Optical Aberration Correction

The first corrected electron-optical SEM was developed by Zach [10]. Eor low-voltage SEM (LVSEM, down to 500 eV electron energy instead of the conventional energies of up to 30 keV) the spot size is extremely large without aberration correction. Combining and correction and a electrostatic objective lens, Zach showed that a substantial improvement in spot size and resolution is possible. The achievable resolution in a LVSEM is now of the order of 1-2 mn. More recently, Krivanek and colleagues succeeded in building a corrected STEM [53,M]. 

At the most basic level, the data can be visualized (rendered) as an image volume viewed at varying angles and manually compressed or stretched to fit. Alternatively, a more mathematical approach can be taken if the optical aberrations can be measured. In 3D fluorescence microscopy, this measurement is known as the point spread function (psf), and a process of deconvolution can be used to correct any aberrations with a known psf. Once corrected, the data can be rendered for viewing and measurement with confidence. 

Batson, P.E. Dellby, N. Krivanek, O.L. Subangstrom resolution using aberration corrected electron optics. Nature 2002, 418, 617-620. 

Figure 12. Optical schematic representation of an aberration-corrected toroidal diffraction grating spectrometer designed for use with an I PDA detector system. (Reproduced with permission from Ref. 12.

UV-visibie-near-iR Plane holographic gratings up to 1400 grooves per mm, or spherical or aspherical types with diode array detection Aspherical types correct for optical aberrations... 

Geckos eyes have 350 times more sensitivity to color in dim light than human eyes. Newer-generation excimer laser systems used for vision-correction surgery have the capacity to measure and correct higher-order optical aberrations of the human eye. Iris recognition with a rotational adjustment is also available on some lasers. 

Fig. 2. Projection lithography, (a) Schematic diagram illustrating the essential components in a projection exposure tool, (b) Optical elements in a modern projection exposure tool used for device manufacturing. The additional components enhance the precision of the projected light image by correcting for optical aberrations and distortions (12).

Some of the new IR microscopes on the market are capable of minimising these problems. An IR-Plan IR microscope (obtained from Spectra-Tech, Stamford, CT) has been used to obtain IR spectra of Kevlar filaments and also to carry out orientation measurements. The microscope uses a 15x IR/visible reflecting lens with Cassegrain optics to focus the IR beam onto the sample (Refleachromat, Spectra-Tech). A lOx reflecting Cassegrain beam condenser is used to collect the transmitted beam after it passes through the sample. The beam condenser also has a sample compensator, which can be used to correct for optical aberrations caused by the fibre samples. The microscope uses its own narrow band liquid... 

Matrix modulators or controlled transparencies could be used in data-processing systems for correction of the optical aberration in real-time images [49], as well as writing and reconstructing the holographic information [49, 50], which is a basic tool for optical pattern classification [51], modeling of neural networks [52], optical associative memory [53], phase conjugation of low-power optical beams [54], etc. 

Development of essentially new methods of GB-elements manufactures which upgrades the existing methods of GRIN-elements for use in special optical devices Creation of inhomogeneous gradient layers on surfaces of spherical lenses with the aim of geometrical aberration corrections ... 

A nice article that describes the importance of oxygen diffusion for silicone-based soft contact lenses may be found online http //www.clspectrum.com/article.aspx7article=12953 Note if the lens contains a hydrophobic surface, it will disrupt the tear flow that results in the deposition of an albumin film on the lens. Not only will this reduce the effectiveness of the lens to correct optical aberrations, but will also cause infection/irritation. Eor more details regarding the lens surface and eye complications, see Rao, J. B., Saini, J. S. Complications of Content Lenses in Contact Lenses. Aquavella, J. V., Rao, G. N., eds. Lippincott Williams and Wilkins Philadelphia, PA, 1987. 

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