Shengqian Chang

Optimal illumination for visual enhancement based on color entropy evaluation

Object visualization is influenced by the spectral distribution of an illuminant impinging upon it. In this paper, we proposed a color entropy evaluation method to provide the optimal illumination that best helps surgeons distinguish tissue features. The target-specific optimal illumination was obtained by maximizing the color entropy value of our sample tissue, whose spectral reflectance was measured using multispectral imaging. Sample images captured under optimal light were compared with that under commercial white light emitting diodes (3000K, 4000K and 5500K). Results showed images under the optimized illuminant had better visual performance such as more subtle details exhibited.

Generation of propagation-invariant and intensity-controlled dark hollow beams by a refractive beam shaping system

Dark hollow beams (DHBs) have great potential in the applications of optical manipulation, and the generation of DHBs is still a challenging and rewarding issue. In this paper, we present a beam shaping system for generating DHBs. The proposed system is composed of a freeform lens array and a non-classical zoom system which has a constant focal length but various image locations. The DHBs with a well-controlled intensity profile generated by the proposed system is not sensitive to the change of the intensity distribution of the incident beam, which allows flexible choices of light sources. Moreover, the annular pattern produced by the DHB remains unchanged when the image plane is moved a long distance of 17mm, and the energy efficiency of the beam shaping system is greater than 90% when Fresnel loss is considered. The proposed beam shaping system endows the generated DHBs with new properties and may have great potential in the field of optical tweezers and atom guides.

High efficient metasurface for broadband achromatic focusing in visible spectrum

Metasurfaces are expected to realize the miniaturization of conventional refractive optics into planar structures; however, they suffer from large chromatic aberration due to the high phase dispersion of their subwavelength building blocks, limiting their real applications in imaging and displaying systems. In this paper, a high-efficient broadband achromatic metasurface (HBAM) is designed and numerically demonstrated to suppress the chromatic aberration in the continuous visible spectrum. The HBAM consists of TiO2 nanofins as the metasurface building blocks (MBBs) on a layer of glass as the substrate, providing a broadband response and high polarization conversion efficiency for circularly polarized incidences in the desired bandwidth. The phase profile of the metasurface can be separated into two parts: the wavelength -independent basic phase distribution represented by the Pancharatnam-Berry (PB) phase, depending only on the orientations of the MBBs, and the wavelength-dependent phase dispersion part. The HBAM applies resonance tuning for compensating the phase dispersion, and further eliminates the chromatic aberration by integrating the phase compensation into the PB phase manipulation. The parameters of the HBAM structures are optimized in finite difference time domain (FDTD) simulation for enhancing the efficiency and achromatic focusing performance. Using this approach, this HBAM is capable of focusing light of wavelengths covering the entire visible spectrum (from 400 nm to 700 nm) at the same focal plane with the spot sizes close to the diffraction limit. The minimum polarization conversion efficiency of most designed MBBS in such spectrum is above 20%. This design could be viable for various practical applications such as cameras and wearable optics.

Designing two freeform surfaces with Monge–Ampère equation method for point-like sources

Two freeform surfaces provide more degrees of freedom in designing illumination optics and can yield a better solution. The existing methods for point-like sources are mostly valid in designing one freeform surface. Designing two freeform surfaces for point-like sources still remains a challenging issue. In this letter we develop a general formulation of designing two freeform lens surfaces for point-like sources. The proposed method is very robust in designing freeform lenses with two elaborately designed surfaces. The examples clearly show that using two freeform surfaces yields better solutions to challenging illumination problems with ultra-high energy efficiency.

High-resolution DMD-FPM system based on ring pattern phase retrieval algorithm

We report an approach to enhance the resolution of the microscopy imaging by using the fourier ptychographic microscopy (FPM) method with a laser source and Spatial Light Modulator (SLM) to generate modulated sample illumination. The performance of the existed FPM system is limited by low illumination efficiency of the LED array. In our prototype setup, digital micromirror device (DMD) is introduced to replace the LED array as a reflective spatial light modulator and is placed at the front focal plane of the 4F system. A ring pattern sample illumination is generated by coding the micromirrors on the DMD, and converted to multi-angular illumination through the relay illumination system. A series of intensity sample images can be obtained by changing the size of the ring pattern and then used to reconstruct high resolution image through the ring pattern phase retrieval algorithm. Finally, our method is verified by an experiment using a resolution chart. The results also show that our method have higher reconstruction resolution and faster imaging speed.

Three dimensional reconstruction using a lenslet light field camera

A novel method is proposed in this paper to accurately reconstruct the three-dimensional scenes by using a passive single-shot exposure with a lenslet light field camera. This method has better performance of 3D scenes reconstruction with both defocus and disparity depth cues captured by light field camera. First, the light field data is used to refocus and shift viewpoints to get a focal stack and multi-view images. In refocusing procedure, the phase shift theorem in the Fourier domain is first introduced to substitute shift in spatial domain, and sharper focal stacks can be obtained with less blurriness. Thus, 3D scenes can be reconstructed more accurately. Next, through multi-view images, disparity depth cues are obtained by performing correspondence measure. Then, the focal stack is used to compute defocus depth cues by focus measure based on gray variance. Finally, the focus cost is built to integrate both defocus and disparity depth cues, and the accurate depth map is estimated by using Graph Cuts based on the focus cost. Using this accurate depth map and all-in-focus image, the 3D structure in real world are accurately reconstructed. Our method is verified by a number of synthetic and real-world examples captured with a dense camera array and a Lytro light field camera.

Arbitrary manipulation of micro-particles in three dimensions by steering of multiple orbital angular momentum modes

Optical tweezers is an increasingly important technique for controlling and probing particles since computer-generated holography (CGH) make steering of multiple traps individually possible. In addition, the dark focus of orbital angular momentum (OAM) beams is increasingly widely used in trapping reflecting, absorbing or low-dielectric-constant objects. In this paper, we present a method to create arbitrary three-dimensional configurations of orbital angular momentum modes to achieve manipulation of micro-particles. Compared with conventional optical tweezers, this method can steer mixed arrays of traps individually and randomly by producing three-dimensional structure of optical vortices. These optical traps we used was formed by a CGH generated complex phase mask, which has three components: 1) a helical phase mask to change the transverse phase structure, 2) a blazed grating phase mask to vary the propagation direction of the incident beams, and 3) a modulated grating phase mask to divert the focal plane from the planar configurations. The latter one ensure that we can form threedimensional trapping patterns. The trap patterns can also be generated dynamically by holographic display system based on liquid crystal on silicon (LCoS). The experimental results show that the refresh frequency of reconfiguring achieves 24fps. Our method is effective and promise an exciting new opportunity to be used as a valuable non-contact manipulation tool in various applications.

Optimising the illumination spectrum for enhancing tissue visualisation

In operations, light reflected from biological tissue can be used for disease detection. In this paper, we used a visual entropy evaluation method to design the optimal illuminant to improve colour discriminability of biological tissue. The optimal spectral power distribution of the illuminant was obtained by maximising the visual entropy value of sample tissue based on the human visual system. In the experiment, multispectral imaging was used to measure the spectral reflectance of the tissue and colour clustering was conducted to extract its colour features. To verify the effectiveness of this method, simulated tissue images under illuminations with optimised spectral power distributions were compared with those under other light sources such as the standard illuminant D65 and white LED light sources. Results show that the sample under optimised illumination has a higher visual entropy value with better perceptual visibility.

Computer-generated holographic near-eye display system based on LCoS phase only modulator

Augmented reality (AR) technology has been applied in various areas, such as large-scale manufacturing, national defense, healthcare, movie and mass media and so on. An important way to realize AR display is using computer-generated hologram (CGH), which is accompanied by low image quality and heavy computing defects. Meanwhile, the diffraction of Liquid Crystal on Silicon (LCoS) has a negative effect on image quality. In this paper, a modified algorithm based on traditional Gerchberg-Saxton (GS) algorithm was proposed to improve the image quality, and new method to establish experimental system was used to broaden field of view (FOV). In the experiment, undesired zero-order diffracted light was eliminated and high definition 2D image was acquired with FOV broadened to 36.1 degree. We have also done some pilot research in 3D reconstruction with tomography algorithm based on Fresnel diffraction. With the same experimental system, experimental results demonstrate the feasibility of 3D reconstruction. These modifications are effective and efficient, and may provide a better solution in AR realization.