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Electron Microscopes
Published in Ravishankar Chityala, Sridevi Pudipeddi, Image Processing and Acquisition using Python, 2020
Ravishankar Chityala, Sridevi Pudipeddi
The electrons enter the magnetic field at point O1 (Figure 16.4). Point O2 is the point where all electrons generated by the electron gun are focused by the magnetic field. The distance O1-O2 is the focal length of the lens. The mathematical relationship that defines focal length is given by f=KVi2 where K is a constant based on the design of the coil at the geometry, V is the accelerating voltage, and i is the current through the coil. As can be seen, either increasing the voltage or reducing the current in the coil can increase focal length. In an optical microscope, the focal length for a given lens is fixed while it can be changed in the case of an electromagnetic lens. Hence, in an optical microscope, the only method for changing the focal length is by either changing the lens (using objective turret) or by changing the spacing between the lenses. On the other hand, in an electromagnetic lens, the magnification can be changed by altering the voltage and current. The electromagnetic lens suffers from aberrations similar to optical lenses. Some of these are astigmatism, chromatic aberration, and spherical aberration. They can be overcome by designing and manufacturing under high tolerance.
Adaptive optics ophthalmoscopes
Published in Pablo Artal, Handbook of Visual Optics, 2017
All optical systems are affected by optical aberrations that degrade imaging performance. Correcting the lower-order aberrations of the eye’s optics such as defocus and astigmatism provides most people with good vision. However, in order to better understand the function of the normal retina and the pathophysiology of retinal disease, one needs to look into the eye. This can, of course, be accomplished by studying histological preparations ex vivo, but the retina has to be studied in vivo in order to diagnose and monitor retinal disease and evaluate efficacy of medical treatment. This was facilitated by the independent inventions of the ophthalmoscope by Charles Babbage in 1847 and the indirect ophthalmoscope by Hermann von Helmholtz in 1851. The functionality and usability of the ophthalmoscope were greatly advanced by the invention of the handheld direct-illuminating ophthalmoscope by Francis Welch and William Allyn in 1915. Modern-day versions of the Welch–Allyn ophthalmoscope are routinely used in eye examinations today.
Optical System and Design
Published in Shen-En Qian, Hyperspectral Satellites and System Design, 2020
There are several inherent problems with a refracting telescope. Refracting lenses lend themselves to what are called chromatic and spherical aberrations. Chromatic aberration occurs when a lens fails to focus all the colors to the same focal point, as shown in Figure 5.2. This defect shows as a fringe of color along the boundaries that separate dark and bright parts of the image. This was originally dealt with by increasing the focal length of the lens, which led to an extremely long telescope. Spherical aberration occurs due to the increased refraction of light rays when they strike a lens near its edge. This causes the outer rays of light to be focused more tightly away from the focal point, which causes the image to be imperfect.
Ultrafast laser micromachining of angled surfaces in fused silica
Published in Journal of Modern Optics, 2023
A 45-degree angled surface is fabricated successfully with high accuracy by applying the femtosecond laser irradiation followed by a chemical etching process. The roughness of the bottom and top faces is studied, and the top surface appears to be smoother as the focal spot has a tail which increases the roughness of the bottom face. However, at the 3.625 µm z increment, top and bottom surface roughness values get closer and reach the minimum. The 45-degree angled ∼1.5 mm2 surface plane is achieved with less than ±0.5° angle deviation and Gaussian filtered surface roughness of ∼300 nm. Thus, it is demonstrated that micron-level 3D angled fused silica structures without any cracking with a deviation less than ±0.01% can be fabricated by optimizing the parameters in this method. In future studies, aberrations at the focus can be corrected using adaptive elements to change the wavefront. Roughness can be further reduced based on the needs through additional procedures like laser polishing. The proposed procedure makes it possible to create complicated 3D structures, as exemplified, which are not possible to acquire with conventional optical fabrication methods.
Designing A Video Laryngoscope Imaging System with A 7-mm Blade for Neonatal Patients
Published in Smart Science, 2018
Ming-Ying Hsu, Wen-Tse Hsiao, Han-Chao Chang
After selecting the initial structure, we used an optical calculation program to calculate all aberrations and light path aberration curves. Data analysis was performed to determine aberration factors that affect the imaging quality of the optical system and identify methods to improve quality and perform aberration correction. Aberration analysis and light path balancing are processes that are repeated until the desired quality of imaging (distortion <0.05%) is achieved. The imaging quality of optical systems depends on the size of aberrations, and the optical design is aimed at correcting aberrations in optical systems. However, in any optical system, a residual aberration always exists, and it is impossible to correct all aberrations to zero. The image quality is not the same for different sizes of residual aberration. Hence, designers must determine the values of optical aberration and tolerance for various optical systems’ residual aberrations, so that the design corresponds to the size of the residual aberration of the desired optical system’s imaging quality. The evaluation method of the optical system imaging quality [7–9] includes modulation transfer function (MTF), chief ray angle, and tolerance analysis.
A velocity map imaging apparatus optimised for high-resolution crossed molecular beam experiments
Published in Molecular Physics, 2021
Vikram Plomp, Zhi Gao, Sebastiaan Y. T. van de Meerakker
Spherical aberrations are caused by a dependence of the focal length of each lens on the radial distance (r) from the lens axis, i.e. the radial field strength is not perfectly linear with respect to r. The extent of the spherical aberrations increases when the ions cross the lens further away from the axis. Since the Newton sphere expands when moving through the VMI-apparatus, the effect is generally largest for the last lens(es) encountered.