# Robot Bionic Vision Technologies: A Review
## 2 Literature Review
### Robot Bionic Vision Development Based on Human Vision  
The human eye changes the shape of the lens through the contraction or relaxation of the ciliary muscle, thereby realizing zooming to obtain clear images of objects at different distances [54]. Referring to the zoom mechanism of the human eye, researchers have designed flexible zoom lenses with different driving modes and physical states [9]. Although the zooming process is different and the zooming effect is slightly different, most zooming processes achieve the zooming purpose by changing the curvature of the lens surface, and a few achieve the zooming purpose by changing the refractive index of the lens constituent material. The human eye changes the aperture of the pupil through the expansion and contraction of the smooth muscle on the iris (shrinks in bright light and expands in dark light), thereby controlling the amount of light entering the pupil to adapt to various lighting environment changes [8]. To enhance the imaging performance of the bionic visual optical system, in recent years, researchers have simulated the iris adjustment mechanism of the human eye, and have carried out many studies related to iris bionic technology. According to the differing focus of research, it can be roughly divided into two categories: the first category focuses on exploring the physiological model of pupil change, studies the relationship between human pupil diameter and brightness, and proposes a mathematical model of pupil diameter change based on adaptive adjustment of brightness. The other type focuses on the application of the iris driving mechanism in engineering. By simulating the telescopic motion mechanism of the smooth muscle on the iris, a freely adjustable optofluidic variable aperture optomechanical device is developed [59]. There are two different types of photoreceptors in the retina of the human eye: cone and rod cells. The two types of photoreceptor cells contribute differently to vision [65]. Cone cells sense strong light and color, have a strong resolution for object details and colors, and are mainly responsible for producing photopic vision in a brighter environment. Rod cells are sensitive to weak light and can only distinguish between light and dark but cannot distinguish the details and colors of objects. Rod cells are mainly responsible for producing scotopic vision in dim environments. These two types of photoreceptor cells are unevenly distributed in the retina [12]. There are approximately 7 million cone cells that are mainly distributed in the fovea area of the retina. The closer to the edge of the retina, the lower the density of cone cells, and the greater the density of rod cells. Based on the distribution and physiological characteristics of the above-mentioned retinal cone cells and rod cells, the details and color resolution of the human retina change with the change in spatial position; therefore, the retina can process information in a large field of view, high resolution, and real-time [13]. A balance between them can be achieved to meet the application requirements on different occasions. Inspired by this, in recent years, researchers at home and abroad have started to work on bionic research on retinal photoreceptors and imaging mechanisms. At present, research related to retinal bionic technology can be divided into two categories: the distribution characteristics of cone and rod cells and the retina's physiological characteristics [51; 52]. According to the uneven distribution of retinal photoreceptor cells, only the target image in the fovea area has the characteristic of high resolution. However, the objects we observe are usually not static, and the scene changes. Therefore, to consider the need of a large field of view and high resolution, the entire eyeball needs to keep rotating, and the head and body movements also assist with changes in eyesight.