Friday, March 28, 2008

Night Vision Without Batteries

© 2006

Without Batteries

At night, vision is limited to the existing light from the stars, moon and other man-made sources. In a disaster, the man-made sources may not be available. If you have a fire, you also have light. What if you don't have a fire?At night, without fire or electricity, when the clouds roll in it is VERY DARK in the woods. The only light would be from an occasional hole in the clouds that may (or may not) allow starlight to peek through. If you are used to sleeping with a night-light you may be in trouble.Fear of the DarkFear of the dark is a close cousin to fear of the unknown. If you can't see, you can't travel. On the plus side, if you are evading a group of desperados they can't see either. If you can't see, you can't shoot - at least not accurately. A gunshot in the dark is like a lighthouse on the ocean - "Here I am, shoot me!" To overcome the fear of the dark - and the fear of the unknown, you need to know how to use all your senses, especially your eyes. I know. You've all been seeing things all your life. But how many times did you really have to use them in almost total darkness? Even on the outskirts of any town, the ambient (reflected) light from the town's lights will make enough light to see objects. But, way out in the woods, at night with an overcast sky, the woods seem a lot larger and scarier than they really are. Small noises seem to be amplified, because there is no city noise to dampen out their movements. You can't hear a raccoon going through your garbage if you have the TV on. In the woods, you'll hear the raccoon walking up on you. The best way to take the "unfamiliarity" of the woods out of the picture, is to walk all around your camp before it gets completely dark. Become familiar with the terrain. Now, you know what's there - what the dangers are - and best of all, what's NOT out there. Most noises in the woods come from three sources:1. The weather. Wind blows things around and into each other. I remember my first visit to Florida after being raised in Indiana. My first night there I was absolutely sure there was a major storm outside my room. In the morning, I got up early to check out the storm damage. All it turned out to be was the wind blowing palm branches together in the dark. It sounds just like rain. The more wind, the more noise in the woods. Snow cover acts like a muffler – it can get eerily quiet in the snow. Ice makes dead branches "snap" off of trees and stepping on frozen grass makes a "crunchy" noise. Rain can block out all the noise in the woods. Including the noise you make. 2. Small animals and bugs. The noise at night in some areas is deafening. Crickets can drive you nuts. Frogs and birds call out to each other all night long. Bugs click and night feeding animals step on leaves and branches. Even deer, as agile and nimble as they are, make noise moving through the woods. Raccoons, the thieves of the forest, will find you and check you out. So what?3. Large animals and humans. On the average, there are very few "large" animals per acre in any woods. Most large animals will avoid you anyway. The real danger is from the human animal - the poorest equipped animal in the forest. If you think large animals are noisy, watch humans walking through the woods. Unless they are trained military troops or long-time hunters, it is almost comical.Walking in the woods at night, without making any noise, is almost impossible. Even the ninjas make noise, if you know what to listen for. The key to being silent in the woods at night is S L O W movement. By placing your every step in a spot you KNOW does not contain twigs or leaves, you can move almost silently. It's not a skill practiced by today's youth who hang out at the mall. Rural kids who hunt may have an idea how to do it – and all ex-Special Forces-types will know. Most haven't practiced the skill since their service days. However, unlike animals, most humans who slip and fall make noise. It could be just a "ummph" when hitting the ground, or escalate to a loud string of curses at everything in general. People are used to talking and communicating, even in pain. In the woods, the "ummph" is a dead giveaway not only that you are there, but where you are. But, in the dark, you cannot "SEE" them...or can you?TOTAL DARKNESSUnless you are underground or in a cave, total darkness is rare. Your eyes, while not as well equipped as an owl, can still see something. It's the interpretation of what you see — and how you use your eyes, that can make all the difference in the world.Adaptation (eye)In ocular (eye) physiology, adaptation is the ability of the eye to adjust to various levels of darkness and light.The human eye can function from very dark to very bright levels of light - its sensing capabilities reach across nine orders of magnitude. However, in any given moment of time, the eye can only sense a contrast ratio of one thousand. What enables the wider reach is the eye adapts its definition of what is black. The light level that is interpreted as "black" can be shifted across six orders of magnitude - a factor of one million.The eye takes approximately 30 minutes to fully adapt from bright sunlight to complete darkness and become one million times more sensitive than at full daylight. In this process, the eye's perception of color changes as well. However, it takes approximately five minutes for the eye to adapt to bright sunlight from darkness.THE HUMAN EYEIt is essential in this discussion that you become familiar with the internal structure of the human eye.Of these structures, I will talk mostly about the Retina, where the rods and cones are locatedCornea - the clear, dome-shaped tissue covering the front of the eye. Iris - the colored part of the eye - it controls the amount of light that enters the eye by changing the size of the pupil. Lens - a crystalline structure located just behind the iris - it focuses light onto the retina. Optic nerve - the nerve that transmits electrical impulses from the retina to the brain. Pupil - the opening in the center of the iris- it changes size as the amount of light changes (the more light, the smaller the hole). Retina - sensory tissue lining the back of the eye. It contains millions of photoreceptors (rods and cones) that convert light rays into electrical impulses that are relayed to the brain via the optic nerve. MORE ON THE RETINAThe retina contains two types of photoreceptors, rods and cones. The rods are more numerous, some 120 million, and are more sensitive than the cones. However, they are not sensitive to color. The 6 to 7 million cones provide the eye's color sensitivity and they are much more concentrated in the central yellow spot known as the macula. In the center of that region is the " fovea centralis ", a 0.3 mm diameter rod-free area with very thin, densely packed cones. The cones are what gives us highly detailed vision – but the rods are what enables us to see in the dark. THE BLIND SPOTThe human eye has a blind spot in its field of vision. This lies on the point of the retina where the optic nerve leads back into the brain. The retina has no light-sensitive rods or cones at this point, and so a small object in the field of vision's blind spot becomes invisible. The diagram shows how the optic nerves for left and right eyes are arranged symmetrically, so the blind spot of the right eye lies somewhere right of the center of vision and the blind spot of the left eye lies somewhere off to the left of center. Since the right eye can see whatever lies in the left eye's blind spot, and vice versa, the two eyes together provide complete vision. The brain fills in the "missing" data. Source: OurWorld.Compuserve.comThe bottom figure shows the distribution of rods and cones in the retina. This data was prepared from histological sections made on human eyes. In the top figure, you can relate visual angle to the position on the retina in the eye.Notice the fovea is rod-free and has a very high density of cones. The density of cones falls off rapidly to a constant level at about 10-15 degrees from the fovea. Notice the blind spot which has no receptors.At about 15°-20° from the fovea, the density of the rods reaches a maximum. Remember this if you want to present peripheral stimuli and you want to avoid the blind spot.Scotopic visionScotopic vision is the monochromatic (one color) vision of the eye in dim light. Since cone cells are nonfunctional in low light, scotopic vision is produced exclusively through rod cells, therefore there is no color perception. What this means to us, the "blind as a bat" survivalist, is only parts of our eyes are functioning in the dark. What we do see, will not be in color, but in black and white (shades of gray). Biological night visionIn biological night vision, molecules of rhodopsin in the rods of the eye undergo a change in shape as light is absorbed by them. The peak rhodopsin build-up time for optimal night vision in humans is 30 minutes. Rhodopsin in the human rods is insensitive to the longer red wavelengths of light, so many people use red light to preserve night vision as it will not deplete the eye's rhodopsin stored in the rods. Aviators and submariners have used red lights to maintain night vision for many years. Many flashlights have red plastic inserts for the lens. This lens should be used at night if you want to maintain your night vision. Red light at night does not travel as far as white light. You must use the eye's cones to see color...and at night, the cones don't work. You can see the red light up close, but it fades out the farther away you move the light. Some animals, such as cats, dogs, and deer, have a structure called the tapetum in the back of the eye that reflects light for even better night vision than humans, in which only 10% of the light entering the eye falls on photosensitive parts of the retina. This is the reason animal's eyes reflect brilliant colors when jack-lighted by a bright light. Source: WikipediaProtecting Night VisionWhile working and performing tasks in daylight, the exposure to light directly affects night vision. Repeated exposure to bright sunlight has an increasingly adverse effect on dark adaptation. Exposure to intense sunlight for two to five hours causes a definite decrease in visual sensitivity, which can persist for as long as five hours. This effect can be intensified by reflective surfaces such as sand and snow. At the same time, the rate of dark adaptation and the degree of night vision capability will be decreased. Since these effects are cumulative and may persist for several days, military neutral density (N-15) sunglasses or equivalent filter lenses should be used in bright sunlight when night operations are anticipated. NIGHT GLASSES (Binoculars) Night glasses are telescopes or binoculars with a large diameter objective. Large lenses can gather and concentrate light, thus intensifying light with purely optical means and enabling the user to see better in the dark than with naked eye alone. Often night glasses also have a fairly large exit pupil of 7 mm or more to let all gathered light into the user's eye. However, many people can't take advantage of this because of the limited dilation of the human pupil. To overcome this, soldiers were sometimes issued atropine eye drops to dilate pupils. Before the introduction of image intensifiers, night glasses were the only method of night vision, and thus were widely utilized, especially at sea. In the Second World War, night glasses usually had a lens diameter of 56 mm or more with magnification of seven or eight. Major drawbacks of night glasses are their large size and weight. MAKING NIGHT VISION WORKThere are two factors to good night vision: Eye movement and peripheral vision. You cannot use your eyes at night like you do during the least not if you expect to actually see anything. If you "stare" at an object you want to see using the normal focal point of the macula, you CANNOT see it because there are no (or very few) rods in this area of the eye. You have to be able to move your eyes slightly off the normal focus point to activate the rods in your interior eyeball. The key to night vision is to keep your eyes moving. Stationary objects are hard to see, and objects or things moving directly at you from the front are also difficult to see. Your best night vision is from some point to the SIDE of your normal eye focus point (peripheral vision). PERIPHERAL VISION DETECTS MOVEMENTPeripheral vision is a part of vision that occurs outside the very center of gaze. There is a broad set of non-central points in the field of view that is included in the notion of peripheral vision. "Far peripheral" vision exists at the edges of the field of view, "mid-peripheral" vision exists in the middle of the field of view, and "near-peripheral", sometimes referred to as "paracentral" vision, exists adjacent to the center of gaze. Peripheral vision is weaker in humans, compared with other animals, especially at distinguishing color and shape. This is because the density of receptor cells on the retina is greatest at the center and lowest at the edges (see visual system for an explanation of these concepts). In addition, there are two types of receptor cells, rod cells and cone cells; rod cells are unable to distinguish color and are predominant at the periphery, while cone cells are concentrated mostly in the center of the retina (the macula).Peripheral vision is good at detecting motion (a feature of rod cells), and is relatively strong at night or in the dark, when the lack of color cues and lighting makes cone cells far less useful. This makes it useful for avoiding predators, which tend to hunt at night and may attack unexpectedly.An object moving directly away from an observer provides fewer visual cues of movement than it would be moving across the field of view. However, changes in retinal-image size are produced that give a clue to its movement. Thus a stationary, but shrinking, luminous object in the dark is seen as if it were receding. Other clues to movement in depth are changes in the convergence…PRACTICAL NIGHT VISIONNight Vision ScanningDark adaptation or night vision is only the first step toward maximizing the ability to see at night. Night vision scanning enables soldiers to overcome many of the physiological limitations of their eyes and reduce the visual illusions that so often confuse them. The technique involves scanning from right to left or from left to right using a slow, regular scanning movement (Figure K-1). Although both day and night searches use scanning movements, at night soldiers must avoid looking directly at a faintly visible object when trying to confirm its presence. Bleach-Out EffectEven when off-center viewing is practiced, the image of an object viewed longer than two to three seconds tends to bleach out and become one solid tone. As a result, the object is no longer visible and can produce a potentially unsafe operating condition. To overcome this condition, the soldier must be aware of this phenomenon and avoid looking at an object longer than two to three seconds. By shifting his eyes from one off-center point to another, he can continue to pick up the object in his peripheral field of vision.If you are trying to evade, keep this chart in mind and avoid using light sources that attract attention:SHAPE OR SILHOUETTEAt night objects must be identified by their shape or silhouette. Familiarity with the architectural design of structures common to the area of operations determines one's success using this technique. For example, the silhouette of a building with a high roof and a steeple can be recognized in the United States as a church, while churches in other parts of the world may have entirely different architecture. Knowing what shape to look for is an important part of intelligence gathering. Let everyone in your group know what it is they may see at night. While your visual acuity may be only 1/7th of normal daylight vision, you can easily identify the shape of vehicles and personnel walking in the woods. SUMMARYPractice night vision techniques in your own neighborhood. Remember these factors:Keep your eyes moving as shown on the scanning diagram.Don't stare at any object longer than 2 seconds.Use your peripheral vision to detect movement.Know what the shape or silhouette is for common items in your area.Protect your night vision. Expect everything you see to be in black and white, not color.Movement directly away from, or directly toward you is the hardest to see. Keep moving your eyes, even if the object is moving directly toward you. It will take longer to recognize the shape or silhouette of the object than in the daytime. Don't get frustrated, this is normal.Don't forget to scan up, down and from side to side. Humans crawling slowly toward you are very hard to see at night.

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