# Retinal Vessels Segmentation Techniques and Algorithms: A Survey
## 2 Retinal Fundus Imaging  
Retina photography is typically conducted via an optical apparatus called fundus camera, as shown in Figure 1. Reference [11] fundus camera can be viewed as a low power microscope that specializes in retina fundus imaging, where the retina is illuminated and imaged via the attached camera. In particular, fundus camera is designed to capture an image for the interior surface of human eye, which is composed of major parts, including macula, optic disk, retina and posterior pole [12]. Fundus photography can be viewed as a sort of documentation process for the retinal interior structure and retinal neurosensory tissues. The retinal neurosensory tissues convert the optical images reflection, that we see, into electrical signals in the form of pulses sent to our brain where it decoded and understood. Retina photography can be conducted based on the idea that the eye pupil is utilized as both an entrance and exit for the illuminating and imaging light rays that are used by the fundus camera. During the fundus photography, patients' foreheads are placed against the bar and their chins placed in the chin rest, as shown in Figure 1. After the oculist aligns the fundus camera, the camera shutter is released so a flash light is fired and a two-dimensional picture for retina fundus has been taken [13], as anatomically illustrated in Figure 2.  
As shown in Figure 2, the fundus of the human eye is the back portion of the interior of the eye ball. The optic nerve resides at the center of retina, which can be seen as a white area of circular to oval shape and measuring about \(3\times 3\) mm across diameter. The major blood vessels of the retina radiate from the center of the area of optic nerve and then radiate and branched to fill the entire area excepting the fovea zone, which is a blood-vessel free reddish spot with an oval-shaped that lies directly to the left of the optic disc and resides in the center of an area that is known by ophthalmologists as the \"macula\" region [14]. In General, the photographic process involves grasping the light that reflected off the subject under consideration. In our case, the subject is the fundus of retina. Since the internal room of the eye has no light source of its own, in the retina photography, we need to flash or shine a light into eye room to capture a good photograph. The ocular fundus imaging has three major photography modes, as elaborated in Figure 3:  
Figure 1: Retinal fundus camera. Adapted with permission from [12], IEEE, 2007. 1. Full-color Imaging
2. Monochromatic (Filtered) Imaging
3. Fluorescence Angiogram  
In the case of full-color photography mode, no light-filtration is used and it is totally non-invasive contrary to other modes of fundus imaging. The resultant retina fundus image is a two-dimensional full color image, as illustrated in Figure 3a. On the other hand, if the fundus is imaged via a monochromatic filter or via particular colored illumination, then the fundus photography is called \"monochromatic\", as shown in Figure 3b. This type of fundus photography is built based on the idea that the visibility of different structures in a retinal image is enhanced if the spectral range of illumination is changed correspondingly. In other words, instead of using white light of a broad scale of wavelengths, we use a light of a specified wavelength that corresponds to a specific color, for example, a red object in an image would appear lighter if the image is taken through a red filter and it would appear darker if it is taken through green filter. As the white light can be divided into red, green and blue lights, the ocular fundus can be photographed via one of these gradient lights where each light has the capability to enhance the visibility of specific retinal anatomical structures based on their colors. For example, blue filter  
Figure 3: Imaging modes of ocular fundus photography: (**a**) full color retinal fundus image; (**b**) monochromatic (filtered) retinal fundus image; and (**c**) fluorescence angiogram retinal fundus image.