Ishihara test for color blindness

Color vision In the retina, there are 2 types of photoreceptor cells: Rods and Cones. Rod cells function only in dim light and are blind to color. Cone cells operate in bright light and are responsible for high acuity vision, as well as color. They contain single visual pigments selective for either red, green, or blue light. Rods generally outnumber cones more than 10 to 1—except in the retina's center, or fovea. The cones are highly concentrated in the fovea. Even though the fovea is essential for fine vision, it is less sensitive to light than the surrounding retina. Thus, if we wish to detect a faint star at night, we must gaze slightly to the side of the star in order to project its image onto the more sensitive rods, as the star casts insufficient light to trigger a cone into action. Colorblindness - is a condition in which certain colors cannot be distinguished, and is most commonly due to an inherited condition. Red/Green color blindness is by far the most common form, about 99%, and causes problems in distinguishing reds and greens. Another color deficiency Blue/Yellow also exists, but is rare. Total color blindness (seeing in only shades of gray) is extremely rare.

Farsightedness Nearsightedness

Vision 20/20 vision – how vision is measured. If you have 20/20 vision, you see things from a distance of 20 feet the same as someone with perfect vision. If you have 20/100 vision, you can read at 20 feet what someone with perfect vision can read at 100 feet. Some people have 20/15 vision. What are they able to do? Nearsightedness - ablity to see near objects well but difficulty seeing objects that are far away. Light rays become focused in front of the retina. This is caused by an eyeball that is too long, or a lens system that has too much power to focus. Nearsightedness is corrected with a concave lens. This lens causes the light to diverge slightly before it reaches the eye Farsightedness - ablity to see distant objects well but difficulty seeing objects that are near. Light rays become focused behind the retina. This is caused by an eyeball that is too short, or by a lens system that has too little focusing power. This is corrected with a convex lens. Astigmatism - an uneven curvature of the cornea and causes a distortion in vision. To correct this, a lens is shaped to correct the unevenness. Presbyopia - As we grow older, the lens becomes less elastic. It loses its ability to change shape. The loss of elasticity prevents the lens from becoming thicker. As a result, we lose the ability to focus on close objects. This usually becomes noticeable when we reach our mid-forties. To correct this, bifocals are required. Bifocals are a combination of a lower lens for close vision (reading) and an upper lens for distance vision.

Eye anatomy The eye is a complex organ that is very similar to a camera in the way that it captures and focuses light. Sclera – the outer, white layer is a tough, protective shell. Cornea - the front, transparent part of the eye through which light enters. The cornea provides the eye with much of its light focusing power. Iris - is the colored part of your eye and is embedded with tiny muscles that dilate (widen) and constrict (narrow) the pupil. Pupil - is the opening in the center of the iris, controls the amount of light that enters the eye. Lens – focuses light rays by adjusting its shape and thickness and projects the image to the retina. Retina - a nerve tissue layer that lines the back of the eye. The retina contains millions of photoreceptors that capture light rays and convert them into electrical impulses. Optic nerve – carries the electrical images to the brain where they are turned into images. Aqueous humor – a clear fluid inside the front part of the eye. The fluid nourishes the lens, iris and cornea. Vitreous humor – a clear jelly-like substance in the center of the eye This substance gives the eye form and shape.