Brittlestars form a large group of sea animals that are similar to starfish. There are more than 2,000 species of brittlestars. However, this article will focus on two of them, Ophiocoma pumila (Figure 1a) and Ophiocoma wendtii (Figure 1b). In spite of their similar appearance, these two kinds of brittlestars have one main difference. While O. pumila is insensitive to light, O. wendtii is highly light sensitive. For example, the latter has different colors at day and night, as shown in Figure 1b, left and right respectively. More interestingly, O. wendtii can sense shadows of predators and quickly move into dark areas such as a cave or underneath a rock.

To understand the mechanisms behind the difference in light sensitivities of these two species, Joanna Aizenberg and her colleagues investigated1 the microstructure of both brittlestars’ outer skeletons with an electron microscope and came up with a striking result: The top surface of O. wendtii’s skeleton has very well ordered lens-like hemi spherical elements (Figure 1f). The cross section image of one of those hemispheres actually looks like a compound lens made up from two hemispheres with different diameters (Figure 1g). On the other hand, O. pumila’s skeleton had a typical stereom (sponge-like calcite) structure (Figure 1e). These images strongly suggest that the lenses in O. wendtii’s skeleton are responsible for the relatively high light sensitivity. However, understanding how that really happens require further investigation.

It is well-known that spherical lenses suffer from a problem called “spherical aberration,” which means that the light rays that are closer to the optical axis are focused at a different point than the ones that are away from the axis. A quick solution to this problem is to use two lenses, whose diameters have a certain ratio, back to back; this helps to correct the aberration originating from the first one with the second one. Interestingly, when Aizenberg et. al. calculated1 the optimum compound lens configuration for O. wendtii’s skeleton, which has the minimum aberration, their result matched the original lens structure perfectly (the orange outline in Figure 1e). They were also able to locate the focal point of these lenses (d = 4-7 um* below the lens) with the same method. Their further electron microscopy studies showed optically sensitive nerve bundles exactly at that location. All these results clearly show that O. wendtii’s skeleton has the perfect geometry to collect and focus light to improve its light sensitivity. However, there is one big question about these lenses: their material.

Calcite, a kind of calcium carbonate (CaCO3), is a common ingredient of the shell or the skeleton of marine organisms. Interestingly, the birefringence property of calcite makes it very unfavorable as a lens material. In a birefringent material the speed of the light depends on the direction it travels with respect to the crystallographic axes of the material. As a result, if one looks through it, they will observe a doubly refracted image (Figure 2). Being the most famous example of birefringent crystals, calcite’s refractive index is 1.64 parallel to one crystallographic axis and 1.49 in the perpendicular direction. Therefore a regular calcite lens cannot focus light on a single spot, unless it is oriented along a special crystallographic axis (c-axis to be specific), which would be along the diagonal of the prism in Figure 2.

At this point we are not surprised to learn that the optical axis of the O. wendtii’s lenses, and the c-axis of the calcite crystal that they are made of, indeed overlap. We are not surprised because we already had a strong feeling that these lenses should work. However, it is quite surprising that these little creatures can grow single crystals of calcite with a specific crystallographic orientation. As Kenneth Towe states in the context of a similar study, “This precise orientation of crystals is the big mystery of biomineralization. Organisms know how to do it; we do not yet know how they know.”3

Biomineralization, the controlled deposit of inorganic minerals by living organisms, is a very active research field attracting many scientists from various disciplines, including biology, physics, chemistry, and material science. In general, controlling crystal structures at small length scales is a very challenging task. Scientists spend millions of dollars to build state-of-the-art facilities for single crystal materials synthesis. They work in clean rooms, under an ultra high vacuum and at extremely high temperatures. On the other hand, from brittlestars to large whales, almost all living creatures have biominerals, such as bones and shells, manufactured in chemically dirty environments and at decent temperatures. Organisms are apparently equipped more efficiently than our laboratories are.

A. Ali Eren has a Ph.D. in Physics and lives in the USA. He studies physical chemistry of biological processes.

References

*1 um (micron) is one thousandth of a millimeter. Human hair is approximately 100 micron thick.

1. Aizenberg, Joanna, et al. “Calcitic microlenses as part of the photoreceptor system in brittlestars.” Nature 412.6849 (2001): 819-822.
2. ttp://jademellor.com/blog/2013/6/21/rainbow-rhombus, accessed 4/20/2014
3. Towe, Kenneth M. “Sea urchins as crystallographers.” Science 311.5767 (2006): 1554-1555.

There are so many blessings in life, granted to us free of charge, which we take for granted. Eyesight, being able to see near and far distances, most certainly tops the list. But we do not have to be deprived of our sight in order to understand its wisdom and functioning, and to contemplate upon its true value and worth.

Years of research and hard work were dedicated to develop cameras and multi-featured objective lenses. Initially, one to three lens objective cameras were used for simple shots, whereas today, objectives with seven to ten lenses are being used to take better photographs from a snow drop falling onto a flower to a buzzing bee resting on a flower. I wonder to what extent human beings are aware of the pair of eyes that has been bestowed upon them by God, and its ability to see different colors and shapes both near and far. Unfortunately, as people who often understand the true value of things once they are lost, we understand the blessing of being able to see near after the age of forty when we cannot read the newspaper without glasses and when we cannot put a thread through a needle.

So why is it that we can still see far after the age of forty but fail to see near? In order to understand this we need to examine the structure of the eye and its functions.

The structure of the eye and the ability to see

The exterior part of the eye is made up of a translucent layer (cornea) at the front and a white protective layer (sclera) behind it. The vascular layer of the eye (uvea) is located in the middle of the sclera. The most inner part of the eye is made up of the retina, the light-sensitive layer of tissue responsible for converting light rays into electrical signals. The hole located in the center of the iris, the colored part of the eye, is called the pupil. Behind the pupil is the crystalline lens. For a clear vision, lights reflected from objects need to be focused on the central part of the retina (fovea). While cameras have lens systems to focus the image on the film, it is the cornea and crystalline lens that are responsible for the same function in the eye.

Refraction power of cornea is constant and around 43 diopters. The refraction power of the eye lens when resting is around 20 diopters. Light rays coming from outside refracts at a set ratio and manages to focus on the retina. The light rays coming at the retina are then coded into electrical signals. Afterwards these signals are routed towards related regions of the brain via optic nerves. Most of the stimuli relayed by the optical nerve arrive at the visual center of the brain (occipital cortex). These coded electrical signals then become vision when they reach the optical lobe of the brain.

The function of the lens and accommodation

The refraction power of both the cornea and the lens (43+20+63 diopters) is sufficient to focus an image on the retina when looking at objects farther than 6 meters. Extra refraction power is needed for closer distances in order to focus images on the retina. Mobile lens systems enable this job to take place in camera objectives. Since refraction power of the cornea in human eye does not change, this additional task of refraction is set to be provided by the ocular lens. It is built as a flexible structure without any blood vessels. Aqueous humor (lens fluid) which is secreted by the ciliary body is responsible for lens nourishment, removal of waste products and toning of the eye since the lens does not contain any blood vessels. This internal fluid has low oxygen concentration therefore the lens is made to derive its energy supply mostly from anaerobic metabolism.

The iris is positioned in a suitable place where it can change the shape of the internal lens behind the pupil. The lens in this special place is suspended into position via zonule of zinn ligaments attached to the eye as a ciliary body. The ciliary body contains ciliary muscles where zinn ligaments are attached. Only 0.5 mm of space exists between the lens and the ciliary body. Zinn ligaments are tight when ciliary muscles are resting and this enables a flatter configuration of the lens. Upon contraction of ciliary muscles, zinn ligaments become relaxed and the diameter of the lens decreases along with an increase in its thickness. Thicker lens becomes more globular and this increases its refractive power, thus enabling vision of the closer distances. This increase in refractive power of the lens in order to see closer objects is called “accommodation.” If the stimuli of the ciliary muscles expire, ciliary muscles then relax making zinn ligaments tighter, reducing thickness of the lens, making it flatter and therefore less refractive. This reshapes it to focus on distant objects for a clearer vision.

Accommodation mechanisms and loss of accommodation during aging

The vision blurs temporarily when one takes an immediate shift from staring at an object in the distance to another object nearby. As soon as this blurry image reaches the occipital cortex, stimuli generated here arrives first at the Edinger-Westphal nucleus via special nerve tracks and then to the ciliary muscles of the eye. In a very short time, this blurry vision is corrected and becomes clearer without us even noticing with optimal increase of refraction in the internal lens. In a time as short as 0.35 seconds, for thousands of times in a day, this mechanism is set to function in such a perfect manner to spur those thoughtful minds into reflection and wonder.

Accommodation ability is at its highest point in children and this feature of the eye decreases with age. Refractive power of the lens can increase up to 34 diopters with a 14 diopters accommodation power along with 20 diopters of resting refraction during childhood. This way, children can clearly see objects as close as 7 centimeters. Accommodation power decreases with age. It reduces to 4-8 diopters after the age of 40 and 2-3 diopters around the age of 50. It is widely accepted that refractive power disappears entirely after the age of 60.

In the advanced stages of aging, the eye lens loses its transparency, becomes cloudy as it develops cataract. Eye lens in this poor transparent stage is removed via cataract surgery, to be replaced with an artificial lens to carry out the refracting task. Unfortunately today, technology is still unable to produce an artificial lens that is capable of all the tasks that a human eye can perform. Artificial internal eye lenses that are used in surgeries today cannot carry out accommodation functions. Majority of these lenses can only focus on one point at a near or far distance. Newly developed multifocal lenses can utilize various mechanisms to see both near and far distances yet they are not in any position to replace the human lens completely.

Ocular motions when looking near and far

Thanks to ocular movements, we do not have to move our head constantly while looking around. The eye movement involving both eyes in which each eye moves in the same direction is referred to as version type movements. Another movement type is called vergence, and this is when both eyes move in opposite directions. Vergence type movements are a type of ocular motility coded in a special center part of the brain. It is called convergence because the eyes get closer to each other when looking at closer distances, and called divergence when both eyes focus on the same spot by directing away from each other. If eyes only moved in the same direction without this convergence mechanism, both eyes would not be able to focus on closer points and would not be able to develop three dimensional visions (depth perception).

In addition to accommodation and convergence, when we look closer, our pupils get smaller (Miosis). Light rays coming from outside objects get improved focus on the retina via this constriction of the pupils. This way, a clearer image is provided.

When we look closer, accommodation, convergence and miosis all happen at the same time in a synchronized manner to provide a clear vision. The details of these complicated chains of events have yet to be understood.

Conclusion

The fineness of refractive power of the eye with a single lens, accommodation ability and sensitive balances of ocular motility is only a few of the blessings of the eye granted to humankind. The ability to see near being at its peak during young ages when learning is most active is another dimension to this miracle. These wisdom-filled capacities given to the eye makes one ponder upon the importance of the eye for survival, in addition to being a reminder to those with an open mind and heart to gaze upon the natural world and contemplate upon the Almighty.

The four main functions required for life, namely the digestive, respiratory, circulatory, and excretory systems are all found in plants but performed by different organs. If these functions are non-existent the dynamism or the essence of existence that we call life will disappear and death is inevitable. If these four functions (digestion, circulation, respiration, and excretion) are in working order, it means an organism is alive but only at the level of plant life. To reach the animal level of life, in addition to the four main functions, functions like senses, nervous and muscular systems are also required.

If these functions fail, life continues, but at the level of plant life. We sometimes hear people saying “in a vegetative state.” When you hear this, people are usually talking about a person who has lost the use of the animal functions. He or she may be unable to see, feel, hear, or move. Intelligence, comprehension, will power, conscience and the many other special aspects of human beings cannot be compared with the essential functions of living; these are additional characteristics of being human which accompany the animal system of functions and emerge in relation to a person’s spirituality. This obviously does not mean that those who have lost their abilities are less human and they do not have any rights; on the contrary, it means to say that they are not responsible any more.

The focal points of sensory functions of sight, hearing, touch, taste, and smell are found within the head in the brain, which is the command center of our body and the most complex form of existence we know of in the universe. The brain is connected to the nervous system which communicates with all the organs in the human body. This is why the head is so precious; it is a very sensitive part of the body and has to be protected like a jeweler guards precious stones. If you tread on a nail, it may hurt for a while, but with treatment the wound can be cured. But if a nail were to penetrate someone’s head, this could damage any of the sensory functions or, God forbid, could even result in death.

As you have probably understood from our introduction, the head is firstly the center of animal functions then the focus point of the addition of the human senses. When you mention the head we are the first thing that comes to mind: the eyes. Did we hear you ask why? It’s because we are what you are reading these words with right now, and because you can see the beautiful creations of the universe with us-that’s why.

If my Creator had not created us and positioned us in the two cavities on your head, you would have no knowledge of the beauties of light, color, insects, flowers, roses or birds. You would be afraid to walk without us because you would have no idea where you are about to tread. The effect of sight can only be sent through us to your brain and reflected into your mind. The development of human knowledge would have been very delayed if God had not created us or the other sense organs because the only way to gain knowledge is through healthy sense organs. The sense organs are the only way of detecting and recognizing the characteristics of objects.

You need us to realize that water is transparent, apples are red, quinces are yellow and violets are purple, you need us to recognize your mothers, fathers and friends. You need us to eat, drink, read and write and so you do not bump into walls. What do you think will happen if we close our lids for ten seconds and you try to walk down the street? Try it if you want!

You see-it was harder than you thought. You were scared in case you bumped into something or fell over. Look, Peter, just take a deep breath and give praise to our Creator while our lids are closed tight, for you could not tolerate being in the dark for a mere ten seconds, so what if you never saw the light? Just think sometimes about people who are not as fortunate as you, who cannot see for one reason or another. Give praise to our Creator for not giving you such a trial, and pray for the patience of those friends who have been deprived of sight.

Yes, now we have come to our characteristics and delicate creation, so pay attention. When Darwin saw God’s magnificent skill in our creation, he realized that we could not have been just a coincidence or a self-made creation, and it was impossible for us to be a creation of unconscious nature. Due to the guilt he felt inside, he found it necessary to say that the idea that “the evolution of complex organs like the brain and the eye could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree.”

There is no artificial optical device that can match the esthetics or precision of our creation. Our operating principles depend on the optical laws God Almighty has determined for the light. As a matter of fact, just by looking at our structure, human beings worked out the rules of optics, you made the simplest of cameras, and you went on to produce the most magnificent photographic cameras possible. But whatever you do, never try to compare one of us to those cameras you have invented or you may become rather embarrassed. Your cameras are a simple toy compared to us. From the time of the invention of the old, wooden box cameras that had to be covered with a black cloth up to the modern digital cameras of the present, 175 years have passed. Many people worked for years to bring cameras to such a perfect state. Can anyone claim that the old camera made of a wooden box and a lens evolved by itself and turned into the high-quality, digital cameras of today? With all the knowledge accumulated by hundreds of scientists over the years, can this invention really be called a coincidence? So, can we be a coincidence? Could the eyes of mollusks or insects make themselves evolve and transform into the eyes of humans? Of course not! But to understand this a little better you must pay attention to our structure.

We are globe-shaped and look like covered capsules with a multi-layered structure which is quite solid and supple (Figure 1). Each of us is approximately 24mm in diameter. We have an outer layer made of something called sclera (hard coating). We are protected by a strong cover made up of dense ligament fibers, and beneath this is the choroid layer (a layer of blood vessels), where the blood vessels nourishing us go into; this layer covers us completely like a network of vessels. In the middle of the eye is the retina which is a layer of film. It is located in the most precious place where our actual receivers of light reside. There are other layers which each have their own duty beneath these layers, but we won’t go into too much detail.

We each have a main casing that is round and has a dome-shaped surface which slightly protrudes. The center of the hard coating, the cornea, is transparent so that it will allow light to pass through. On the outer part of the transparent area is what they call the white of the eye, and the whole areas seen from the front is covered in a clear membrane (the conjunctiva) with mucus cells. This keeps us lubricated. So as to focus light rays, our cornea section is more curved than other sections. There is a tiny chamber behind this curved front and this is actually where the lens, which separates the main chamber, is found. In the front chamber between our lens and cornea is a transparent liquid, the iris, which gives us our color. The black hole in the center of the iris is called the pupil. The iris, which has a special structure of muscles, works like a curtain contracting and expanding our pupil in response to the brightness of light. If the light is powerful, it contracts to protect the retina from any damage, whereas if the light is dim, the iris expands the pupil to allow more light into the retina.

The fibers (zonules) that hold the lens suspended in place and the cluster of muscles (corpus ciliare), which changes our lens according to the focus distance, are in front of a layer of blood vessels. Our lens, which plays a role in focusing, changes shape and adjusts according to whether the focus point is near or at a distance by thinning and thickening. We do this with the help of the fibers that keep the lens suspended.

Behind the lens is a larger chamber filled with a jellylike, transparent liquid (vitreous humor). The pressure and consistency of the jellylike liquid ensure that we keep our round shape. There are photoreceptive cells in the shape of rods and cones which are sensitive to light in the dark chamber behind our retina. The visual images formed by the rays which pass through the cornea and lens to the retina are upside-down. There is a small pit in my retina where almost every cell has a light receptor cell. This is where your sharpest vision is formed, but that doesn’t mean that it is where you actually perceive the object you are looking at. Sight is what happens when a group of cells in the brain’s visual center is stimulated, and the images on our lens are comprehended. It is unbelievable how fast is the effect of the chemical and electro events on our light receptor cells. The effect of light is conveyed to your brain through the stimulation of electric signals in our receptors’ optic nerve where the actual vision is produced in the brain, so in a way we are just the means of vision.

Because we are such delicate and sensitive organs, our Creator placed us in the cavities within the bone structure of your head for protection. We fit in the very strong and secure structure comprising your chin bone, cheek bones, forehead bone, orbital bone (just around us, your eyes), nasal bone and occipital bone (at the rear and bottom of your skull), but this is not our only protective mechanism. We have top and bottom eyelids that we close to protect ourselves from oncoming dangers. The frequent blinking of our eyelids prevents our cornea from becoming dirty, just like the windscreen wipers of a car. Our eyelids are not just simple folds of skin, they are a secretion system of glands which continuously lubricate the inner part of your lashes and seize dirt and dust, turning them into harmless particles. When you feel emotion, the secretion produced by the tear glands between us and the nose fills the tear ducts, passes through the two canals, and gives us a good wash. But when you cry too much, the excess secretion empties through another canal, which also washes your nose.

As this system is complex, it can also go wrong sometimes. If you consider our many parts and our millions of cells you will clearly see the possibility that any one of our components may fail. However, our Creator has formed the eyes in most people’s heads without any defect or failure, so we can serve you with vision of the universe.

The Creator gives us something called illness so we are reminded of our weakness, a defect or failure which arises as an act of wisdom. Some illnesses, like diabetes, deficiency of vitamin A or atherosclerosis, may have a negative effect on us and even render us useless. We eyes also have some defects which occasionally appear, such as not being able to see at a distance, or close up. Focus defects are easily remedied with spectacles or lenses, but faults in the sensitive light receptor cells in the retina are more difficult to amend. The pressure of the liquid in our larger chamber must be correctly balanced. If this pressure increases too much, we will give you a great deal of pain due to what doctors call glaucoma. If we lose our transparency, your vision will become clouded by what is called a cataract. Apart from this, there are many viruses and bacteria that can cause infections and diseases, but the cells of your immune system are like soldiers who, with the Creator’s help, protect you from those bacteria and viruses.

Look, Peter! It would take pages and pages for us to explain ourselves to you, but we don’t really want to confuse you with even more anatomical information. Our whole aim is to explain the reasons for the creation of our parts, to astonish you with the wisdom and fine art of the Divine, so you will contemplate the wonders of creation and give praise to the Almighty for the blessings he has bestowed upon you. If we have been successful in achieving this, that would be the greatest reward we could ask for.

Irfan Yilmaz is a professor of biology at Dokuz Eylul University, Izmir, Turkey.

In order to get a sharp image, ideally one point on the image (film) should receive light rays from only one point on the object. In practice, this ideal case can not be achieved in case of a large pinhole. If we have a large pinhole, as seen in Fig. 4, the light rays emerging from one point on the object reaches multiple points on the film. Also, multiple points on the object can arrive at a single point on the film. The result of both cases is a blurry image. For a small pinhole, however, there are light rays emerging from a point on the object in all directions, and only a very small amount of light is received on the film. To produce a decent image on the film, the exposure time needs to be very long, especially when a very small pinhole is used. Of course, we can make the pinhole a little larger to capture more light. But, wait! This would make the image blurry. Therefore, for pinhole photography, the size of the pinhole sets the quality of the image and the minimum exposure time. Also, we can take the picture of a still object, but a moving object cannot be captured easily by a pinhole camera due to the long exposure time. One important question is: Can we make reduce the size of the pinhole size to increase the quality of the image? Well, physics says: “No!” Fig. 5 shows a comparison of the images of a filament taken by a pinhole camera.⁶ As the size of the pinhole gets smaller and smaller, the effects of the diffraction phenomenon are more and more pronounced, and the image becomes blurry again. We also notice that the image is barely formed with a very small pinhole, indicating that very little light is received.

How can we gather more light and still make the image sharp? Can we achieve this with just the pinhole? Apparently not. That is why we mention lenses when talking about any imaging system. This is what a lens basically does. A lens gathers more light, and still preserves the one-to-one correspondence between the points on the object and those on the image. So, a lens is very useful for imaging. Most cameras have one or multiple lenses. Our eyes have lenses. We should remember, though, a pinhole camera takes a picture, but the compromise is the exposure time. Now, we see that a lens solves the exposure time problem, but is there a price to pay? To answer this question, let’s take a look at once again Fig. 2, where we see two pictures, one taken by a pinhole camera, and the other one by a lens camera. Look at the pictures carefully, and try to understand the difference before proceeding. As you probably observed, in the pinhole camera image everything in the picture is in sharp focus, from the close-by plants to the far distant beacon, and the clouds in the sky. However, in the lens camera image, only the closest daisy is in focus, and the other daisies, only a few meters away, are blurry. Indeed, we lose the depth of field in our images when using a lens camera. Depth of field can be defined as “The distance between the nearest and farthest points that appear in acceptably sharp focus in an image.” This is actually something we live with everyday. Try to focus your eyes on a mountain far away; the objects that are very close will not be in focus anymore. So, our eyes, consisting of lenses, also have limited depth of field. A nautilus eye, on the other hand, has an infinite depth of field, as it has a pinhole eye. Therefore, the price paid for gathering more light with a lens is that not everything will be in focus.

We can understand why lens cameras have a limited depth of field while a pinhole camera has nearly infinite depth of field if we consider the focusing mechanism of a lens. Given a pre-determined film position, a lens can only form sharp images of an object at a certain distance from the lens. Other points that are farther from or closer than same section of the object will be out of focus. However, if the size of the object is not large, we usually do not notice this effect. When we want to take pictures of close-by objects, then this effect is clearly seen, as Fig. 2 (b) shows.

Actually, we can correct this problem. The image of the farther points of the object forms at a farther point on the film. So, multiple light rays coming from one point will end up on the film. If we place an aperture next to the lens, then we can block some of these light rays. If we make the size of the aperture sufficiently small, we can get a sharp image of the farther point. Our original question about the lenses can be posed once again at this point: By using the aperture we make the image sharper, but what do we lose? Of course, since we block some of the light rays, we lose the light-gathering ability of the lens. Now, if we make the aperture size smaller and smaller, we finally reach the pinhole, and an almost infinite depth of field!

Our eyes also use similar mechanism of placing an aperture. The amount of light allowed to enter each eye is controlled by the iris, a circular diaphragm that opens wide at low light levels and closes to protect the pupil (the aperture) and retina (light detector of the eye) at very high levels of illumination. As illumination changes, the diameter of the pupil (positioned in front of the crystalline lens) reflexively varies between a size of about 2 to 8 millimeters. When illumination is very bright, the pupil narrows and light rays from the side are excluded from the optical pathway.⁷ The result is a sharper image on the retina. A very narrow pupil (approximately 2 millimeters) produces diffraction artifacts that spread the image of a point source on the retina, similar to the image of the filament captured by a very tiny pinhole (Fig. 5)

In conclusion, a pinhole camera is a very instructive tool for learning about imaging concepts. A pinhole camera has an infinite depth of field and the ability to produce sharp images regardless of the distance of the objects. This comes with a price, though: the small size of the pinhole limits the amount of light received by the film, so long exposure times are needed. A lens helps to gather more light, but compromises the depth of field. These concepts are used in imaging technologies, and can be found in the eyes of living organisms.

Notes

1. E. Hecht, Optics, 2nd edition, pp. 199. Addison-Wesley Publishing Co.
2. http://images.encarta.msn.com/xrefmedia/aencmed/targets/illus/ilt/T045986A.gif.
3. http://www.kosara.net/gallery/.
4. http://www.eyedesignbook.com/.
5. http://www.spylife.com/pinholecam.html.
6. http://www.umiacs.umd.edu/~ramani/cmsc426/Lecture3.pdf
7. http://www.olympusmicro.com/primer/lightandcolor/humanvisionintro.html.