Description of Terminology
The refractive index is the most critical element in the process of producing slim lenses. Light refraction refers to changes in the direction of movement of light caused by boundary surfaces of different media.
The refractive index is a value unique to a certain material and is used to indicate the degree to which the material in question is capable of refracting light. Therefore, the greater the refractive index of a material used, the thinner the resulting lens of identical power will be. Currently available plastic lenses have been developed with a refractive index of up to 1.49 -1.74.
The term "specific gravity of a substance" refers to the ratio of "the mass of the substance in question" to "the mass of a standard substance of the same volume." Normally, water of a temperature of 4 degrees Celsius is used as the standard substance. Since lenses become lighter the lower the specific gravity value, lenses with a low specific gravity tend to have a low refractive index, and, in some cases, this necessitates thickening of the lens, resulting in increased lens weight.
It is common knowledge that light from the sun (white light) comprises a mixture of various colors, and when this light passes through a spot in the lens with a prism effect, it divides into spectra (in wavelength sequence from red to blue) as shown in Fig. 2. This phenomenon is referred to as "light dispersion" and gives rise to the problem of chromatic aberration in a lens. Abbe's number is a value that shows the degree of light dispersion: The smaller the value of Abbe's number, the greater the degree of chromatic aberration, the greater the value, the lower the degree of chromatic aberration. When chromatic aberration occurs, the object viewed becomes colored and appears blurred. Taking the prism on the lens to be P (Δ) and the Abbe's value of the lens material to be v, the amount of chromatic aberration Pc is derived from the equation Pc = P/v, as shown in Fig. 3, and, while there are differences depending on the individual, normally, eyesight deteriorates when the Pc value is greater than 0.2.
Progressive Power Corridor
Progressive lenses comprise of a "distance portion" for viewing objects in the distance and a "near portion" for viewing objects at close quarters and these two are divided by an "intermediate portion" that progressively changes the strength of the lens. The progressive power corridor is the length from the bottom edge of the distance portion to the top edge of the near portion. Therefore progressive lenses enable the wearer to view objects in the distance by raising the eyes a little and objects nearby by lowering the eyes slightly. While a short power corridor reduces the degree by which eyes need to be lowered to view objects nearby, providing a wider range of close vision, the shorter the progressive power corridor, the greater the degree of distortion, known as astigmatism, at the sides, resulting in discomfort for the wearer. Although this depends on factors such as the application the wearer requires and becoming accustomed to the spectacles, the usual progressive power corridor length is approximately 14 mm for habitual, daily use.
The strength of an aspheric design lens is determined by the combination of its front and back surface curvatures. Although, previously, both front and back surface curvatures featured spherical designs, thanks to advances in technology, "aspheric design lenses" with an "aspheric" front surface, have become more widely used. This type of lens offers the advantage of being able to produce a thin, lightweight lens.
Double-sided Aspheric Design
Seiko succeeded in realizing the world's first "Double-sided Aspheric design" that features aspheric design on both the front and back surfaces. The result is a lens that is even thinner and lighter.
Basically, refractive correction of presbyopia requires two pairs of spectacles, one for viewing objects in the distance and one for viewing objects nearby, but constantly having to change from one pair of spectacles to the other is an unnecessary burden. To solve this problem, the multifocal lens was developed to enable the wearer to view objects in the distance and at close range with a single pair of spectacles. The simplest form of multifocal lens if the bifocal lens, known as the EX type, that features a lens for distance viewing and a lens for near viewing joined at the middle to form a single lens. In addition, trifocal lens are also available that enable the wearer to see not only objects nearby and in the distance, but also in the intermediate distance. Addition of finer gradations in the segment for intermediate distance vision results in a lens capable of graded variation from the distance lens strength to the near lens strength. This is the basic concept of the progressive lens.
The greatest advantage of the progressive lens is its absence of a conspicuous boundary line seen in the bifocal lens. In addition, the intermediate portion enables non-graduated changes in the lens strength, making this an outstanding lens that allows the user to focus on objects at any distance from close to long range.