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Conjunctiva:
Thin, Transparent tissue that covers the outer surface of the eye. It begins at the outer edge of the cornea, covering the visible part of the sclera, and lining the inside of the eyelids. It is nourished by tiny blood vessels that are almost invisible to the naked eye.
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3. Optic Nerve:
Transmits electrical impulses from the retina to the brain. The retina’s sensory receptor cells are absent from the optic nerve. Because of this reason, humans have a bind spot.
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4. Pupil:
Is the opening in the center of the iris. The size of the pupil determines de amount of light that enters the eye. The pupil size is controlled by the dilator and sphincter muscles of the iris. The sphincter muscle contracts when the eye is exposed to bright light, causing the pupil to constrict. The dilator muscles runs radially through the iris. This muscle dilates the eye in dim lighting.
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5. Iris:
The colored part of the eye is called the iris. The iris controls light levels inside the eye. The circular opening in the center of the iris is called the pupil. The iris has tiny muscles that dilate and constrict the pupil size. The iris is flat, and divides the front (anterior chamber) of the eye, with the back (posterior chamber).
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6. Cornea:
The cornea is the clear protective coating in the front of the eye that allows light to pass through it without distortion. The cornea covers the colored iris, and it’s the first part of the eye that refracts the light. The cornea provides most of the focusing power.
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7. Lens:
The crystalline lens is located behind the iris. The lens purpose if to focus light onto the retina. The nucleus of the lens is surrounded by softer material called the cortex. The lens is encased in a elliptical shaped bag, and is suspended within the eye by tiny wires called zonules.
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8. Ciliary Body:
The ciliary body lies behind the iris. One function of the ciliary body is the production of aqueous humor, which is a fluid that fills the front of the eye. It also controls accommodation by changing the shape of the lens. When the ciliary body contracts, the lens thickens, increasing the eyes ability to focus up close. When the ciliary body relaxes, the lens becomes thinner adjusting the eye focus for distance vision.
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9. Sclera:
The sclera is the white part of the eye. The sclera serves as an eye protective outer coat. Six tiny muscles connect to it around the eye and controls the eye movements. The optic nerve is attached to the sclera at the back of the eye. The sclera becomes yellow as we age.
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10. Retina:
The retina is the multi layered sensory tissue that lines the back of the eye. It contains millions of photoreceptors that capture light and convert them into electrical impulses. These impulses travel along the optic nerve to the brain where they are turned into images. There are two types of photoreceptors, rods and cones. Rods are spread out through the peripheral retina, and function best in dim light. Cones are more densely packed in the fovea, and function best in bright light.
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11. Fovea:
The concave center of the retina. The fovea is the region of highest visual acuity and cone cell density. There are approximately six million cones.
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12. Periphery:
There are hardly any cones in the peripheral retina. In contrast, there are many rods. These rods are more sparse here than closer to the fovea. The rods here are also shorter and wider than in the central retina. Receptive fields at the periphery are very large with many rods converging into one ganglion cell.
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13. Light Stimulus
All the visual stimuli that humans can perceive can be thought as a wave of electromagnetic energy. The wave is represented as a point in the visual spectrum. Each wave has two properties which are wavelength (nm) and amplitude. The hue perceived is determined by the wavelength, and the brightness is determined by the amplitude. The hue perceived is characterized by the combination of the blue, red, and green primary colors. The wavelengths visible to the human eye range from 400 nm (observed as red-violet) to a wavelength of 700 nm (observed as red). Mixtures of different wavelengths often act as stimuli, like in the case of purple which is a mixture of the red and blue wavelength. Pure wavelengths are rarely encountered. This pure wavelengths are often “diluted” by mixing them with different amounts of gray or white (which is light with no dominant hue). When wavelengths are not diluted, they are said to be saturated. A light stimulus can therefore be characterized by three values: hue, brightness, and saturation.

14. Receptor system The lens
Rays of light first pass through the cornea, which is a protective surface that absorbs some of the light energy. These rays of light then pass through the pupil which opens when its dark, to allow more light to come in, and closes as it gets brighter, to allow the right amount of light through the pupil. The lens of the eye changes its shape (accommodation). What accommodation does is that it brings images to precise focus on the retina (back part of the eyeball). The accommodation of the lens is accomplished by the use of the ciliary muscles. Receptors located in the ciliary muscles send information related to the accommodation of the lens to the brain. This is how accommodation is accomplished. The ciliary muscles accommodate, depending on the location of the object. If the object is close, the muscles must accommodate by changing the shape of the lens to a rounder shape. In contrast, when the location of the image is far, the muscles accommodate the lens, so that it becomes flatter. The accommodation of the lens depends on distance and can be described in terms of diopters, which is equal to 1/viewing distance.

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The shape of the lens takes some time to adjust, and there are some problems that many people encounter related with the accommodation of the lens. One problem is called Myopia (nearsightedness) which happens when the lens cannot flatten, and therefore objects at a large distance cannot be brought into focus. In contrast, Presbyopia (farsightedness) results when the lens cannot accommodate to focus objects that are up close (lens does not curve). Accommodation can also be affected by the amount of light. If the lighting is not enough, it may be harder to accommodate the lens, which makes it harder to focus a specific image.
The Visual Receptor System
Any given image is characterized by its luminance, wavelength, and size. The image size is expressed by its visual angle. This angle can be calculated by using this formula where H is the height of the object, seen at a distance D. If the visual angle is less than 10 degrees, the angle may be expressed in minutes of arc using the formula VA = 5.7 x 60 x (H/D).

16. The image can also be characterized by its position on the back of the retina, since depending on this position , different types of visual receptor cells will be used to transform images into electrical impulses which will be further processed by the brain. There are two types of receptors which are called rods and cones which have six different properties that are:
1. Location: The middle section of the retina (fovea) has a visual angle (VA) of around 2 degrees which contains cones. Outside the fovea, the periphery is mainly occupied by both rods and cones. Concentration of cones declines rapidly while moving farther away from the fovea.
2. Acuity: The amount of fine detail is greater when the image falls on the closely spaced cones than on the sparse spaced cones.
3. Sensitivity: Cones have an advantage over the rods when it comes to acuity, but the rods have an advantage over the cones when it comes to sensitivity. Therefore the fovea is very poor for picking up dim illumination since there are not rods in the fovea. Rods are good for Scotopic Vision (night vision) Photopic Vision refers when both rods and cones are used.
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17. 4. Color Sensitivity: Rods cannot discriminate different wavelengths, and therefore they are “color blind.” Therefore, the extent to which hues can be resolved declines in peripheral and night vision.
Adaptation: Rods lose their sensitivity when they are exposed to light Time is needed to regain this sensitivity. It can take up to half an hour to regain sensitivity and adapt to darkness.
Differential Wavelength Sensitivity: Cones are sensitive to all wavelengths, while rods are only sensitive to long wavelengths. This is the reason why color with long wavelengths look very black at night. The rods sensitivity can be maintained if objects are illuminated by red light .

18. Limitations Contrast Sensitivity
The ability to detect contrast is necessary in order for the eye to detect shapes. The contrast of a given pattern is given by the difference between the luminance of light (L), and dark (D) areas divided the addition of L and D. C = (L - D)/(L + D).
The higher the contrast sensitivity, the smaller the value of the minimum amount of contrast that can just be detected (CM). CM describes the contrast threshold with the formula: CS = 1/CM.
Contrast sensitivity can be affected by several factors. One of these factors is concerned with lower contrasts and how they are less easily discerned. Color contrast is important to distinguish objects. As an example, it is easier to read a word with black font printed in a white paper, than on a dark gray paper. Another influence on color sensitivity is the level of illumination of the stimulus (L + D). Lower illumination reduces the contrast sensitivity, and this sensitivity is affected more dramatically for sensing high spatial frequencies, than for low frequencies. An example is that it is hard to read fine printed words when the illumination is low.

19. Other factors that affects contrast sensitivity are the resolution of the eye and the dynamic characteristics of the viewing conditions. Increasing age reduces the amount of light that passes though the cornea, and therefore reduces contrast sensitivity. The accommodation of the lens is also affected with age, and when it is combined with the reduction of total light passing through the cornea results in a poor contrast sensitivity when there is low illumination. Motion also affects contrast sensitivity. The higher and faster the motion, the lower the contrast sensitivity.
Color Sensation
Color vision can be well employed in a well illuminated area. Color can be distinguished by the different hues. Many people cannot distinguish color even if the level of illumination is sufficient or high. Approximately 7% of the male population is color deficient (unable to discriminate certain hues from each other). The majority of the 7% cannot distinguish between the red-green hues, since the wavelengths create identical sensations if they are of the same luminance. Simultaneous contrast is the tendency of some hues to appear different when viewed adjacently with other hues. Color should not be used for any type of selection since some people are color blinded. Redundancy is needed.

20. Night Vision
The loss of contrast sensitivity at all spatial frequencies affects the perception of print, and also the detection and recognition of objects by their shapes or color in poorly illuminated viewing conditions.

21. Depth Perception
Humans rely on several depth cues, In order to judge distance from objects. The first three cues described are all dependent on the physiological structure and wiring of the visual sensory system. These cues are accommodation, binocular convergence and binocular disparity . These cues are effective for judging distance, slant and speed of object within a few meters.
Accommodation is related to the change in shape of the lens that makes the eye focus images at different distances. Sensory receptors in the ciliary muscles causes the lens to change shape depending on the object’s distance.
Convergence is based on the amount of inward rotation that the muscles in the eye must accomplish in order to bring an image to rest on the retina, and in both eyes. The closer the distance to the object, the greater the inward movement of the eyeball. The farther away the less inward rotation.
Binocular disparity results that the closer an object if to the person, the greater the amount of disparity received by each eye. The brain uses this measure of disparity to compute a location where the signals from both eyes are combined in the brain, and therefore we can estimate the distance of an object.

22. The judgment of depth and speed at larger distances is related to some “Pictorial” cues. These cues are linear perspective, relative size, interposition, light and shading, textural gradients, and relative motion.
Linear perspective is based that parallel lines converge toward more distant points. An example can be a road which converges to single point.
Relative size is based that if two objects are known to be the same size, then the object which occupies the smaller visual angle is farther away.
Interposition is a cue which defines that nearer objects tend to obscure the contours of objects that are farther away (nearer objects block farther objects).
Light and shading is a property that 3D objects have. The shadow can provide evidence of the objects location in 3D form.
Textural gradient is based that some textured surfaces which are viewed at an angle will show a change in texture density. The finer density signals the farther part of the object.

23. Relative motion (parallax) is a cue that describes that objects which are farther away show smaller movement on the visual field of the observer. An example can be of a person traveling on a car. If the person looks out the window the objects at a distance will move slower on the persons visual field, compared to the closer objects.
All of these cues combined provide humans with a sense of their position in the 3D space.

24. Search and Detection How the eye moves
The movement of the eye is key for the searching of visual fields. These movements can be divided into two important classes which are pursuit and saccadic. Pursuit is related to the movement of the eye at constant velocity. In contrast saccadic movements are more abrupt. The movement time of the eye is usually 50 ms, and does not vary much with movement.
Visual Search
When it comes to the search of an object, people usually have a target or destination, and they have to distinguish between targets and non-targets. Sometimes non-targets need to be inspected in order to determine if the object is the desired target or not. Most of the searches are serial, and this means that each object is inspected (one at a time) to see if it is the target object. The search yields a average time until detection which is modeled by the formula T = (N x I)/2, where N is the total number of items in the search field and I is the average inspection time per object.

25. People usually tend to search from top to bottom, and left to right, since in most of the times people tend to move their eyes naturally in those directions. If the search is not organized from top to bottom, or left to right, then the eye searches randomly until they examine all the locations (an example is a map).
Visual search has two important influences which are conspicuity, and expectancies. Certain objects are said to be conspicuous since they catch the eye’s attention not mattering where they are in the visual field. If this happens non-target items do not need to be inspected. This search can be described as “parallel” since all items can be examined at once.
Expectancies, on the other hand, are based upon prior knowledge. Sometimes people search depending on where they expect to find the desired target. An example of this influence is when a secretary is looking for a phone number with a specified last name. The secretary expects to find the phone number depending on the first letter of the last name, therefore she can search for that phone number easily.

26. Detection
After a target is located in the visual search, then it is necessary to confirm that the object is really the desired target. This process is sometimes trivial, but in many cases it is a difficult task. Signal detection is often critical, and varies from person to person.
Signal Detection Theory (SDT) is used to model the process of signal detection, and assumes that a signal can be either present or absent, and there is noise that makes detection harder to accomplish, and detection time longer. The goal when detecting a signal is to discriminate it from noise. When detecting, and observer can choose between two different answers which are: “Yes (there is a signal)” and “No (there is only noise).” the combination of the two states and the two responses yields four joint events which are labeled: hit, false alarms, misses, and correct rejections. Hits and correct rejections represent good outcomes, the remaining two are the ones we need to avoid. The probability of an observer to get a hit is given by the formula p(hit) = #hits/#signals. The probability of a miss is p(miss) = 1 – p(hit). The probability of a false alarm is given by P(FA) = #FA/#no-signal encounters. The probability of a correct rejection is p(CR) = 1 – p(FA).

27. The data from a signal detection is usually represented in a 2 x 2 matrix given below:
Signal Present
Signal Absent
Yes (Signal seen)
No (No signal perceived)
Hits
False Alarms
Miss
Correct Rejection

28. Discrimination
Very often humans have the ability to discriminate between several signals, instead of detecting a desired signal. Sometimes humans fail to discriminate, and this happens when the signals create a similar stimuli. Failing to distinguish between two colors is an example of humans failing to discriminate. Not only signals with similar stimuli can cause persons to fail to discriminate, discrimination is also greatly affected by the outside conditions such as lighting.

29. Conditions
Usually the eye is susceptible to many diseases and disorders. Below are the five most common eye disorders.
Dry eye syndrome
Macular Degeneration
Glaucoma
Cataracts
Diabetic Retinopathy

30. About Net Vision
Net Vision gives detailed information about how we see and the way the eye works. Net Vision is the leader in information about the mechanics of the human eye.
To learn more about Net Vision, we invite you visit our site page on contact Net Vision, there you can ask questions, give us a call, or mail us at our business.

31. References
St. Luke's Cataract and Laser Institute
Eye Anatomy
Terms and Definitions Related to Macular Degeneration
Visual Search and Attention: A Signal Detection Theory Approach

32. Contact Net Vision By Phone 555-1010 By Email questions@netvision.com
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