Search results
Results From The WOW.Com Content Network
These distortions generally do not possess figure-of-revolution symmetry and are thus astigmatic, and slowly become permanently polished into the surface if the problems causing the distortion are not corrected. Astigmatic, distorted surfaces potentially introduce serious degradations in optical system performance.
[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 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).
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 ...
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:
Refractive surgery is an optional eye surgery used to improve the refractive state of the eye and decrease or eliminate dependency on glasses or contact lenses.This can include various methods of surgical remodeling of the cornea (keratomileusis), lens implantation or lens replacement.
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]
The surgeon can ascertain the astigmatic, or steepest, meridian in a number of ways, including manifest refraction or corneal topography. Manifest refraction is the familiar test where the eye doctor rotates lenses in front of the eye, asking the patient, "Which is better (or clearer), this one or this one?"