Zibang Zhang

Single-pixel imaging by means of Fourier spectrum acquisition

Single-pixel imaging techniques enable to capture a scene without a direct line of sight to the object, but high-quality imaging has been proven challenging especially in the presence of noisy environmental illumination. Here we present a single-pixel imaging technique that can achieve high-quality images by acquiring their Fourier spectrum. We use phase-shifting sinusoid structured illumination for the spectrum acquisition. Applying inverse Fourier transform to the obtained spectrum yields the desired image. The proposed technique is capable of capturing a scene without a direct view of it. Thus, it enables a feasible placement of detectors, only if the detectors can collect the light signals from the scene. The technique is also a compressive sampling like approach, so it can reconstruct an image from sub-Nyquist measurements. We experimentally obtain clear images by utilizing a detector not placed in direct view of the imaged scene even with noise introduced by environmental illuminations.

Applicability analysis of wavelet-transform profilometry

The applicability of the wavelet-transform profilometry is examined in detail. The wavelet-ridge-based phase demodulation is an integral operation of the fringe signal in the spatial domain. The accuracy of the phase demodulation is related to the local linearity of the phase modulated by the object surface. We present a more robust applicability condition which is based on the evaluation of the local linearity. Since high carrier frequency leads to the phase demodulation integral in a narrow interval and the narrow interval results in the high local linearity of modulated phase, we propose to increase the carrier fringe frequency to improve the applicability of the wavelet-transform profilometry and the measurement accuracy. The numerical simulations and the experiment are presented.

Three-dimensional single-pixel imaging with far fewer measurements than effective image pixels.

Typical single-pixel imaging techniques inherently consume a large number of measurements to reconstruct a high-quality and high-resolution image. Three-dimensional (3-D) single-pixel imaging with both high sampling efficiency and high depth accuracy remains a challenge. We implement fringe projection virtually by exploiting Helmholtz reciprocity. Depth information is modulated into a deformed fringe pattern whose Fourier spectrum is sampled by using sinusoidal intensity pattern illumination and single-pixel detection. The fringe pattern has a highly focused first-order component in its Fourier spectrum, which allows us to efficiently acquire the depth information from measurements far fewer than illumination pattern pixels. The 3-D information is retrieved through Fourier analysis. We experimentally obtained a 3-D reconstruction of a complex object with 599×599 effective pixels, achieving a measurement-to-pixel ratio of 5.78%. The depth accuracy is evaluated at sub-millimetric level by using a test object.

Hadamard single-pixel imaging versus Fourier single-pixel imaging

Single-pixel imaging which employs active illumination to acquire spatial information is an innovative imaging scheme and has received increasing attentions in recent years. It is applicable to imaging at non-visible wavelengths and imaging under low light conditions. However, single-pixel imaging has once encountered problems of low reconstruction quality and long data-acquisition time. Hadamard single-pixel imaging (HSI) and Fourier single-pixel imaging (FSI) are two representative deterministic model based techniques. Both techniques are able to achieve high-quality and efficient imaging, remarkably improving the applicability of single-pixel imaging scheme. In this paper, we compare the performances of HSI and FSI with theoretical analysis and experiments. The results show that FSI is more efficient than HSI while HSI is more noise-robust than FSI. Our work may provide a guideline for researchers to choose suitable single-pixel imaging technique for their applications.

Spatial quasi-phase-shifting technique for single-frame dynamic fringe analysis

Phase demodulation from carrier-frequency fringe patterns is the core of many optic measurements. We propose spatial quasi-phase-shifting technique by expressing the fringe signal in the frequency-modulated form, which requires only one frame fringe pattern for instantaneous and dynamic measurements. In an area smaller than a fringe period, there substantially exists an approximately constant phase shift between spatially adjacent sample points. The technique is capable of demodulating the phase with such intra-frame phase shifts, which makes the instantaneous and dynamic measurement possible. The technique implements demodulation within three spatially adjacent neighbors, achieving spatial localization as good as a several-point level. Both numerical simulation and experiment are presented to verify its performance.

Fast Fourier single-pixel imaging via binary illumination

Fourier single-pixel imaging (FSI) employs Fourier basis patterns for encoding spatial information and is capable of reconstructing high-quality two-dimensional and three-dimensional images. Fourier-domain sparsity in natural scenes allows FSI to recover sharp images from undersampled data. The original FSI demonstration, however, requires grayscale Fourier basis patterns for illumination. This requirement imposes a limitation on the imaging speed as digital micro-mirror devices (DMDs) generate grayscale patterns at a low refreshing rate. In this paper, we report a new strategy to increase the speed of FSI by two orders of magnitude. In this strategy, we binarize the Fourier basis patterns based on upsampling and error diffusion dithering. We demonstrate a 20,000 Hz projection rate using a DMD and capture 256-by-256-pixel dynamic scenes at a speed of 10 frames per second. The reported technique substantially accelerates image acquisition speed of FSI. It may find broad imaging applications at wavebands that are not accessible using conventional two-dimensional image sensors.

Single-frame rapid autofocusing for brightfield and fluorescence whole slide imaging

A critical consideration for whole slide imaging (WSI) platform is to perform accurate autofocusing at high speed. Typical WSI systems acquire a z-stack of sample images and determine the best focal position by maximizing a figure of merit. This strategy, however, has suffered from several limitations, including low speed due to multiple image acquisitions, relatively low accuracy of focal plane estimation, short axial range for autofocusing, and difficulties in handling transparent samples. By exploring the autocorrelation property of the tissue sections, we report a novel single-frame autofocusing scheme to address the above challenges. In this approach, we place a two-pinhole-modulated camera at the epi-illumination arm. The captured image contains two copies of the sample separated by a certain distance. By identifying this distance, we can recover the defocus distance of the sample over a long z-range without z-scanning. To handle transparent samples, we set an offset distance to the autofocusing camera for generating out-of-focus contrast in the captured image. The single-frame nature of our scheme allows autofocusing even when the stage is in continuous motion. We demonstrate the use of the our autofocusing scheme for fluorescence WSI and quantify the focusing performance on 1550 different tissue tiles. The average autofocusing error is ~0.11 depth-of-field, 3 folds better than that of conventional methods. We report an autofocusing speed of 0.037 s per tile, which is much faster than that of conventional methods. The autofocusing range is ~80 µm, 8 folds longer than that of conventional methods. The reported scheme is able to solve the autofocusing challenges in WSI systems and may find applications in high-throughput brightfield/fluorescence WSI.

13-fold resolution gain through turbid layer via translated unknown speckle illumination

Fluorescence imaging through a turbid layer holds great promise for various biophotonics applications. Conventional wavefront shaping techniques aim to create and scan a focus spot through the turbid layer. Finding the correct input wavefront without direct access to the target plane remains a critical challenge. In this paper, we explore a new strategy for imaging through turbid layer with a large field of view. In our setup, a fluorescence sample is sandwiched between two turbid layers. Instead of generating one focus spot via wavefront shaping, we use an unshaped beam to illuminate the turbid layer and generate an unknown speckle pattern at the target plane over a wide field of view. By tilting the input wavefront, we raster scan the unknown speckle pattern via the memory effect and capture the corresponding low-resolution fluorescence images through the turbid layer. Different from the wavefront-shaping-based single-spot scanning, the proposed approach employs many spots (i.e., speckles) in parallel for extending the field of view. Based on all captured images, we jointly recover the fluorescence object, the unknown optical transfer function of the turbid layer, the translated step size, and the unknown speckle pattern. Without direct access to the object plane or knowledge of the turbid layer, we demonstrate a 13-fold resolution gain through the turbid layer using the reported strategy. We also demonstrate the use of this technique to improve the resolution of a low numerical aperture objective lens allowing to obtain both large field of view and high resolution at the same time. The reported method provides insight for developing new fluorescence imaging platforms and may find applications in deep-tissue imaging.

Spatially-incoherent annular illumination microscopy for bright-field optical sectioning

Numerous advanced microscopic imaging techniques have been proposed for optical sectioning, but they generally employ a complex and costly optical system. Here we report a microscopy termed spatially-incoherent annular illumination microscopy (SAIM). It allows for simple, effective, non-fluorescence, and bright-field optical sectioning. The proposed technique is implemented by installing an annular array of light emitting diodes (LEDs) on a standard bright-field microscope for illumination. The LED array produces distinctive illumination, that is, each LED provides coherent, large-angle oblique illumination while all LEDs generate spatially-incoherent annular illumination. Such a distinctive illumination can improve both lateral resolution and axial resolution. The improvement of lateral resolution is due to the coherent and large-angle oblique illumination. The spatially-incoherent annular illumination can improve the axial resolution. It is because, for defocused structures, each LED results in a blurred image with a different lateral shift and all LEDs result in an incoherent stagger superposition of the defocused images. The superposition looks much more blurred, which improves the contrast of the in-focus image remarkably. We experimentally demonstrate that SAIM is able to provide bright-field optical sections with 600-nm axial resolution and 150-nm lateral half-pitch resolution by using a 525-nm wavelength LED array and an objective with 100X, numerical aperture (NA) 1.25.

Phase-sensitivity-doubled surface plasmon resonance sensing via self-mixing interference

Conventional phase-based surface plasmon resonance (SPR) sensing can achieve 10-8  RIU, but commonly requires two-beam interference. It therefore leads to complexity in terms of utilized devices, poor anti-noise ability, and demand for fine working conditions. With these requirements imposed, conventional SPR sensing has difficulties in commercial use. In this Letter, we report a simple, compact, and phase-sensitivity-doubled self-mixing interference (SMI)-based SPR sensing approach. The reported approach employs SMI and, therefore, needs only one optical path, enabling the advantages of compactness and simplicity in experimental setup, and strong anti-vibration property. With the proposed setup, the phase of light from the prism to the sample changes twice. Consequently, the sensitivity of phase is doubled. For experimental demonstration, we monitor the refractive index change of NaCl solution by using the proposed technique. The experimental results coincide with the theoretical analysis and simulation results.