Understanding the Eyes
Proper vision depends upon the health of a number of organs, including the eyes themselves, the muscles that support and move the eyes, the optic nerve, and various portions of the brain that sense and interpret visual input (Merck 2013).
The eye consists of the following parts (Merck 2013; Swartz 2014):
- Sclera: Outer layer of the eye; it is fibrous and white in appearance, often referred to as the “white of the eye.”
- Conjunctiva: Thin transparent membrane located in the front of the eye over the sclera.
- Cornea: Dome-shaped region in the front-center of the eye through which light enters and is partially focused by the cornea.
- Pupil: Central black dot of the eye through which light is transmitted.
- Iris: Circular colored area of the eye which controls the size of the pupil in response to ambient light conditions (the iris connects to small muscles that dilate the pupil in response to dim light and constrict the pupil in response to bright light).
- Lens: Situated behind the pupil, the lens refracts light and helps focus it on the part(s) of the eye that transmit visual stimuli to the brain; it can change its shape to allow for both near and far vision.
- Vitreous Humor: Transparent, gel-like substance comprised largely of water that fills the cavernous inner part of the eye between the lens and the retina (Bishop 2000).
(WikiMedia Commons 2013)
The retina contains 2 types of photoreceptors called rods and cones. The rods are much more numerous than cones and are responsible for low-light vision, peripheral vision, and black and white vision. Cones are responsible for color vision and are grouped mostly in the center of the retina. In the photoreceptor cells, the light that enters the eye is converted into electrical nerve impulses, which enter the optic nerve, cross the optic chiasm, and travel to the back of the brain where they are interpreted (Merck 2013).
Photoreceptor cells are protected against light damage by the macular pigments (lutein, zeaxanthin, and meso-zeaxanthin) (Krinsky 2003; Berg 2002). The macular pigments sit above the photoreceptor cells to absorb blue light and neutralize free radical chemicals (Yanoff 2011; Chucair 2007; Ehrlich 2008; Nolan 2013; Carpentier 2009; Krinsky 2003). Macular degeneration is characterized by a number of factors including reduced levels of retinal pigments and reduced blood flow to the macula (Ehrlich 2008; Ferri 2014).
Understanding Visual Acuity
Visual acuity is typically reported as one number over another number; for example, 20/20 is considered “normal” vision. The number is usually determined based upon an individual’s ability to read a Snellen chart (the chart in the eye doctor’s office with letters that decrease in size with each progressive line). But what exactly does this mean?
The first number in the ratio represents the individual’s distance in feet from the eye chart at which he or she can clearly read the letters. The second number represents the distance in feet at which a person with normal vision could read the chart.
So, a visual acuity rating of 20/40 means that an individual has to be 20 feet away from the chart to read lines that a person with normal vision could read from 40 feet away. Conversely, if a person’s vision is better than average, he/she might have a visual acuity rating of 20/10, meaning that individual can read the chart from a distance of 20 feet, but a person with normal vision would need to stand 10 feet from the chart to read it (A.D.A.M. 2013; AOA 2014a).