract changes in refractive index within accuracy close to 0.001 on the submicrometer length scale. This is demonstrated with simulated holograms and with detailed measurements on a number of different micrometer-sized samples such as suspended drops, optical fibers, as well as organisms of biological interest such as E. coli bacteria, HeLa cells, and fibroblast cells.     

Pattern recognition with a digital holographic microscope working in partially coherent illumination

We describe the implementation of the automatic spatial-frequency-selection filter for recognition of patterns obtained with a digital holographic microscope working with a partially coherent source. The microscope provides the complex-optical-amplitude field that allows a refocusing plane-by-plane of the sample under investigation by numerical computation of the optical propagation. By inserting a correlation filter in the propagation equation, the correlation between the filter and the propagated optical field is obtained. In this way, the pattern is located in the direction of the optical axis. Owing to the very weak noise level generated by the partially coherent source, the correlation process is shift invariant. Therefore the samples can be located in the three dimensions. To have a robust recognition process, a generalized version of the automatic spatial-frequency-selection filters has been implemented. The method is experimentally demonstrated in a two-class problem for the recognition of protein crystals.     

Hologram reconstruction corrected for measurements through layers with different refractive indices in digital in-line holographic microscopy

Digital in-line holographic microscopy (DIHM) using point sources has been shown to be a versatile technique, especially for three-dimensional tracking of particles or microorganisms. However, the spherical source wave is altered when measurements are performed through layers with different refractive indices, such as water cuvettes. The situations where a layer of medium with a refractive index different than that of the predominant surrounding propagation medium (usually air) is situated behind or in front of the plane to be reconstructed are analyzed in detail, and a general approach for reconstruction under such circumstances is developed. The proposed refractive index correction is tested experimentally and compared to conventional reconstruction algorithms. Using 3D traces of swimming algal spores, the influence on the velocity calculation is also shown.     

Time-averaged in-line digital holographic interferometry for vibration analysis

Time-averaged in-line digital holography is applied for vibration analysis. In particular, by use of a double-exposure approach, simultaneous determination of vibration mode shape and mean static state deformation during a vibration cycle are obtained. The subtraction of two numerically reconstructed digital holograms recorded at the same resonant frequency but with a small difference in amplitude shows the mixing of Bessel-type time-averaged fringes owing to vibration and of the double-exposure fringes owing to differences in the mean deformation of the object. It is shown that separation of these fringe patterns can be readily accomplished numerically. An experimental demonstration of this effect by use of in-line digital holography for relatively small membranes is demonstrated.     

Efficient light harvesting from flexible perovskite solar cells under indoor white light-emitting diode illumination

This is the first report of an investigation on flexible perovskite solar cells for artificial light harvesting by using a white light-emitting diode (LED) lamp as a light source at 200 and 400 lx,values typically found in indoor environments.Flexible cells were developed using either low-temperature sol-gel or atomiclayer-deposited compact layers over conducting polyethylene terephthalate (PET)substrates,together with ultraviolet (UV)-irradiated nanoparticle TiO2 scaffolds,a CH3NH3PbI3-xClx perovskite semiconductor,and a spiro-MeOTAD hole transport layer.By guaranteeing high-quality carrier blocking (via the 10-40 nm-thick compact layer) and injection (via the nanocrystalline scaffold and perovskite layers) behavior,maximum power conversion efficiencies (PCE) and power densities of 10.8％ and 7.2 μW.cm-2,respectively,at 200 lx,and 12.1％ and 16.0 μW·cm-2,respectively,at 400 lx were achieved.These values are the state-of-the-art,comparable to and even exceeding those of flexible dye-sensitized solar cells under LED lighting,and significantly greater than those for flexible amorphous silicon,which are currently the main flexible photovoltaic technologies commercially considered for indoor applications.Furthermore,there are significant margins of improvement for reaching the best levels of efficiency for rigid glass-based counterparts,which we found was a high of PCE ～24％ at 400 lx.With respect to rigid devices,flexibility brings the advantages of being low cost,lightweight,very thin,and conformal,which is especially important for seamless integration in indoor environments.     

Digital holographic microscopy with sequential off-axis illumination directions based on spatial light modulator

Digital holographic microscopy (DHM) with time-sequential off-axis illumination directions can synthesize a large aperture, and thus have a higher spatial resolution than the one with on-axis illumination. In this paper, time-sequential off-axis illumination directions are generated by a spatial light modulator (SLM) in DHM, and the residual phases along different illumination directions are suppressed by using the phase compensation technique, as such the image with resolution enhancement is reconstructed. The usage of SLM enables shifting the illumination for different orientations and phase shifts without mechanical motion. The experiments have been conducted to verify the feasibility of this method on the residual phase suppression.     

Compensation of the inherent wave front curvature in digital holographic coherent microscopy for quantitative phase-contrast imaging

An approach is proposed for removing the wavefront curvature introduced by the microscope imaging objective in digital holography, which otherwise hinders the phase contrast imaging at reconstruction planes. The unwanted curvature is compensated by evaluating a correcting wave front at the hologram plane with no need for knowledge of the optial parameters, focal length of the imaging lens, or distances in the setup. Most importantly it is shown that a correction effect can be obtained at all reconstruction planes. Three different methods have been applied to evaluate the correction wave front and the methods are discussed in detail. The proposed approach is demonstrated by applying digital holography as a method of coherent microscopy for imaging amplitude and phase contrast of microstructures.     

Designing light-emitting diode arrays for uniform near-field irradiance

We analyze the first-order design of light sources consisting of multiple light-emitting diodes (LEDs) to uniformly illuminate a near target plane by considering each single LED as an imperfect Lambertian emitter. Simple approximate equations and formulas are derived for the optimum LED-to-LED spacing, i.e., the optimum packaging density, of several array configurations to achieve uniform near-field irradiance.     

Schlieren systems with coherent illumination for quantitative measurements

Schlieren systems with a coherent light source were investigated by the Fourier optics technique. The imaging properties of the systems with various cutoff filters were studied. Systems with a graded piecewise linear filter and a Gaussian step function convolution (graded) filter are considered, demonstrating that the image can be approximated by the geometrical-optics theory of conventional schlieren systems. A nonlinear phase contribution was estimated, allowing for the measurement of strong phase objects. Within the framework of linear approximation the results are described by the phase derivative point-spread function, introduced in this paper as the schlieren point-spread function. In addition, modification of the Lopez cutoff filter is proposed, demonstrating its superiority over the piecewise linear and the Gaussian step convolution filters. Simulations of coherent schlieren imaging as well as phase derivative measurements were performed. Finally, the imaging properties of the schlieren systems with the different filters are compared.     

Hybrid normal-reverse prism coupler for light-emitting diode backlight systems

For the first time to our knowledge, a hybrid normal-reverse prism coupler was formed on the bottom surface of a light guide in a LED backlight system to achieve a thin, lightweight, LED backlight system. The hybrid prism coupler (HPC) simultaneously exhibits two functions: extraction of guided light from the light guide and focusing the radiated light from the light guide, corresponding to the optical functions of the prism and diffusive sheets used in conventional LED backlight systems. Therefore, using a HPC eliminates the prism and diffusive sheets that have been indispensable optical elements in conventional LED backlight systems, which consequently reduces the thickness of the LED backlight system by 40% compared with conventional systems.     

Topology optimization for optical microlithography with partially coherent illumination

This article revisits a topology optimization design approach for micro‐manufacturing and extends it to optical microlithography with partially coherent illumination. The solution is based on a combination of two technologies, the topology optimization and the proximity error correction in microlithography/nanolithography. The key steps include (i) modeling the physical inputs of the fabrication process, including the ultraviolet light illumination source and the mask, as the design variables in optimization and (ii) applying physical filtering and heaviside projection for topology optimization, which corresponds to the aerial image formulation and the pattern development processes, respectively. The proposed approach results in an effective source and a binary design mask, which can be sent directly to fabrication without additional post‐processing steps for proximity error correction. Meanwhile, the performance of the device is optimized and robust with respect to process variations, such as dose/photo‐resist variations and lens defocus. A compliant micro‐gripper design example is considered to demonstrate the applicability of this approach. Copyright © 2016 John Wiley & Sons, Ltd.     

Space-bandwidth conditions for efficient phase-shifting digital holographic microscopy

Microscopy by holographic means is attractive because it permits true three-dimensional (3D) visualization and 3D display of the objects. We investigate the necessary condition on the object size and spatial bandwidth for complete 3D microscopic imaging with phase-shifting digital holography with various common arrangements. The cases for which a Fresnel holographic arrangement is sufficient and those for which object magnification is necessary are defined. Limitations set by digital sensors are analyzed in the Wigner domain. The trade-offs between the various holographic arrangements in terms of conditions on the object size and bandwidth, recording conditions required for complete representation, and complexity are discussed.     

Design of an in-line, digital holographic imaging system for airborne measurement of clouds

We discuss the design and performance of an airborne (underwing) in-line digital holographic imaging system developed for characterizing atmospheric cloud water droplets and ice particles in situ. The airborne environment constrained the design space to the simple optical layout that in-line non-beam-splitting holography affords. The desired measurement required the largest possible sample volume in which the smallest desired particle size (～5 μm) could still be resolved, and consequently the magnification requirement was driven by the pixel size of the camera and this particle size. The resulting design was a seven-element, double-telecentric, high-precision optical imaging system used to relay and magnify a hologram onto a CCD surface. The system was designed to preserve performance and high resolution over a wide temperature range. Details of the optical design and construction are given. Experimental results demonstrate that the system is capable of recording holograms that can be reconstructed with resolution of better than 6.5 μm within a 15 cm(3) sample volume.     

High-speed holographic microscopy for fast-propagating cracks in transparent materials

A method of high-speed holographic microscopy is developed to take three successive microscopic photographs of a crack tip propagating in a transparent poly(methyl methacrylate) specimen at a speed of several hundred meters per second. When a crack is propagating in a specimen, three Q-switched ruby lasers emit three laser pulses successively. The time interval between each laser pulse and the next is 1 mus or longer. An optical system of angle-multiplexing holography records the crack as three successive holograms on one photographic plate. Crack images are reconstructed and photographed through a conventional microscope. The spatial resolution of the reconstructed images is approximately 114 lines/mm. From the photographs, one can measure crack speed, crack opening displacement, and the dynamic stress intensity factor. The high-speed holographic microscopy makes it possible to study rapid crack propagation in microseconds.     

Analysis of coherent symmetrical illumination for electronic speckle pattern shearing interferometry

In an effort to find a non-contact technique capable of providing measurements of in-plane strain, a speckle shearing interferometer was designed using symmetrical coherent illumination. This paper presents an analysis of the sensitivity to displacement and strain of this interferometer, together with an analysis of the phase-stepping of the resultant fringe patterns. New notation is introduced alongside this analysis to define the interference components in speckle shearing interferometers using multiple illumination beams. Experimental results show fringe patterns and phase stepping in support of the theoretical analysis.