Qinxiao Liu

Free-form reflector optimization for general lighting

A slope angle-based reflector profile optimization method is introduced at the first time. Compared to the traditional profile-based optimization method, better optimization results are obtained with this method. Design examples of reflectors for general lighting applications are given to compare both types of optimization methods. Disadvantages, such as range definition, range overlapping, and parameter-dependent problems, which existed in the traditional profile-based optimization method can be totally overcome with the proposed method.

Compact LED based LCOS optical engine for mobile projection

With the development of high power LED (light emitting diode) technology and color filter LCOS (liquid crystal on silicon) technology, the research on LED based micro optical engine for mobile projection has been a hot topic recently. In this paper one compact LED powered LCOS optical engine design is presented, which is intended to be embedded in cell phone, digital camera, and so on. Compared to DLP (digital light processor) and traditional color sequential LCOS technology, the color filter based LCOS panel is chosen for the compact optical engine, this is because only white LED is needed. To further decrease the size of the optical engine, only one specifically designed plastic free form lens is applied in the illumination part of the optical engine. This free form lens is designed so that it plays the roles of both condenser and integrator, by which the output light of LED is condensed and redistributed, and light illumination of high efficiency, high uniformity and small incident angle on LCOS is acquired. Besides PBS (polarization beam splitter), LCOS, and projection lens, the compact optical engine contains only this piece of free form plastic lens, which can be produced by plastic injection molding. Finally a white LED powered LCOS optical engine with a compact size of less than 6.6 cc can be acquired. With the ray tracing simulation result, the light efficiency analysis shows that the output flux is over 8.5 ANSI lumens and the ANSI uniformity of over 80%.

Image restoration based on generalized minimal residual methods with antireflective boundary conditions in a wavefront coding system

The point spread function (PSF) is asymmetric in a wavefront coding (WFC) system with cubic phase mask (CPM). The image formation of the WFC system is described as the generalized Sylvester matrix equation. With Tikhonov regularization, a global generalized minimal residual method (Gl-GMRES) algorithm is used to obtain the restored sharp image. For this large-scale, linear, and discrete, ill-posed problem, we introduce a Kronecker product approximation of the blurring operator to reduce the computation consumption. To eliminate ringing effect, four boundary conditions (BCs) are considered in the image restoration: periodic BCs, zero BCs, reflective BCs, and antireflective BCs. Analysis and numerical results show that the antireflective BCs provide better results than others. The experiment results show that the Gl-GMRES algorithm with antireflective BCs is more effective than the classic Wiener filter.

Pupil design based on Fisher information optimization to extend field depth in practical optical system

Wavefront coding (WFC) is used to extend the field depth of an incoherent optical system by employing a phase mask on the pupil. We uses a Fisher information (FI) metric based optimization method to design a phase mask by taking the modulation transfer function (MTF) of the practical optical system into consideration. This method can modulate the wavefront so that the point spread function and optical transfer function are insensitive to the object distance. The simulation results show that the optimized phase mask based on the proposed method can further improve the defocusing image quality while maintaining the focusing image quality.

A new polarization multiplexing method for the micro LCOS projector optical system

In this paper we present a new polarization multiplexing method for the micro LCOS projector optical system. We use calcite to separate the P and S light with respect to the PBS used in the traditional way. The specific method is described as follow: the special lens group or TIR is used to collimate emission ray from the light source. The ray from the lens is changed into approximate parallel light. A convergent component is used to get the convergence light on the exit facet of the calcite. Because of the structure characteristic of the calcite, the angel between optical axis of crystal and cleavage surface is 45 degree. Then we can get two separate convergence light spot on the exit facet of the calcite which stands for the P and S light respectively. The linear half wave plate is used to convert the state of the polarization. Then light pipe and relay imaging lens are used to meet the requirements of the uniformity and incident angle on the micro LCOS chip.

Fly-eyes illumination analysis

The Fly-eyes element has been widely applied to acquire uniform illumination in microscope, printing, projection, and so on. With large number of fly-eyes pieces and specified aperture shape, illumination of high efficiency, high uniformity, and specified area shape can be acquired. In this paper, mathematical expressions for fly-eyes illumination are derived for free form incoming light distribution. With these expressions, the reason why uniform illumination can be acquired with fly-eyes is given, and the defects of fly-eyes illumination is also discovered. According to traditional design experience, the illumination uniformity acquired by fly-eyes illumination will be increased with the increment of fly-eyes number. But with the derived expressions, it can be found that there are some limitations for this experience. For some odd symmetrical illumination cases, uniform illumination can not be acquired even with large number of fly-eyes pieces. This feature of fly-eyes illumination should be pay good attentions when dealing with non-symmetrical illumination cases. Another feature of fly-eyes illumination is that discrete angular distribution instead of continuous angular distribution of output light is acquired, and this discrete angular pattern varies with numerical aperture of the incoming light. This may be a bad feature for microscope with fly-eyes illumination, but this is a good feature for projection. With these derived mathematical expressions, guidelines for the application of fly-eyes illumination can be found and a deep view into the fly-eyes illumination can be acquired.

Defocus insensitive projection lens for CF-LCOS pico-projector

Micro-projector has found wide application in the field of mobile phone, pico-projector et al. However, because of the depth of the field of the projection lens, precise focus mechanical design is needed for the optical engineer to get fine and clear image. Wavefront coding technique is a novel technique which is introduced by Dowski and Cathey in 1995. By adding an aspheric phase mask at the pupil of the optical system, wavefront coding technique can make the imaging performance of the optical system insensitive to the defocus, and can also make the system insensitive to the defocus related aberrations, such as spherical aberration, chromatic aberration, field curvature, astigmatism and so on. In this paper, a projection lens with long depth of field for the CF-LCOS projector is developed to satisfy the new requirement. We first design the projection lens with the traditional optical design software to let it project the image to the average projection distance. For example, if the lens is designed with in the range of 0.65-2.0m, then about 1.0m is selected as the preliminary projection distance. After the lens (with a 3mm plane plate at the stop surface) is optimized with the CODE V in this distance, the plane plate is replaced with cubic phase plate. Then the lens is optimized again (the phase plate parameter varied for optimization) to get the final lens. In the new optimization, the resolution should be 3 times of the real resolution according to the LCOS resolution. This new optimized lens has the same MTF or PSF for different fields between 0.65 and 2m projection distances, this ensures the high resolution with no focus operation. Because of the MTF satisfies the LCOS requirement between the distance of 0.65 to 2m, no resolution is damaged and also no focus operation is need in this effective projection distance , thus no mechanical focus system is needed.

Simple color sequential LCoS illumination system used for projection display

Illuminating system gives attention to the transmission efficiency and spatial uniformity. Various non-imaging components were used as the illuminating system for the projection system to collect the light emitted by Light-Emitting Diodes, such as light pipe, CPC and flyeyes lens. For the micro projection, the key is to find a useful method to keep the system compact which causes little loss of the efficiency and spatial uniformity. Kohler illumination is an efficient method to get the uniformity lighting zone on the pupil plane of the optical system. According to the principle of the Kohler illumination, in this paper, we proposed to set the stop on the LCoS front surface for the first time. It can be found that this method can give satisfied results by using two simple lens. In order to get the white light from the R, G, B LEDs, the color matching method by using the actual measurement chromaticity coordinate is also given. Simulation results show that both the solutions can give efficient illumination and the system with aspherical lens can give more uniformity.

The research of underwater imaging based on the wavefront coding system

Current research on the underwater imaging often needs particular techniques. Since the specificity of the underwater imaging system, it must overcome the impact of the turbulent and turbid of the water. In this paper we present a new application of the wavefront coding (WFC) system in the underwater imaging. Wavefront coding system is a two-step imaging system, by using a cubic phase mask with optimized parameters; we get the defocus-insensitive intermediate image in the imaging plane. Then the final sharp image can be obtained by deconvoluted the intermediate image with the point spread function (PSF). Initially the wavefront coding system is introduced to extent the depth of the field of the optical system and the allowance of the system can be great improved by using the optimized cubic phase mask. Because of the defocus-insensitive property, this system can eliminate the aberration brought by the turbulent of the water. The deconvolution can improve the imaging quality by deblurring intermediate image and reduce the impact brought by the turbid of the water. In order to get excellent result, we propose to use the global generalized minimal residual method as the deconvolution algorithm. Antireflective boundary conditions (BCs) are considered to eliminate ringing effect, and Kronecker product approximation is introduced to reduce the computation consumption. This method can give excellent deconvolution result. There is little ringing effect on the image border and little vibration on the edge. Theory analysis and the experiment show that this is an effective system in the underwater imaging. It is an economical system.