Terminology

Myopia - Nearsightedness, or myopia, is the most common refractive error of the eye, and it has become more prevalent in recent years. If you are nearsighted, you typically will have difficulty reading road signs and seeing distant objects clearly, but will be able to see well for close-up tasks such as reading and computer use.

Hyperopia - Hyperopia, or farsightedness, is a common vision problem, affecting about a fourth of the population. People with hyperopia can see distant objects very well, but have difficulty focusing on objects that are up close.

Astigmatism - Astigmatism is probably the most misunderstood vision problem. Astigmatism usually causes vision to be blurred or distorted to some degree at all distances.

Presbyopia - Presbyopia usually occurs beginning at around age 40, when people experience blurred near vision when reading, sewing or working at the computer. Presbyopia is caused by an age-related process. These age-related changes occur within the proteins in the lens, making the lens harder and less elastic over time.

Corneal Topography -  A non-invasive medical imaging technique for mapping the surface curvature of the cornea, the outer structure of the eye. Since the cornea is normally responsible for some 70% of the eye's refractive power, its topography is of critical importance in determining the quality of vision and corneal health.

Index of Refraction - The index of refraction of an eyeglass lens material is a number that is a relative measure of how efficiently the material refracts (bends) light, which depends on how fast light travels through the material. The higher the refractive index of a material, the slower light moves through it, which results in greater bending (refracting) of the light rays. So the higher the refractive index of a lens material, the less lens material is required to bend light to the same degree as a lens with a lower refractive index. In other words, for a given eyeglass lens power, a lens made of a material with a high refractive index will be thinner than a lens made of a material with a lower refractive index.

Abbe Value - The Abbe value (or Abbe number) of a lens material is an objective measure of how widely the lens disperses different wavelengths of light as light passes through it. Lens materials with a low Abbe value have high dispersion, which can cause noticeable chromatic aberration — an optical error visible as colored halos around objects, especially lights. When present, chromatic aberration is most noticeable when looking through the periphery of eyeglass lenses. It is least noticeable when looking directly through the central optical zone of the lenses.Abbe values of eyeglass lens materials range from a high of 59 (crown glass) to a low of 30 (polycarbonate). The lower the Abbe number, the more likely the lens material is to cause chromatic aberration.

Dry Eye Syndrome - Dry eye syndrome is caused by a chronic lack of sufficient lubrication and moisture on the surface of the eye. Persistent dryness, scratchiness, red eyes and a burning sensation are common symptoms of dry eyes. 

Cornea - The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. 

Retina -  The Retina is the third and inner coat of the eye which is a light-sensitive layer of tissue. It is a layered structure with several layers of neurons interconnected by synapses. The only neurons that are directly sensitive to light are the photoreceptor cells. These are mainly of two types: the rods and cones. Rods function mainly in dim light and provide black-and-white vision, while cones support daytime vision and the perception of color. 

Optic Nerve - The optic nerve is located in the back of the eye. It is also called the second cranial nerve or cranial nerve II. It is the second of several pairs of cranial nerves. The job of the optic nerve is to transfer visual information from the retina to the vision centers of the brain via electrical impulses. The optic nerve is made of ganglionic cells or nerve cells. It consists of over one million nerve fibers. Our blind spot is caused by the absence of specialized photosensitive (light-sensitive) cells, or photoreceptors, in the part of the retina where the optic nerve exits the eye.

Cataracts - A cataract is a clouding of the eye's natural lens, which lies behind the iris and the pupil. Cataracts are the most common cause of vision loss in people over age 40 and is the principal cause of blindness in the world.

Intraocular Lens - An intraocular lens (IOL) is a lens implanted in the eye used to treat cataracts or myopia. The most common type of IOL is the pseudophakic IOL. These are implanted during cataract surgery, after the cloudy crystalline lens (otherwise known as a cataract) has been removed. The pseudophakic IOL replaces the original crystalline lens, and provides the light focusing function originally undertaken by the crystalline lens.

Melanoma - Melanoma is a type of cancer that develops in the cells that produce melanin — the pigment that gives your skin its color. Your eyes also have melanin-producing cells and can develop melanoma. Eye melanoma is also called ocular melanoma. Eye melanoma may not cause signs and symptoms. When they do occur, signs and symptoms of eye melanoma can include:

• A growing dark spot on the iris
• A sensation of flashing lights
• A change in the shape of the dark circle (pupil) at the center of your eye
• Poor or blurry vision in one eye
• Loss of peripheral vision
• Sensation of flashes and specs of dust in your vision (floaters)

Corneal Endothelium - The corneal endothelium is a monolayer of 350,000 to 500,000 specialized cells that cover the posterior surface of the cornea. One of the endotheliums physiological functions is to secrete a collagen matrix that forms Descemets membrane.1 But, the primary physiological function of the corneal endothelium is to maintain the health and transparency of the corneal stroma. Because the cornea is avascular, the supply of nutrients occurs via diffusion of glucose and other solutes from the anterior chamber across the cornea endothelium. To facilitate diffusion, intraocular pressure constantly forces aqueous into the stroma from the anterior chamber. Although the influx of aqueous into the stroma is necessary to maintain corneal health, the level of corneal hydration must be controlled to prevent edema.

Amblyopia - Amblyopia can be caused by the misalignment of the two eyes—a condition called strabismus. With strabismus, the eyes can cross in (esotropia) or turn out (exotropia). A common cause of amblyopia is the inability of one eye to focus as well as the other one. Amblyopia can occur when one eye is more nearsighted, more farsighted, or has more astigmatism. The condition is also referred to as lazy eye.