**2.3 Retinal vasculature**

*Applications of Pattern Recognition*

**2.2 Anatomy of the retina**

Human eye consists of many working parts such as sclera, cornea, pupil, lens, iris, ciliary body, retina, optic nerve, choroid, etc. *Sclera*, the white colored outer layer of the eye, works as a protector of the eye. *Cornea*, a transparent circular part, refracts the light entering the eye onto the lens. *Lens* is a crystalline part located directly behind the pupil. Its task is to focus light onto the retina. *Pupil* is the dark spot at the center of a healthy iris. It acts as the shutter of a camera since the amount of light entering the human eye is regulated by the diameter of the pupil. *Iris* is the colored, visible part of the eye located in front of the lens. It regulates the amount of light entering the eye by widening (dilation) and narrowing (constriction) of the pupil. *Ciliary body* delivers oxygen and nutrients to the lens and cornea. It contains the ciliary muscle, which changes the shape of the lens when our eyes focus on an object. *Optic nerve* transfers all the visual information from the retina to the brain. *Choroid* is a thin vascular layer between the retina and the sclera. It provides oxygen and nourishment to the outer layers of the retina. It also

The retina is located at the back of the eye and is the only part of the central nervous system that is non-invasively observable. It is responsible for light rays sensing and vision at all. It is a very thin light-sensitive layer that is transparent. Retinal cameras capture deeper layers of the eye behind the retinal layer. Its thickness is 0.2-0.4 mm and contains two types of light-sensitive cells, rods and cones. The rods, which are approximately 75 to 150 million [3, 4], are used for more sensitive vision in low light. However, the rods only send information to the brain in the grayscale. On the contrary, the cones perceive incident light rays in color, but they need more light to function than rods. There are approximately 7 million of them in one eye and are divided according to the perception of colors red, green, and blue. The peripheral part of the retina is rod-dominated, whereas the yellow spot is conedominated. Low density of rods in the yellow spot makes it less sensitive to the light.

contains a pigment that absorbs excess light (**Figure 1**) [2].

Retina would be compared with 157-megapixel camera [3, 4].

**2. The retina**

**2.1 Eye anatomy**

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**Figure 1.**

*Anatomy of an eye [1].*

The structure of retinal vessels is much like the brain and remains unchanged throughout life. Two main sources of blood supplying the retina are the retinal artery and the vessels. The blood vessels nourish the retina's outer layer with photoreceptors. Meanwhile, four major branches of the retinal artery called terminal arterioles, provide blood supply to the retina, nourishing in the first place the inside of it.

One type of retinal scanners is the fundus camera, which is a specialized low power microscope with an attached camera. **Figure 3** shows the fundus camera used in the biometric laboratory of Faculty of Information Technology (FIT) at Brno University of Technology (BUT) in Czech Republic (CZ). No matter which kind of retinal scanners is used, without user's conscious cooperation the retinal image acquisition is not possible.

Blood vessels coming out from the blind spot and make a tree shape on the surface of the retina (shown in **Figure 2**). This tree shape hardly ever changes over the lifetime of an individual except due to some severe eye diseases such as hard glaucoma and cataracts. It is not affected by external environment, since the retina is not an external organ such as a fingerprint [8, 9]. Moreover, it differs from person to person due to many factors such as the thickness of vessels, the distance from

**Figure 2.** *Retinal image [5].*

**Figure 3.**

*Slit lamp example (left) [6]; example of a non-mydriatic fundus camera (middle) [7]; (right) fundus camera used in the biometric laboratory at FIT BUT (right).*

each other, the presence of bifurcations (division points of a single vessel), crossings (intersection points of two or multiple vessels), ending points of vessels etc., which are all in different locations and in various numbers.
