How Do Glasses Work?
The eyes are complex organs that must work together to respond to light and produce clear vision. In the presence of light, an individual is able to see because light reflects of objects and enters the eyes through the cornea (the clear, front surface of the eye).
The cornea focuses light onto the back of the eye. It comprises five main layers:
Epithelium – the thin surface layer. This layer contains fast-growing, easily regenerated cells. It is kept moist with tears.
Bowman’s layer - a tough layer comprising mainly protein fibres. It stops the cornea from swelling forwards.
Stroma – the thick middle layer making up the majority of the cornea. It contains highly arranged protein fibres and supporting cells called keratocytes.
Descemet’s membrane – the thin inner layer that is important for the health of the endothelial cells in the endothelium of the cornea.
Endothelium – the layer that is rich in cells that regulate fluid. It prevents the cornea from getting too wet.
Light passes from the cornea to the aqueous humour (an area of clear fluid) and the pupil (the circular opening in the centre of the iris, the coloured part of the eye). The iris can contract, limiting the amount of light that can enter the eye, or dilate, increasing the amount of light that can enter the eye. Light then passes from here through a clear, curved structure called the crystalline lens. The lens further focuses light.
With the help of ciliary muscles, the lens can change its shape to focus on light reflecting from near or distant objects. This process is called accommodation. Light then passes through vitreous (clear jelly) and onto its final destination, the retina (the light-sensitive tissue at the back of the eye). The retina contains cells called photoreceptors, which send signals via nerve fibres to the optic nerve at the back of the eye.
The optic nerve then sends these signals to the visual cortex, an area in the back of the brain that controls an individual’s sense of sight. The visual cortex interprets these signals to build a representation of the object in question.
Problems that result when the eyes do not focus properly
Sometimes, the eyes are not able to focus properly. This can cause a number of problems, including:
Myopia, also known as near-sightedness or short-sightedness, occurs when the eye ball is too long from front to back. This means that light rays do not reach the retina. Instead, they stay in the front of the eye, causing distant objects to look blurry.
Myopia is usually the result of a combination of genetic and environmental factors that disrupt the normal growth of the eye. Symptoms often start around puberty and gradually worsen until the eye is fully grown (by around the age of eight).
Hyperopia, also known as far-sightedness or long-sightedness, occurs when the eye ball is too short from front to back, the cornea is not curved enough or the lens is not thick enough. This means that light rays focus behind the retina rather than directly on it, causing close up objects to look blurry.
Hyperopia has various causes, including increasing age, genetics and underlying medical conditions such as diabetes. When hyperopia occurs in people over the age of 40, it is known as presbyopia.
Unlike myopia, it is possible for an individual to overcome hyperopia.
Astigmatism occurs when the cornea or lens is not perfectly curved in shape. The cornea and the lens should be curved like the surface of a football, but in people with astigmatism, one or more of these structures has an irregular curve, similar to that of a rugby ball. This means that light rays do not focus properly, causing objects to look distorted.
Astigmatism may be present at birth, or it may develop after an eye injury or as a complication of eye surgery. It can cause symptoms such as headaches, tiredness and eye strain, particularly after tasks that involve focusing the eyes for long periods.
How corrective lenses work
The aforementioned eye conditions can be treated with corrective lenses. Lenses fall into two main types: convex and concave.
A convex lens is a type of lens used to correct hyperopia. It is thicker at the centre than at the edges. When it is placed in front of a long-sighted eye, it increases the refraction (bending) of light and shortens the focal length (the distance over which light travels) so that the image is focused on the retina. A convex lens is said to have a positive refractive power as it causes light rays to converge (come together).
A concave lens is a type of lens used to correct myopia. It is thicker around the edges than at the centre. When it is placed in front of a near-sighted eye, it reduces the refraction of light and increases the focal length so that the image is focused on the retina. A concave lens is said to have a negative refractive power as it causes light rays to diverge (spread apart).
Different types of lenses
Lenses can be further categorised by their type or types of focus:
Monofocal lenses, also known as single-vision lenses, have only one vision-correcting prescription across their surface. These lenses are suitable for individuals with only one type of vision problem.
Multifocal lenses have two or more vision-correcting prescriptions across their surface. These lenses are suitable for individuals with several vision problems.
The most common type of multifocal lens is the bifocal lens, which is split into two sections. The upper section is for distance vision and the lower section is for near vision. Bifocal lenses are usually prescribed for people with presbyopia.
Another type of multifocal lens is the trifocal lens, which is split into three sections. The upper section is for distance vision, the middle section is for viewing objects in the intermediate zone, such as computer screens, and the lower section is for near vision. Trifocal lenses are usually prescribed for people who require help with seeing objects that are within an arm's reach.
When multifocal lens are made to look seamless, they are referred to as progressive lenses.
Adjustable lenses are lenses with an adjustable focal length. This means that they provide variable focusing, allowing wearers to adjust their focus to see clearly at any distance.
Aspheric lenses are lenses that have differing degrees of curvature over their surface. In comparison to standard lenses, they are thinner, flatter and offer a larger area for vision-correcting prescriptions.
Plano lenses, also known as fashion lenses, are plain lenses suitable for use by those who do not require a vision-correcting prescription. These lenses have no vision-correcting properties.
How lenses are made
A prescription lens begins its life as a lens blank, which has a flat back surface and a curved front surface. Using an individual's prescription as a guide, a lab technician selects a lens blank that is as close as possible to the segment (ADD) and base curve selected by the optician. The ADD is the amount of additional correction required for an individual with presbyopia to be able to focus at close distances. It is measured in dioptres and generally ranges from +0.75 to +3.00. The base curve is the location in which a type of power called prismatic power is required in an individual’s lenses.
To ensure the power of the finished lens matches an individual’s prescription, the lab technician grinds a curve on the back of the lens blank. The grinding process requires painstaking precision to ensure that the lens is not only made to the exact prescription but that it is free from scratches that can affect the lens’ function.
The finished lens must have a power that matches that in the sphere (SPH) section of an individual’s prescription. The term "sphere" means that the correction required is spherical (equal in all meridians of the eye). The higher the spherical power, the stronger the curve required. If the SPH power is negative, it means an individual has myopia; if it is positive, it means an individual has hyperopia. The SPH power is always measured in dioptres.
If astigmatism is present, the lab technician must ensure that the finished lens helps to correct this. The cylinder (CYL) section of an individual’s prescription indicates the amount of power required to correct astigmatism. The term "cylinder" means that the correction power required is not spherical, but shaped so one meridian contains no added curvature and the meridian perpendicular to this contains the maximum power and lens curvature. Like the SPH power, the CYL power is measured in dioptres. A low number, such as 0.20, means that the eye is shaped more like a football than a rugby ball. A high number, such as 3.00, means that the eye is shaped more like a rugby ball than a football. The orientation of the astigmatism, measured in degrees, is found in the AXIS section of an individual’s prescription. It aids the lab technician in determining the angle at which the lens should be placed inside the frame.
If an individual’s eyes do not work well as a pair, the lab technician must apply prismatic power to the lens so that the lens can bend the path of light without altering its focus. The amount of prismatic power required is found in the prism section of the prescription, measured in prism diopters (P.D.)
Following grinding, the lens is edged to fit the frame the individual chose. The lens may also be tinted or coated for ultraviolet (UV) ray protection. A groove is then cut into the lens to ensure that it fits snugly into the frame.
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• Online Supplier of Glasses