Medicine

A scleral lens is a large type of contact lens that rests on the sclera and creates a tear-filled vault over the cornea. Scleral lenses are designed to treat a variety of eye conditions which do not respond to other forms of treatment.

Modern scleral lenses are made of a highly oxygen permeable polymer. They are also unique in their design in that they fit onto and are supported by the sclera, the white portion of the eye. The cause of this unique positioning is usually relevant to a specific patient, whose cornea may be too sensitive to support the lens directly. In comparison to regular contact lenses, scleral lenses bulge outward considerably more. The space between the cornea and the lens is filled with artificial tears. The liquid, which is contained in a thin elastic reservoir, conforms to the irregularities of the

Special effect scleral lenses have also been used to produce eerie eye effects in films, such as the whited-out eyes of the monsters in Evil Dead, or blacked-out eyes in Underworld and Underworld: Evolution. These lenses tend to be uncomfortable and sometimes impede the actors' vision, but the visual effects produced can be striking. The lenses cost around $300, are custom-made to fit the wearer's eyes and can also be custom-painted, although most companies only sell lenses with a pre-designed look. There are many different designs available, from standard black lenses to flame-effect eyes, as well as a lens that

A lens clock is a mechanical dial caliper that is used to measure dioptric power of a lens. It is a specialized version of a spherometer. A lens clock measures the curvature of a surface, but gives the result as an optical power in diopters, assuming the lens is made of a material with a particular refractive index.

A lens clock can also be used to estimate the thickness of thin objects, such as a hard or gas-permeable contact lens. Ideally, a contact lens dial thickness gauge would be used for this, but a lens clock can be used if a dial thickness gauge is not available. To do this, the contact lens is placed concave side up on a table or other hard surface. The lens clock is then brought down on it such that the center prong contacts the lens as close to its center as possible, and the outer prongs rest on the table. The

where n is the index of refraction for which the lens clock is calibrated, regardless of the actual index of the lens being measured. If the lens is made of glass with some other index n2, the true optical power of the surface can be obtained using

The Lens placode is a thickened portion of ectoderm which serves as the precursor to the lens. SOX2 and Pou2f1 are involved in its development.[1]

Eugène Kalt (1861-1941) was a French ophthalmologist who developed the first known application of a contact lens for the correction of keratoconus. In 1888, he worked on a crude flat-fitting glass scleral lenses designed to "compress the steep conical apex thereby correcting the condition". His first lenses were crafted from the bottoms of glass test tubes. Kalt was born in Landser in the Alsace region of France.

The lens clock has three pointed probes that make contact with the surface of the lens. The outer two probes are fixed while the center one moves, retracting as the instrument is pressed down on the lens's surface. As the probe retracts, the hand on the face of the clock turns by an amount proportional to the distance. The optical power φ of the surface is given by where n is the index of refraction of the glass, s is the vertical distance (sagitta) between the center and outer probes, and D is the horizontal separation of the outer probes. A typical lens

A biconcave lens made of flint glass with an index of 1.7 is measured with a lens clock calibrated for crown glass with an index of 1.523. If the lens clock gives surface powers of −3.0 and −7.0 diopters (dpt), the optical power of the lens is obtained as follows: Next, the optical powers of each surface are obtained: Finally, if the lens is thin the powers of each surface can be added to give the approximate optical power of the whole lens: −13.4 diopters. The actual power, as read by a vertometer or lensometer, might differ by as much as 0.1

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