According to DIN EN 13666: 2013:10, a single-vision lens “is designed to provide a single dioptric power”. Single-vision lenses usually offer visual correction for just one distance, be it for general distance vision, for a specific distance or for near vision. They are classified in spherical, aspherical, astigmatic and prismatic lenses.
Spherical lenses have the same refractive power in all meridians. Aspherical lenses have different refractive powers in every point of the lens regardless of the meridian. This results in a thinner lens design, less weight and significantly better image quality. Astigmatic lenses feature two meridians perpendicular to each other that have different refractive powers. Prismatic lenses use the prismatic effect which results from the prismatic deviation and base position. Prismatic deviation refers to the change in direction imposed on a ray of light as a result of refraction.
Multifocal lenses are lenses designed to provide two or more visibly divided portions of different focal powers.
Bifocal lenses enable clear vision at two different distances, usually for near and distance vision. The upper part is designed for distance use and the lower part for reading. Bifocal lenses eliminate the need to swap between different pairs of glasses for different visual tasks.
Trifocal lenses are multi-power lenses with three points of focus. Most have one zone for distance vision, one for intermediate vision and one for near vision. The intermediate zone usually provides 50% of the added magnifying power of the near zone.
Unlike bifocals or trifocals, progressive lenses offer the right dioptric power for every distance. They allow smooth vision at any distance. Progressive lenses are roughly divided into three vision zones: distance zone, progression channel (intermediate zone) and near zone. The transition between these zones is smooth and invisible on the outside. The width of the progression channel depends on the design of the progressive lens and the power of the addition. The peripheral areas on the right and left-hand side of the progression channel limit the useable zones of the progressive lens. The deviation from the required correction is usually so pronounced in the peripheral areas that the wearer cannot use them for direct vision.
Different types of light protection lenses are available to accommodate individual requirements. Selecting the appropriate lens depends, for instance, on the degree of latitude or where the wearer wants to use the spectacles, whether the correction eyewear is used only indoors or also outdoors, or whether the wearer wants to avoid irritating water reflections when fishing. An overview of the tint and filter options available is presented in the following.
Ophthalmic lenses are available in different colours and tint densities. People experiencing asthenopia may find relief by using slightly tinted lenses (10-20%) that are also often worn for cosmetic reasons. Spectacle lenses with a light tint can be worn all day. Traditional colours for highly tinted sunglass lenses are brown, grey and green. These colours have the least distorting effect on colour vision. Tinted lenses also increase comfort, reduce glare and protect the eyes from UV radiation.
Blue blocking filters
Standard light protection lenses evenly reduce the visible light across the entire spectrum. By contrast, blue blocking filters cut off a specific portion of the short wavelengths of light at a defined point in the light spectrum. This is why blue blocking filters considerably reduce glare and improve contrast vision. However, the more blue light is absorbed, the higher the risk of colour distortion.
Polarising filters help eliminate unwanted light reflections off shiny surfaces such as glass or glossy coatings. Polarising filters work like a narrow grid, allowing only waves oscillating parallel to the grid bars to pass. Light rays hitting the grid at an angle of 90° are fully blocked. Light reflected off horizontal planes is 100 percent polarised. Polarising filters take advantage of this effect. The more the oscillating angle deviates from the grid orientation, the less light passes through the filter.
Photochromic or self-tinting lenses
A photochromic lens changes in its transmission when exposed to UV light. The following factors influence the light transmission and darkening speed: type of light, light intensity, exposure time and lens temperature. The darkening technology is based on self-tinting molecules that change their structure. Since these molecules constantly react to the presence of UV light, the spectacle lenses perfectly adapt the tint to the light conditions.