Conventional Point Spread Function Research Articles

Relative truth value acquisition method of heterogeneous surface based on the Improved Point Spread Function

Abstract. The acquisition of pixel-scale relative truth value is important for remote sensing validation.The Point Spread Function (PSF) has been widely used in the field of the acquisition of relative truth value of heterogeneous surface. In this study, we propose an Improved Point Spread Function (IPSF) based on the Median Pixel Variability Weighted (MPVW) method and PSF to acquire relative truth value of heterogeneous surface. Firstly, the size of variance and clustering window are confirmed based on the pixel scale information of the satellite product to be validated. Secondly, the PSF suitable for heterogeneous surface is selected from 5 PSFs. Thirdly, the IPSF is constructed according to the MPVW and the PSF suitable for heterogeneous surface. Finally, the IPSF is used to acquire relative true value at pixel-scale of heterogeneous surface from airborne hyperspectral image. This study shows: (1) Good correlation between relative true value obtained using IPSF and reference value is indicated as R2 values among 256 channels reach 0.985, structural similarity (SSIM) ranges from 0.992 to 0.998, and peak signal noise ratio (PSNR) more than 35dB. (2) Better accuracy performance is observed in the acquisition of relative truth value of heterogeneous surface for IPSF than PSF. Compared with conventional PSF, the R2, PSNR and SSIM of the result of IPSF are increased by about 1.34%, 9% and 0.7% on the average. (3) Significant advantage of IPSF is also reported in reducing the deviation compared to PSF, as the average root mean square error (RMSEave) is reduced by 30.37% and the average mean absolute error (MAEave) is reduced by 35.98%, respectively. (4) Comparing the RMSE and MAE of PSF and IPSF result, the RMSE and MAE of the result of IPSF become smaller. The RMSE decreases from 3 ~ 195 to 2 ~ 131. The MAE changes from 2 ~ 138 to 1 ~ 89. Overall, the IPSF proposed in this paper can effectively calculate the relative true value of heterogeneous surface, which can provide reference for the validation of multi-spectral and hyper-spectral satellite products.

High-resolution light-field microscopy with patterned illumination.

Light-field fluorescence microscopy can record large-scale population activity of neurons expressing genetically-encoded fluorescent indicators within volumes of tissue. Conventional light-field microscopy (LFM) suffers from poor lateral resolution when using wide-field illumination. Here, we demonstrate a structured-illumination light-field microscopy (SI-LFM) modality that enhances spatial resolution over the imaging volume. This modality increases resolution by illuminating sample volume with grating patterns that are invariant over the axial direction. The size of the SI-LFM point-spread-function (PSF) was approximately half the size of the conventional LFM PSF when imaging fluorescent beads. SI-LFM also resolved fine spatial features in lens tissue samples and fixed mouse retina samples. Finally, SI-LFM reported neural activity with approximately three times the signal-to-noise ratio of conventional LFM when imaging live zebrafish expressing a genetically encoded calcium sensor.

Point-Spread-Function (PSF) Characterization of a 340-GHz Imaging Radar Using Acoustic Levitation

In this paper, resolution characterization and beam analysis of a 340-GHz imaging radar are demonstrated by means of a point-spread-function (PSF) study by imaging an acoustically levitated point-like target. It is shown that at terahertz frequencies, conventional PSF measurement techniques are limited by the presence of strong scattering response of background objects, such as suspension threads, within the imaging field-of-view (FOV). Using acoustic levitation, it is possible to eliminate secondary objects within the FOV and achieve a pure PSF characterization of the radar. It is shown that the PSF patterns obtained using acoustic levitation exhibit high fidelity and are free from artifacts. We demonstrate this using a small water droplet suspended in air at the focus of the 340-GHz radar. The measured PSF characteristics of the radar are in excellent agreement with physical optics (PO) simulations and analytical results.

Aberration correction method based on double-helix point spread function.

Point spread function (PSF) engineering has met with lots of interest in various optical imaging techniques, including super-resolution microscopy, microparticle tracking, and extended depth-of-field microscopy. The intensity distributions of the modified PSFs often suffer from deteriorations caused by system aberrations, which greatly degrade the image contrast, resolution, or localization precision. We present an aberration correction method using a spiral-phase-based double-helix PSF as an aberration indicator, which is sensitive and quantitatively correlated to the spherical aberration, coma, and astigmatism. Superior to the routine iteration-based correction methods, the presented approach is iteration-free and the aberration coefficients can be directly calculated with the measured parameters, relieving the computing burden. The validity of the method is verified by both examining the intensity distribution of the conventional Gaussian PSF in three dimensions and observing muntjac skin fibroblast cells. This iteration-free correction method has a potential application in PSF engineering systems equipped with a spatial light modulator.

Airy pattern approximation of a phased microphone array response to a rotating point source.

Deconvolution of phased microphone array source maps is a commonly applied technique in order to improve the dynamic range and resolution of beamforming. Most deconvolution algorithms require a point spread function (PSF). In this work, it is shown that the conventional definition of the PSF, based on steering vectors, is changed when the source is rotating. The effect of rotation results in an increase in the resolution and aperture of the array. The concept of virtual array positions created by source rotation is used to derive an approximation of the PSF based on an Airy pattern. The Airy pattern approximation is suitable for use in deconvolution of rotating source maps as it is more accurate and computationally less expensive than the conventional PSF definition. The proposed Airy pattern approximation was tested with both CLEAN and DAMAS deconvolution algorithms. On the same hardware, it was significantly faster when compared to the conventional definition. The limitations of the Airy pattern approximation are shown in a synthesized broadband test case with a high dynamic range. However, in most practical beamforming applications, the proposed Airy pattern approximated PSF for deconvolution is a suitable option considering its accuracy and speed.

Spatial specificity in spatiotemporal encoding and Fourier imaging

PurposeUltrafast imaging techniques based on spatiotemporal encoding (SPEN), such as RASER (rapid acquisition with sequential excitation and refocusing), is a promising new class of sequences since they are largely insensitive to magnetic field variations which cause signal loss and geometric distortion in EPI. So far, attempts to theoretically describe the point-spread-function (PSF) for the original SPEN-imaging techniques have yielded limited success. To fill this gap a novel definition for an apparent PSF is proposed. TheorySpatial resolution in SPEN-imaging is determined by the spatial phase dispersion imprinted on the acquired signal by a frequency-swept excitation or refocusing pulse. The resulting signal attenuation increases with larger distance from the vertex of the quadratic phase profile. MethodsBloch simulations and experiments were performed to validate theoretical derivations. ResultsThe apparent PSF quantifies the fractional contribution of magnetization to a voxel's signal as a function of distance to the voxel. In contrast, the conventional PSF represents the signal intensity at various locations. ConclusionThe definition of the conventional PSF fails for SPEN-imaging since only the phase of isochromats, but not the amplitude of the signal varies. The concept of the apparent PSF is shown to be generalizable to conventional Fourier-imaging techniques.

Point Spread Function of Specular Reflection and Gonio-Reflectance Distribution

In this article, the specular reflection phenomenon is analyzed by the concept of the point spread function (PSF). The conventional PSF, which is a sharpness criterion, is used for the transfer function in image science. In general, print (an ink image on paper) is observed as diffuse reflection light, and the light source image, i.e., gloss, is observed as specular reflection. We first introduce the point spread function of specular reflection (SR-PSF). A measurement principle for the SR-PSF is proposed, and a measurement apparatus for the SR-PSF is developed by using a collimator optical system. The experimental results show that the measured SR-PSF works as a transfer function for a specular reflection image. Each detected position is in agreement with the angle of reflected light. The apparatus works as a goniophotometer within a narrow solid angle. The relation between the SR-PSF and the specular reflection angle distribution is discussed in terms of surface geometry. It is concluded that the measured data are the SR-PSF and they have a gonio-reflectance distribution. The reflection angle is caused by the surface normal, and the surface normal distribution can be calculated from the gonio-reflectance distribution by using the mathematical model. We plan to apply our proposed principle to the development of an evaluation technique for the visual gloss. In addition, the high-resolution goniophotometer can be further developed by this measuring technique.

Characterization of a three-dimensional double-helix point-spread function for fluorescence microscopy in the presence of spherical aberration

We characterize the three-dimensional (3-D) double-helix (DH) point-spread function (PSF) for depth-variant fluorescence microscopy imaging motivated by our interest to integrate the DH-PSF in computational optical sectioning microscopy (COSM) imaging. Physical parameters, such as refractive index and thickness variability of imaging layers encountered in 3-D microscopy give rise to depth-induced spherical aberration (SA) that change the shape of the PSF at different focusing depths and render computational approaches less practical. Theoretical and experimental studies performed to characterize the DH-PSF under varying imaging conditions are presented. Results show reasonable agreement between theoretical and experimental DH-PSFs suggesting that our model can predict the main features of the data. The depth-variability of the DH-PSF due to SA, quantified using a normalized mean square error, shows that the DH-PSF is more robust to SA than the conventional PSF. This result is also supported by the frequency analysis of the DH-PSF shown. Our studies suggest that further investigation of the DH-PSF's use in COSM is warranted, and that particle localization accuracy using the DH-PSF calibration curve in the presence of SA can be improved by accounting for the axial shift due to SA.

Impacts of point spread function accuracy on patterning prediction and proximity effect correction in low-voltage electron-beam–direct-write lithography

Electron-beam–direct-write lithography at lower accelerating voltages has been considered as a candidate for next-generation lithography. Although long-range proximity effects are substantially reduced with the voltage, proximity effect correction (PEC) is still necessary since short-range proximity effects are relatively prominent. The effectiveness of model-based PEC can be limited severely if an inaccurate point spread function (PSF) characterizing electron scattering within resist is adopted. Recently, a new PSF form using a promising calibration method has been developed to more accurately characterize the electron scattering and thus significantly improve patterning fidelity at 5 keV. However, influences of adopting the conventional and new PSF forms for the usage of patterning practical circuit layouts have not been intensively studied. This work extensively investigates impacts of PSF accuracy on patterning prediction and PEC under different resist thickness conditions suitable for various lithographic half-pitch nodes, where the critical features of practical circuit layouts are used to quantitatively evaluate their performance. In addition, patterning fidelity limitation suffered from proximity effects is examined to determine whether PEC should be applied. Simulation results indicate that the new PSF form can significantly improve the fitting accuracy, patterning prediction, and PEC results over the conventional PSF forms, especially for circuit layouts with smaller feature sizes.

Combination of the resolution enhancing image inversion microscopy with digital holography

We propose a method to combine image inversion microscopy (IIM) with digital holography. By using this technique we are able to avoid some disadvantages of the IIM, like the use of a stabilized image inversion interferometer or the scanning process. Furthermore, we present a setup and some measurements as proof of principle. The point spread function (PSF) of a holographic image inversion microscope is compared to the conventional PSF. We demonstrate that the distance between the first zeros is reduced by a factor of about two. Additionally, we recorded images of 10 gratings to demonstrate the enhanced resolution and to measure a part of the optical transfer functions of the coherent, the incoherent and the image inverted case.

New parametric point spread function calibration methodology for improving the accuracy of patterning prediction in electron-beam lithography

Electron-beam-direct-write lithography has been considered a candidate next-generation technique for achieving high resolution. An accurate point spread function (PSF) is essential for reliable patterning prediction and proximity-effects correction. It can be derived via an effective parametric PSF calibration methodology, typically involving the fitting of the absorbed energy distribution (AED) from an electron-scattering simulation. However, the existing parametric PSF calibration methodology does not employ a systematic approach to obtain a new PSF form that is both compact and accurate when conventional PSF forms are not satisfactory. Only the AED fitting quality (rather than its patterning-prediction quality) is considered during the conventional calibration methodology. It also lacks a process to consider whether the predicted deviation (as simulated using the chosen PSF form) is satisfactory. This paper proposes a new parametric PSF calibration methodology to systematically obtain a PSF form consisting of the smallest number of terms, with a better combination of basis functions and that optimizes pattern accuracy. The effectiveness of using the new methodology is demonstrated in terms of fitting accuracy, patterning-prediction accuracy, and patterning sensitivity.

A novel easy-to-use phantom for the determination of MTF in SPECT scanners

To evaluate modulation transfer function (MTF) in single photon emission computed tomography (SPECT) systems using the line spread function (LSF) method and a novel flood source which can be easily fabricated with materials accessible in hospital facilities. A Tc-99m-based flood source (E(γ) = 140 keV) consisting of a radiopharmaceutical bound to the grains of a radiographic film was prepared in laboratory. Various films and radiopharmaceuticals were examined, in order to obtain a thin homogenous and reproducible flood source. The source showing best uniformity and reproducibility was placed between two PMMA blocks and images were obtained by using the brain tomographic acquisition protocol (brain) and the myocardial perfusion tomographic acquisition protocol (heart). MTF was evaluated by determining the LSF for various reconstruction methods and filters. MTF calculation was obtained by the utilization of a custom made software in which a method similar to the one proposed by Boone [Med. Phys. 28, 356-360 (2001)] was implemented. All imaging experiments were performed in a Siemens e-Cam γ-camera. Furthermore, MTF was assessed through the point spread function (PSF) following conventional methods. The optimum homogeneity was obtained by immersing an Agfa MammoRay HDR Medical x-ray film in a solution of dithiothreitol (DTT, 10(-3) M)/Tc-99m(III)-DMSA (DMSA: trivalent technetium-99m-dimercapto-succinic acid, 40 mCi/40 ml) for 30 min in the dark. These films exhibited better uniformity (CV < 1.9%). Higher MTF values were obtained for the brain scan protocol with iterative 3D with eight iterations reconstruction method. MTF of the brain protocol was in all cases better than the heart protocol. MTFs derived from LSF were more precise compared with those obtained from PSF since their reproducibility was better in all cases, providing a mean standard deviation of 0.0065, in contrary to the PSF method which gave 0.0348. The method presented here is novel and easy to implement, requiring materials commonly found in clinical practice. Furthermore, this technique which is based on the LSF method reduces measurement noise levels due to the larger amount of data averaging than in the conventional PSF method. Furthermore, MTF can be assessed easily, in three dimensions (3D), by placing the flood source either in sagittal or coronal direction.

The point‐spread function of a confocal microscope: its measurement and use in deconvolution of 3‐D data

SUMMARYWe have measured the point‐spread function (PSF) for an MRC‐500 confocal scanning laser microscope using subresolution fluorescent beads. PSFs were measured for two lenses of high numerical aperture—the Zeiss plan‐neofluar 63 × water immersion and Leitz plan‐apo 63 × oil immersion—at three different sizes of the confocal detector aperture. The measured PSFs are fairly symmetrical, both radially and axially. In particular there is considerably less axial asymmetry than has been demonstrated in measurements of conventional (non‐confocal) PSFs. Measurements of the peak width at half‐maximum peak height for the minimum detector aperture gave approximately 0·23 and 0·8 μm for the radial and axial resolution respectively (4·6 and 15·9 in dimensionless optical units). This increased to 0·38 and 1·5 μm (7·5 and 29·8 in dimensionless units) for the largest detector aperture examined. The resulting optical transfer functions (OTFs) were used in an iterative, constrained deconvolution procedure to process three‐dimensional confocal data sets from a biological specimen—pea root cells labelled in situ with a fluorescent probe to ribosomal genes. The deconvolution significantly improved the clarity and contrast of the data. Furthermore, the loss in resolution produced by increasing the size of the detector aperture could be restored by the deconvolution procedure. Therefore for many biological specimens which are only weakly fluorescent it may be preferable to open the detector aperture to increase the strength of the detected signal, and thus the signal‐to‐noise ratio, and then to restore the resolution by deconvolution.