Vision.pdf

Vision By: Abdullah Munqith The eye produces a real, inverted image of the object being viewed. The image is produced on the retina the lightsensitive region at the back of the eye. The cornea: It is a fixed focusing element, and it is a clear transparent bump in front of the eye, it does≈ 2/3 of th...

Vision By: Abdullah Munqith The eye produces a real, inverted image of the object being viewed. The image is produced on the retina the lightsensitive region at the back of the eye. The cornea: It is a fixed focusing element, and it is a clear transparent bump in front of the eye, it does≈ 2/3 of the focusing, it focuses by refracting the light rays. The pupil: Is the opening in the center of the iris where light enters the lens. Under normal condition of light intensity the opening is about 4mm in diameter. It can change from about 3mm in diameter in bright light to about 8mm in diameter in dim light. Vision The eye produces a real, inverted image of the object being viewed. The image is produced on the retina the lightsensitive region at the back of the eye. The cornea: It is a fixed focusing element, and it is a clear transparent bump in front of the eye, it does≈ 2/3 of the focusing, it focuses by refracting the light rays. The pupil: Is the opening in the center of the iris where light enters the lens. Under normal condition of light intensity the opening is about 4mm in diameter. It can change from about 3mm in diameter in bright light to about 8mm in diameter in dim light. Vision The iris: Is a (pigmented) diaphragm that controls the intensity of the light reaching the retina. It does this by adjusting the size of the pupil in response to signal from the retina. In other words Iris regulates the amount of light entering the eye by controlling the diameter of the pupil. The sclera: (the white of the eye) is the eyes tough outer cover. Its inner lining, the choroid, provides the blood supply for the retina. It contains a large amount of black pigment in order to reduce reflection of the light within the eye and so prevent blurring of the image. Vision The aqueous humor: It is a fluid fills the space between the cornea and the lens. This fluid is mostly water. It is continuously being produced and the surplus escapes through a drain tube (canal of schlemm). Blockage of the drain tube result in increased pressure in the eye, this is called glaucoma. It maintains the internal pressure of the eye at about 20mmHg. The vitreous humor: Is a clear jelly – like substance that fills the large space between the lens and retina. It helps keep the shape of the eye fixed and permanent. It is sometimes called the vitreous body. Vision The lens: Lens contributes only about 25% of the refractive power of the eye. Ciliary muscles: The shape, and therefore the focal length, of the eye lens can be altered by the action of the ciliary muscles attached to it. This makes it possible for light from objects which are at different distances from the eye to be brought to a focus on the retina which is at a fixed distance from the lens. Retina consists of millions of photo-receptors called rods (1.3x108) and cones (7x106) converts light energy into electrical energy. These structures send electrical impulses to the brain via the optic nerve. Vision Fovea is the most sensitive part of retina slightly off center and it contains only cones. Macula is a larger area including and surrounding the fovea and contains rods and cones. Blind spot on retina at optic disk. At the optic disk there are no photo-receptors. Blind spot not noticeable because each eye compensates by seeing what the other doesn’t. Blind spot is the point at which the optic nerve joints the eye (from the brain). Q- Why do we call the blind spot? - Because: there are no light-sensitive receptors there. Vision Vision Accommodation Lens to retina distance is fixed. Focal length (and hence the power) of the lens must vary to ensure objects at various distances can be brought to a focus at the retina process called Accommodation Normal eye is capable of focusing on objects over a range from infinity (far point) (eye muscles relaxed) to the near point of 25 cm. Near point increases with age. Eye defects Farsightedness (hyperopia) Object at normal near point, eye lens cannot be made sufficiently converging to bring object to focus on retina; lens too weak (focal length too long). Converging eyeglass lens required to ensure focus at retina. Nearsightedness (myopia) Eye lens cannot relax sufficiently to focus a very distant object on the retina, lens too strong (focal length too short). Diverging eyeglass lens required to ensure focus at retina. Eye defects Astigmatism Results from an asymmetry in the cornea (irregular curvatures). Cornea: different curvature in different directions. Astigmatism can be corrected using cylindrical lenses, correctly orientated Presbyopia Loss of accommodation (presbyopia) Inability for eye to accommodate increases with age. Lens becomes less flexible and can no longer change its strength over the normal range. Eventually affects nearly everyone; difficulty in seeing near and distant objects clearly. Visual Acuity Visual acuity is ability of the eye to see fine detail; also known as spatial resolving power. The factors that limit visual acuity are: - Illumination - Contrast - Location of image on retina. Someone with 6/6 or 20/20 vision (visual acuity) is just able to decipher an object (letter) that another one with 6/9 vision is not. 6/6 not perfect. Young adults 9/6 or 10/6 Hawk 60/6 Electromagnetic Waves Visible waves Wavelength: ≈ 400 nm -700 nm The only waves the eye can see Red light has the highest wavelengths and violet has the lowest. Color Vision Isaac Newton (1643—1727) demonstrated that white light is composed of several colors. White light passing through a prism is separated into its constituent colors. Reason: refractive index of the glass in the prism varies with wavelength (color) This variation of refractive index with wavelength is called Dispersion Objects have a particular colour because of their reflection/absorption characteristics. For example; an apple appears green because all wavelengths except green are absorbed; green is reflected. Various levels of absorption will result in an object having more complex shades of color. Color Vision Rods and Cones are light sensitive cells in the retina: Rods Highly sensitive Low light level vision Peripheral vision Almost no color information Cones Less sensitive Color vision Concentrated in fovea 3 kinds of cones: each one sensitive to a different array of wavelength. Color Blindness Color blindness results from absence of one or more types of cones The sensitivities of the three cones overlap and the perceived color is due to the relative response of the three cones. Color matching Color matching impaired by color blindness which results from absence of one or more types of cones as mentioned. Color fatigue is resulted from prolonged exposure to one color reduces the response of the eye to that color. Perceptual Psychology: For example appearance of an object depends on the background color. Dark backgrounds make materials appear brighter. In dentistry color matching must be taken in account in restorative work. Tooth enamel is translucent when the light passes through it. So the main color of tooth is that of dentine color. Color matching Color matching Color matching