Compact multi-band fluorescent microscope with an electrically tunable lens for autofocusing

Continuous optical zoom telescopic system based on liquid lenses

Telescopes play an essential important role in the fields of astronomical observation, emergency rescue, etc. The traditional telescopes achieve zoom function through the mechanical movement of the solid lenses, usually requiring refocusing after magnification adjustment. Therefore, the traditional telescopes lack adaptability, port-ability and real-time capability. In this paper, a continuous optical zoom telescopic system based on liquid lenses is proposed. The main components of the system consist of an objective lens, an eyepiece, and a zoom group composed of six pieces of liquid lenses. By adjusting the external voltages on the liquid lenses, the zoom telescopic system can achieve continuous optical zoom from ∼1.0× to ∼4.0× operating with an angular resolution from 28.648" to 19.098", and the magnification switching time is ∼50ms. The optical structure of the zoom telescopic system with excellent performance is given, and its feasibility is demonstrated by simulations and experiments. The proposed system with fast response, portability and high adaptability is expected to be applied to astronomical observation, emergency rescue and so on.

Simple, non-mechanical and automatic calibration approach for axial-scanning microscopy with an electrically tunable lens

We describe a simple and robust calibration approach for axial-scanning microscopy that realizes axial focus shifts with an electrically tunable lens (ETL). We demonstrate the calibration approach based on a microscope with an ETL placed close to the rear stop of the objective lens. By introducing a target-consisted of repeating lines at one known frequency and placed at a ~45° angle to the imaging path, the calibration method captures multiple images at different ETL currents and calibrates the dependence of the axial focus shift on the ETL current by evaluating the sharpness of the captured images. It calibrates the dependence of the magnification of the microscope on the ETL current by measuring the period of the repeating lines in the captured images. The experimental results show that different from the conventional calibration procedure, the proposed scheme does not involve any mechanical scanning and can simultaneously calibrate the dependence of the axial focus shift and the magnification on the ETL current. This might facilitate imaging studies that require the measurement of fine structures in a 3D volume. We also show the calibration procedure can be used to estimate the radius of a conner-arc sample, fabricated using laser micromachining. We believe that this easy-to-use calibration approach may facilitate use of ETLs for a variety of imaging platforms. It may also provide new insights for the development of novel 3D surface measurement methods. RESEARCH HIGHLIGHTS: The proposed calibration scheme does not involve any mechanical scanning and can simultaneously calibrate the dependence of the axial focus shift and the magnification on the electrically tunable lens (ETL) current. It might facilitate imaging studies that require the measurement of fine structures in a 3D volume, and the use of ETLs for a variety of imaging platforms. It may also provide new insights for the development of novel 3D surface measurement methods.

Fast and stable Fourier ptychographic microscopy based on improved phase recovery strategy

Fourier ptychographic microscopy (FPM) imaging is a computational imaging technology that can reconstruct wide-field high-resolution (HR) images. It uses a series of low-resolution images captured by a camera under different illumination angles. The images are stitched in the Fourier domain to expand their spectral range. Under high-angle illumination, a dark-field image is noisy with a low signal-to-noise ratio, which significantly reduces the reconstruction quality of FPM. Conventional reconstruction algorithms often have low FPM imaging performance and efficiency due to optimization strategies. In response to these problems, this paper proposes an FPM imaging method based on an improved phase recovery strategy to optimize the alternating iterative algorithm. The technique uses an improved threshold method to reduce noise in the image preprocessing stage to maximize the retention of high-frequency sample information. Moreover, an adaptive control factor is added in the subsequent iterative update process to balance the sample spectrum function. This study verifies the effectiveness of the proposed method on both simulation and experimental images. The results show that the proposed method can effectively suppress image background noise and has a faster convergence speed and higher robustness. In addition, it can be used to reconstruct HR complex amplitude images of objects under wide field-of-view conditions.

Continuous optical zoom microscopy imaging system based on liquid lenses

In this paper, a continuous optical zoom microscopy imaging system based on liquid lenses is proposed. Compared with traditional microscopes, which have discrete magnification, requiring manual conversion of the objective lens to change the magnification, the proposed microscope can continuously change the magnification of the targets in real-time. An adaptive zoom microscope, a liquid lens driving board, a microscope bracket, an adjustable three-dimensional stage and a light source are stacked to form the main framework of the continuous optical zoom microscopy imaging system. The adaptive zoom microscope which is composed of four electrowetting liquid lenses and six glass lenses form the main imaging element of the microscope. By changing the driving voltage which is applied to the four liquid lenses, the focal length of the liquid lenses can be modulated to achieve continuous zooming. By contrast, in traditional microscopes, the zooming process can only be achieved by rotating the eyepieces at different magnifications. At a fixed working distance, the magnification of the proposed microscope can change continuously from ∼9.6× to ∼22.2× with a response time of ∼50ms. Moreover, an axial depth scanning of ∼1000µm can be achieved without any mechanical movement. Our experiments proved that the microscope has stable performance and high consistency during zooming. Therefore, the proposed microscope has obvious advantages over the traditional microscopes in observing dynamic samples with different magnifications and can be commercialized for further expanding the applications in biochemical and pathological analysis.

Autofocusing technologies for whole slide imaging and automated microscopy

Whole slide imaging (WSI) has moved digital pathology closer to diagnostic practice in recent years. Due to the inherent tissue topography variability, accurate autofocusing remains a critical challenge for WSI and automated microscopy systems. The traditional focus map surveying method is limited in its ability to acquire a high degree of focus points while still maintaining high throughput. Real-time approaches decouple image acquisition from focusing, thus allowing for rapid scanning while maintaining continuous accurate focus. This work reviews the traditional focus map approach and discusses the choice of focus measure for focal plane determination. It also discusses various real-time autofocusing approaches including reflective-based triangulation, confocal pinhole detection, low-coherence interferometry, tilted sensor approach, independent dual sensor scanning, beam splitter array, phase detection, dual-LED illumination and deep-learning approaches. The technical concepts, merits and limitations of these methods are explained and compared to those of a traditional WSI system. This review may provide new insights for the development of high-throughput automated microscopy imaging systems that can be made broadly available and utilizable without loss of capacity.

Large zooming range adaptive microscope employing tunable objective and eyepiece

The conventional microscope has discrete magnification and slow response time in zoom process, which is difficult to capture the dynamic activity of the live specimen. We demonstrate an adaptive microscope employing a tunable objective and a tunable eyepiece with large zooming range. The tunable objective consists of three glass lenses and four electrowetting liquid lenses. The tunable eyepiece consists of an achromatic eyepiece and an electrowetting liquid lens. The focal point between the objective and the eyepiece is designed to be tunable, which are controlled by voltages. Thus, the tuning range is relatively large. We fabricate the adaptive microscope and observe the specimen. In the experiment, the magnification of the microscope changes continuously from ~ 59.1 × to ~ 159.2 × , and the largest numerical aperture is ~ 0.212. The tunable eyepiece can release the back focal length of the tunable objective, which increases the zoom range of the microscope. No mechanical movement is required and the aberrations can be corrected over a wide wavelength range. Thus, the proposed adaptive microscope has a potential application in biological research and clinical medical examination.

Three-dimensional fluorescence imaging using the transport of intensity equation

We propose a nonscanning three-dimensional (3-D) fluorescence imaging technique using the transport of intensity equation (TIE) and free-space Fresnel propagation. In this imaging technique, a phase distribution corresponding to defocused fluorescence images with a point-light-source-like shape is retrieved by a TIE-based phase retrieval algorithm. From the obtained phase distribution, and its corresponding amplitude distribution, of the defocused fluorescence image, various images at different distances can be reconstructed at the desired plane after Fresnel propagation of the complex wave function. Through the proposed imaging approach, the 3-D fluorescence imaging can be performed in multiple planes. The fluorescence intensity images are captured with the help of an electrically tunable lens; hence, the imaging technique is free from motion artifacts. We present experimental results corresponding to microbeads and a biological sample to demonstrate the proposed 3-D fluorescence imaging technique.

Accelerating wavefront-sensing-based autofocusing using pixel reduction in spatial and frequency domains

The wavefront-sensing-based autofocus method can precisely determine the focal plane only with few captured images; however, the required phase retrieval, numerical wavefront propagation, and in-focus determination are often time consuming, inevitably limiting its high-speed applications. To accelerate its processing speed, the pixel-reduced wavefront-sensing-based autofocus (PRWSA) method is proposed: with field of interest selection as pixel reduction in the spatial domain and image compression as pixel reduction in the frequency domain, the wavefront with fewer pixels can be used for autofocusing, significantly decreasing the processing time. With simulations, pixel reduction criteria in both the spatial and frequency domains are first determined and tested; next certificated by experiments, the PRWSA method is proved to be well implemented for different specimens. Considering it can precisely locate the focal plane with simple setup, and accelerate the processing speed, this PRWSA method can be a potential tool for high-speed autofocusing.

Large-scale 3D imaging of insects with natural color

High-resolution 3D imaging technology has found a number of applications in many biological fields. However, the existing 3D imaging tools are often too time-consuming to use on large-scale specimens, such as centimeter-sized insects. In addition, most 3D imaging systems discard the natural color information of the specimens. To surmount these limitations, we present a structured illumination-based approach capable of delivering large field-of-view three-dimensional images. With this approach, 580nm lateral resolution full-color 3D images and 3D morphological data in the size range of typical insect samples can be obtained. This method provides a promising approach that can be used to support many different types of entomological investigations, including taxonomy, evolution, bionics, developmental biology, functional morphology, paleontology, forestry, etc.

Study of an image autofocus method based on power threshold function wavelet reconstruction and a quality evaluation algorithm

As a key component in optical microscopy imaging systems, autofocus technology has a significant effect on imaging quality. In this paper, an optical microscopy autofocus method that includes a wavelet denoising algorithm based on a power threshold function and a Brenner image quality evaluation algorithm is presented. Experimental results show that the power threshold function wavelet denoising algorithm, which can be adopted to obtain more realistic optical images, is superior to the traditional soft, hard, hyperbolic, and exponential threshold functions in terms of peak signal-to-noise ratio, signal-to-noise ratio, mean squared error, and histogram indicators; moreover, compared to the Roberts, sum modulus difference (SMD), and energy gradient functions, the Brenner image quality evaluation algorithm can be used to quickly and accurately lock onto the focal plane. By integrating and applying these two core algorithms in the autofocus image acquisition system of a microscope, the image sharpness and focusing quality are greatly improved, which benefits the further evaluation of images.

Displaceable and focus-tunable electrowetting optofluidic lens

A conventional optofluidic lens usually has one liquid-liquid (L-L) interface, which can be deformed to achieve variable focal length. Such a single lens cannot be used alone to realize optical zooming because its back focal distance keeps changing. Here, we report a novel displaceable and focus-tunable electrowetting optofluidic lens. In comparison with the conventional optofluidic lens, our new lens has a different working principle and it can function as an optical zoom lens. The L-L interface can be displaced by a voltage. The object distance and image distance can be adjusted by shifting the L-L interface position to achieve the desired magnification, yet the lens can refocus the image by reshaping the L-L interface with another voltage. Under such condition, only one lens is adequate to realize the zooming functionality. To prove the concept, we fabricate an optofluidic lens whose largest displaceable distance is ~8.3 mm and the zooming ratio is ~1.31 ×. The proposed optofluidic lens greatly simplifies the zoom lens system. Widespread application of such an adaptive zoom lens is foreseeable.

Subwavelength resolution Fourier ptychography with hemispherical digital condensers

Fourier ptychography (FP) is a promising computational imaging technique that overcomes the physical space-bandwidth product (SBP) limit of a conventional microscope by applying angular-varied illuminations. However, to date, the effective imaging numerical aperture (NA) achievable with a commercial LED board is still limited to the range of 0.3-0.7 with a 4 × /0.1NA objective due to the geometric constraint with the declined illumination intensities and attenuated signal-to-noise ratio (SNR). Thus the highest achievable half-pitch resolution is usually constrained between 500-1000 nm, which cannot meet the requirements of high-resolution biomedical imaging applications. Although it is possible to improve the resolution by using a high-NA objective lens, the FP approach is less appealing as the decrease of field-of-view (FOV) will far exceed the improvement of spatial resolution in this case. In this paper, we initially present a subwavelength resolution Fourier ptychography (SRFP) platform with a hemispherical digital condenser to provide high-angle programmable plane-wave illuminations of 0.95NA, attaining a 4 × /0.1NA objective with the final effective imaging performance of 1.05NA at a half-pitch resolution of 244 nm with the incident wavelength of 465 nm across a wide FOV of 14.60 mm², corresponding to a SBP of 245 megapixels. Our work provides an essential step of FP towards high-throughput imaging applications.

All-optical microscope autofocus based on an electrically tunable lens and a totally internally reflected IR laser

Microscopic imaging at high spatial-temporal resolution over long time scales (minutes to hours) requires rapid and precise stabilization of the microscope focus. Conventional and commercial autofocus systems are largely based on piezoelectric stages or mechanical objective actuators. Objective to sample distance is either measured by image analysis approaches or by hardware modules measuring the intensity of reflected infrared light. We propose here a truly all-optical microscope autofocus taking advantage of an electrically tunable lens and a totally internally reflected infrared probe beam. We implement a feedback-loop based on the lateral position of a totally internally reflected infrared laser on a quadrant photodetector, as an indicator of the relative defocus. We show here how to treat the combined contributions due to mechanical defocus and deformation of the tunable lens. As a result, the sample can be kept in focus without any mechanical movement, at rates up to hundreds of Hertz. The device requires only reflective optics and can be implemented at a fraction of the cost required for a comparable piezo-based actuator.

Wavefront-sensing-based autofocusing in microscopy

Massive image acquisition is required along the optical axis in the classical image-analysis-based autofocus method, which significantly decreases autofocus efficiency. A wavefront-sensing-based autofocus technique is proposed to increase the speed of autofocusing and obtain high localization accuracy. Intensities at different planes along the optical axis can be computed numerically after extracting the wavefront at defocus position with the help of the transport-of-intensity equation method. According to the focus criterion, the focal plane can then be determined, and after sample shifting to this plane, the in-focus image can be recorded. The proposed approach allows for fast, precise focus detection with fewer image acquisitions compared to classical image-analysis-based autofocus techniques, and it can be applied in commercial microscopes only with an extra illumination filter.