The rods and  cones (film) of  the retina are  the receptors  which record  the image and transmit it  through the  optic nerve to the brain for  interpretation.  Rods and  cones  Fovea  (all cones)  Lens  Retina  Optic nerve  Optic disk  (blind spot)  Fovea centralis  Iris  The  pupil  (aperture) is the opening at  the center of the iris. The size of the  pupil is adjusted to control the amount  of light entering the eye.  PUPIL  Light passes through the  cornea  (the  transparent window on the front of the eye) and then through the  lens  to  focus on the retina.  CORNEA  Rod concentration Figure 17-12.  The human eye.  light energy into electrical impulses that travel through nerves  to the brain. The brain interprets the electrical signals to form  images. There are two kinds of light-sensitive cells in the  eyes: rods and cones.  [Figure 17-12]  The cones are responsible for all color vision, from  appreciating a glorious sunset to discerning the subtle shades  in a fine painting. Cones are present throughout the retina, but  are concentrated toward the center of the field of vision at the  back of the retina. There is a small pit called the fovea where  almost all the light sensing cells are cones. This is the area  where most “looking” occurs (the center of the visual field  where detail, color sensitivity, and resolution are highest).  While the cones and their associated nerves are well suited  to detecting fine detail and color in high light levels, the  rods are better able to detect movement and provide vision  in dim light. The rods are unable to discern color but are  very sensitive at low-light levels. The trouble with rods is  Photopic vision provides the capability for seeing color and  resolving fine detail (20/20 or better), but it functions only  in good illumination. Photopic vision is experienced during  daylight or when a high level of artificial illumination exists.  There are three types of vision: photopic, mesopic, and  scotopic. Each type functions under different sensory stimuli  or ambient light conditions.  [Figure 17-13]  that a large amount of light overwhelms them, and they take  longer to “reset” and adapt to the dark again. There are so  many cones in the fovea that are at the very center of the  visual field but virtually has no rods at all. So in low light,  the middle of the visual field is not very sensitive, but farther  from the fovea, the rods are more numerous and provide the  major portion of night vision.  Vision Types  Photopic Vision  Figure 17-13.  Types of vision.  17-20  Types of Vision  Types of vision used  Photopic  Mesopic  Scotopic  Light level  High  Medium/Low  Low  Technique of viewing  Central  Both  Scanning  Color perception  Good  Some  None  Receptors used  Cones  Cones/Rods  Rods  Acuity best  20/20  Varies  20/200  Blind spot  Day  Day/Night  Day/Night 