PhoropterA phoropter or refractor is an ophthalmic testing device. It is commonly used by eye care professionals during an eye examination, and contains different lenses used for refraction of the eye during sight testing, to measure an individual's refractive error and determine their eyeglass prescription. It also is used to measure the patients' phorias and ductions, which are characteristics of binocularity.
Toric lensA toric lens is a lens with different optical power and focal length in two orientations perpendicular to each other. One of the lens surfaces is shaped like a "cap" from a torus (see figure at right), and the other one is usually spherical. Such a lens behaves like a combination of a spherical lens and a cylindrical lens. Toric lenses are used primarily in eyeglasses, contact lenses and intraocular lenses to correct astigmatism.
LensmeterA lensmeter or lensometer (sometimes even known as focimeter or vertometer), is an ophthalmic instrument. It is mainly used by optometrists and opticians to measure the back or front vertex power of a spectacle lens and verify the correct prescription in a pair of eyeglasses, to properly orient and mark uncut lenses, and to confirm the correct mounting of lenses in spectacle frames. Lensmeters can also verify the power of contact lenses, if a special lens support is used.
LorgnetteA lorgnette (lɔːˈnjɛt) is a pair of spectacles with a handle, used to hold them in place, rather than fitting over the ears or nose. The word lorgnette is derived from the French lorgner, to take a sidelong look at, and Middle French, from lorgne, squinting. Their precise origin is debated: some sources describe English scientist George Adams the elder as their inventor, while others cite his son George Adams the younger. The lorgnette was usually used as a piece of jewelry, rather than to enhance vision.
Abbe numberIn optics and lens design, the Abbe number, also known as the V-number or constringence of a transparent material, is an approximate measure of the material's dispersion (change of refractive index versus wavelength), with high values of V indicating low dispersion. It is named after Ernst Abbe (1840–1905), the German physicist who defined it. The term V-number should not be confused with the normalized frequency in fibers.
Defocus aberrationIn optics, defocus is the aberration in which an image is simply out of focus. This aberration is familiar to anyone who has used a camera, videocamera, microscope, telescope, or binoculars. Optically, defocus refers to a translation of the focus along the optical axis away from the detection surface. In general, defocus reduces the sharpness and contrast of the . What should be sharp, high-contrast edges in a scene become gradual transitions. Fine detail in the scene is blurred or even becomes invisible.
BifocalsBifocals are eyeglasses with two distinct optical powers. Bifocals are commonly prescribed to people with presbyopia who also require a correction for myopia, hyperopia, and/or astigmatism. Benjamin Franklin is generally credited with the invention of bifocals. He decided to saw his lenses in half so he could read the lips of speakers of French at court, the only way he could understand them.
Far pointIn visual perception, the far point is the farthest point at which an object can be placed (along the optical axis of the eye) for its image to be focused on the retina within the eye's accommodation. It is sometimes described as the farthest point from the eye at which images are clear. The other limit of eye's accommodation is the near point. For an unaccommodated emmetropic eye, the far point is at infinity, but for the sake of practicality, infinity is considered to be because the accommodation change from 6 m to infinity is negligible.
