Focometer: Instrument mainly used to measure the top focal (D) and prism (△) of eye lenses (including corneal contact lenses), determine the orientation of the column axis of the column lens, print marks on the uncut lens and check whether the lens is correctly mounted in the frame.
Notes:
Top focus: the reciprocal of the focal length of the lens near the axis, measured in meters. A lens contains two top focal points.
Back top focal length: the reciprocal of the paraxial back top focal length of the lens measured in meters.
Front top focal length: the reciprocal of the front top focal length of the lens, measured in meters.
Top focal point of the lens: Usually the top focal point of the lens is set as the top focal point of the lens. The unit of the top focal is the reciprocal of the meter (m-1), the unit name is called "diopter", and the symbol is "D".
Prismatic degree: The deviation of light through a specific point on the lens. The expression unit of prism degree is centimeter per meter (cm/m), the unit name is called "prism diopter", and the symbol is "△".
Spherical lens: the posterior top focus of the spherical lens is called the spherical lens; A spherical lens is a lens that causes paraxial parallel rays to converge on a focal point, and this definition also applies to single-line-of-sight aspherical lenses.
Cylindrical lens: The difference between the two main focal points in a cylindrical lens. Cylindrical lenses are lenses in which paraxial parallel rays converge on two separate and orthogonal focal lines and, unlike spherical lenses, contain two main top focal points. One of the main top focal points can be zero, with the corresponding focal line located at infinity. Complex curved lenses, spherical and cylindrical combined lenses and cylindrical lenses are column lenses. Column lenses are also called astigmatic lenses.
Focometer classification: Focometer according to the display mode is divided into two categories: continuous display and digital display. According to the working principle, the focometer can be divided into two types: focometer based on the principle of focusing imaging and focometer based on the principle of autofocus. The focometer based on the principle of focusing imaging can be divided into two kinds according to the different observation methods: visual and projection. The visual focometer uses the reading telescopic system for observation. The projection focometer uses the projection objective lens and the projection screen to observe. The focus meters in use on the market mainly have two categories: automatic digital display and manual (digital display and direct reading)