Lens and its forms Faculty Aravind School of Optometry

Refraction through a Prism DEFG is the path of the ray through the prism. So ‘D’ appears to be at H. i.e. towards the apex. So any object seen through a prism will appears to be sifted towards apex 1  is defined as the object at 1meter shifted 1 cm towards the apex.

Basic lens construction It can be helpful to think of very basic lens forms in terms of prisms. Recall, as light passes through a prism it is refracted toward the prism base. Minus lenses therefore resemble two prisms apex to apex spreading light rays outward as they pass through the lens, while plus lenses resemble two prisms base to base converging light rays as they pass through the lens.

Convex lens Two prisms placed base to base can bring two rays of light, originally parallel to a focus. Concave lens Two prisms placed apex to apex refract light in a diverging manner.

Vergence Divergence Convergence

Common lens forms A lens has two curved surfaces of consequence to the vision of the wearer: the front surface and the back surface. Common lens shapes based on front and back curves are described in the figure below.

At the centre the sides are parallel and straight, so no Refraction takes place. The path of the ray is known as principal axis. Principal Axis

A small element whose side are parallel but not straight a ray passing through this element will also come out as parallel. If the lens is very thin PQRS will be a straight axis. Principal axis, secondary axis meet at one point that is known as optic center. Secondary Axis

Refraction by Spherical Lens A spherical lens is a portion cut off from a sphere. All the rays passing through a spherical lens will converge to a point known as principal focus Focal Length Distance between the lens and the principle focus is known a the focal length if the lens.

Refraction by Cylindrical Lens Action of a Convex Cylinder Rays of light striking the cylinder perpendicularly to the axis A’ A” are brought to a focus in the focal line F’ F” Refraction by a Concave Cylinder A point of light is brought to a focal lens after refraction through a cylinder.

A dioptre / Diopter A unit of measurement of the optical power of a lens or curved mirror, which is equal to the reciprocal of the focal length measured in meters. The same unit is also sometimes used for other reciprocals of distance, particularly radii of curvature and the vergence of optical beams. The term was coined by French ophthalmologist Felix Monoyer in 1872.

Dioptre The reciprocal if the focal length in meter is known as Diopter. ( i.e., a unit for measuring the power of the lens.) Power = 1/focal length in meters Example I. F = ½ meter  P = 1/ ½ = 2.0D II. F = 10 cm P = 1/ 10/100 = 100/10 = 10.0D

Corrective power of a lens Is determined by adding the front curve to the back curve. F1 + F2 = F from this equation for any given corrective power, an infinite number of curve combinations may be used to achieve the same result. Example: D D = D D D (Plano) = D D D = D

best form spectacle lens A best form spectacle lens is one whose surface powers have been specially computed to eliminate, or minimize, certain inherent defects in its image- forming properties. Practically speaking, the laboratory has a limited number of curve combinations with which to work. Lens blanks come from manufacturers with a limited selection of front curves, also known as base curves, with suggested power ranges for each. Furthermore, since aberrations occur as the eye moves away from the optical center of the lens, the lab will choose curves that minimize aberrations. Lenses with curves chosen to minimize aberrations are called "corrected curve" or "best form" lenses

Why do we choose best form?

Points to remember… A cylinder curve is curved along a single axis and flat along the perpendicular axis. While the focus of a spherical curve is a single point, the focus of a cylinder curve is a line. The meridian along which there is no cylinder power in the lens and consequently the meridian of the cylindrical focus is the cylinder axis which is expressed in degrees between 0 and 180. A lens that combines spherical and cylinder curves is called a compound lens or toric. The power cross is a representation of the two major meridians of the lens surface. The simplest combination to visualize is a plano with D cylinder. Â

Factors involved in BC selection Manufacturer recommendations, Frame selection, Aesthetics, Lens material, and Patient history.

Geneva Lens measure A lens measure has three points of contact which are placed on the lens surface to measure its curve. The outer two points are stationary while the inner point moves in or out to measure the sagittal depth of the lens. From the sagittal depth the instrument indicator displays the curve in diopters, with plus (+) curves shown in one direction and minus (-) curves in the other. The lens measure can also be used to determine whether a lens surface is spherical or toric by placing the lens measure on the optical center of a lens and rotating the instrument about the center. If the indicator does not move while rotating, the surface is spherical. If the indicator changes when the lens measure is rotated, the lens surface is toric, with the minimum and maximum readings corresponding to the meridians of power.

Power calculation

Thank You