Optical illusions, the eye's blind spot and personal identification by iris recognition are all special effects that reveal the physical limits and explore the vast potential of the human eye.
Optical illusions
Optical illusions are fun tools that reveal the limits of human visual perception.
People receive visual information through their optical senses, but interpret that information with their brain. The brain relies on past experiences to figure out what people are seeing. Sometimes, it interprets this information incorrectly. Optical illusions are good examples of the brain’s misinterpretation of visual information.
Illusion no. 1
Which person is tallest?
From an early age, most people learn that distant objects look small. The person on the far left of this illustration seems more distant from the center of the converging lines (far right) than the person on the right (closest to the converging lines). Therefore, the person on the far left looks smaller. In fact, all three people are the same size!
People who live in the jungle often have trouble judging distances in open country. They have not learned that distant objects look small. This visual cue is missing from their personal experiences.
Illusion no. 2
Sometimes, there is more than one way the brain can interpret an image. For example, do you see a white vase or two black faces? To view the faces, visualize white as the background. To view the vase, visualize black as the background. Which image did you see first?
Illusion no. 3
Which square is bigger?
Sometimes, the brain distorts reality because it incorrectly interprets the environmental clues that surround an object. Here a square is surrounded, in one case, by large rectangles and, in another case, by small rectangles. When the surroundings are large, the square appears small and compact. When the surroundings are small, the square appears more expansive. Both central squares are the same size. The brain has been fooled into making a false judgment about their comparative size. A variation of this illusion was first presented in 1892.
Why does the moon look bigger at the horizon than it does when overhead?
Obviously, the moon’s size doesn’t change. When the moon is close to the horizon, experts explain, it is nearer to familiar objects, such as mountains or skyscrapers. The brain erroneously interprets this information and makes the moon appear larger than it really is. Without the benefit of surrounding objects, the moon seems to decrease in size as it climbs through the night sky.
Illusion no. 4
Are the long lines parallel?
The long, diagonal lines that run through this illustration look like randomly scattered feathers. But all of these lines are actually parallel to each other. They only look as if they are bending toward or away from each other because the short crossing lines confuse the brain. This happens because the brain tries to organize visual information into recognizable patterns. In fact, if you look closely, long lines that look the same appear to be parallel — for example, all of the long lines with short, horizontal crossing lines look parallel to each other.
Are the small lines crooked or straight?
A second optical illusion is evident in this illustration. When the short lines cross the long ones, they look bent. In reality, all are straight. The perception is altered by the nearness of other short lines, which go in an opposite direction — confusing the brain.
When people close one eye and look around, they don’t see a blind spot. But it is there! Everyone has a blind spot — one in each eye — where a part of our picture of the outside world is missing.
Images of the world around us are projected through the eye’s lens onto the retina, which is covered with light-sensitive photoreceptors. These specialized nerve cells or neurons act like a sheet of film coupled to a digital camera. They record images, then translate them into electrical impulses for the brain.
These signals are relayed to the brain by the chain of neurons which form the optic nerve. This relay system starts at the optic nerve head, a round disk on the retina where the optic nerve “plugs into” the sheet of photoreceptors that blankets the back of the eye. The optic nerve head can’t “see” anything, because this round disk has no photoreceptors. In effect, it’s a black hole in the retina’s sheet of color film.
Why don’t people have a hole in their vision? The brain fills in the gaps, experts explain. It “makes up” images to bridge the blind spot. It uses previous experience and what the opposite eye sees to create a complete, uninterrupted view of the world.
How can people find the blind spot?
Close the left eye, then look at the small white cross on the left of this diagram with the right eye. Look at the diagram from about a foot away, then gradually move nearer. About 7 to 10 inches away, if your right eye is still fixed on the cross, the white dot on the right side will disappear.
Notice that the brain fills in the blind spot. It evaluates what’s in the whole picture then guesses what’s in the area that it can’t see. In this case, the visual gap is filled with black — like the area immediately around the dot.
Another example of how the brain fills the blind spot is a variation of this diagram. If you draw a straight line across the diagram — through the center of the white cross and dot — when you repeat the above exercise, the dot will disappear, but the line will look straight and uninterrupted. Even though you don’t actually see part of the line, the brain guesses that it’s there!
How big is the blind spot?
The size of the blind spot varies from one person to another and even one eye to another, state experts. To discover how big your blind spot is, make a cross on the left side of a 3- by 5-inch (8- by 13-cm) card. Hold the card exactly 9.75 inches (25 cm) away from your face. With the left eye closed, look at the cross with your right eye.
Using a pen, you can now find the blind spot. Move the pen along the card (to the right of the cross) until the pen tip disappears. By moving the pen tip in a circle in that vicinity, you can map the blind spot. The following mathematical equation determines the exact size of the blind spot in your right eye:
Diameter of blind spot (on card) divided by 2 = spot size divided by 9.75 inches (25 cm)
The iris’s random patterns are unique to each individual — a human “bar code” or living passport. No two irises are alike. Each person has a distinct pattern of filaments, pits and striations in the colored rings surrounding the pupil of each eye. This pattern is stable throughout life.
Unlike fingerprints, iris “prints” are not subject to environmental damage. An internal organ, the iris is protected by the cornea and aqueous humor. Because these structures are transparent, the iris can be easily identified with a high degree of certainty up to three feet away.
From James Bond to ATMs
In 1936, American ophthalmologist Frank Burch first proposed the idea of using iris patterns for personal identification. His concept lay dormant until the 1980s, when iris scans were first depicted as a security protocol in James Bond movies.
Two American ophthalmologists, Leonard Flom and Aron Safir, attempted to transform Burch’s idea from science fiction into reality in 1987. But they were unable to develop a way to classify the differences in irises.
They approached John Daugman, then a professor at Harvard (now at Cambridge University), who developed a mathematical way to analyze the iris’s random patterns. He used mathematical algorithms to create iris codes. Today, this groundbreaking work is the basis of all iris recognition systems.
Eye on the future
In America and Japan, iris recognition systems are used to protect high-security areas from unauthorized access. In Britain, the Nationwide Building Society began to use iris recognition instead of PIN codes in automatic teller machines (ATMs) in 1998.
Iris recognition sounds like something that happens in Star Trek — in the blink of an eye, a digital camera scans a person’s iris from a distance of one to three feet, then compares it to a computerized file of personal identification codes. Today’s iris recognition systems can search a nation-sized database for a single iris code at speeds of up to 100,000 codes per second.
Experts predict that iris recognition will soon be used for international travel, e-commerce, police and forensic purposes, computer access, entry codes, access to privileged information and other transactions that now use passwords, keys, PIN numbers or bar code identification cards.
Sources
Biometric Personal Identification System Based on Iris Analysis. U.S. Patent No. 5,291,560 (March 1, 1994). J. Daugman
