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[25] [26] [27] An alternative technique with the phoropter requires the use of a "clock dial" or "sunburst" chart to determine the astigmatic axis and power. [28] [29] A keratometer may also be used to estimate astigmatism by finding the difference in power between the two primary meridians of the cornea.
A recent development are computer-controlled stigmators, which usually use the Fourier transform of the image to find the ideal stigmator setting. The Fourier transform of an astigmatic image is usually elliptically shaped. [13] For a stigmatic image, it is round, this property can be used by algorithms to reduce the astigmatic aberration. [4]
Beam astigmatism: The beam is astigmatic when the vertical and horizontal parts of the beam focus in different locations along the beam path. Beam wander or jitter: The amount that the centroid or peak value of the beam profile moves with time. Instruments and techniques were developed to obtain the beam characteristics listed above. These include:
The interval O'O", termed the astigmatic difference, increases, in general, with the angle W made by the principal ray OP with the axis of the system, i.e. with the field of view. Two astigmatic image surfaces correspond to one object plane; and these are in contact at the axis point; on the one lie the focal lines of the first kind, on the ...
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A pencil-beam radar A pencil-beam radar. In optics, a pencil or pencil of rays is a geometric construct used to describe a beam or portion of a beam of electromagnetic radiation or charged particles, typically in the form of a narrow beam (conical or cylindrical).
A camera aperture Definitions of Aperture in the 1707 Glossographia Anglicana Nova [1] Aperture icon In optics , the aperture of an optical system (including a system consisted of a single lens) is a hole or an opening that primarily limits light propagated through the system.
block. Also blocking high and blocking anticyclone. A nearly stationary pattern in the atmospheric pressure field overlying a large geographic area, which effectively "blocks" or diverts the movements of cyclones and other convective systems. These blocks can remain in place for days or weeks, causing the areas affected by them to experience ...