The human visual organs are mainly composed of the eyes, visual nerves, and related parts of the brain. The structure of the human eye is shown in the figure. The eye is like a camera, and its pupils are like the aperture of a camera. The iris around the pupils acts as an aperture, which can adjust the size of the pupils according to the intensity of light and control the amount of light entering the eye. The lens, on the other hand, is like the lens of a camera, focusing and imaging external scenery onto the retina. The photosensitive cells on the retina transmit the information they perceive to the visual center of the brain through the visual nerve, forming visual perception of light and dark, color, shape, dynamics, distance, and other aspects to obtain information about the external world. There are two types of photoreceptor cells in the retina, with dense cone-shaped cells located in the central part facing the pupil, and mainly rod-shaped cells around the central part. There are about 8 million cone-shaped cells, and three more, which react to the light in the red, green, and blue regions of the spectrum. And there are about 120 million rod-shaped cells, which have high sensitivity and can sense weak light.
In a bright environment, when the light intensity is more than a few cd/m2 (candela/m2), the rod cells lose their activity, and the cone cells play a major role in photosensitivity. At this time, vision is called bright vision. The characteristic of bright vision is the ability to distinguish visual colors and the ability to accurately distinguish the details of objects. At this point, the eye has the highest sensitivity to green light with a wavelength of 555nm. As the wavelength gradually decreases and increases, the sensitivity gradually decreases until 380nm and 780nm, where the sensitivity drops to zero, i.e. no light perception is generated. When the light intensity is less than a few percent cd/m2 in a very dark environment, the cone cells lose their activity and the rod cells restore their photosensitive function. This is called scotopic vision. Its characteristic is that it can only distinguish between light and dark without color perception, and its ability to discern details of objects is greatly reduced. Scotopic vision has the highest sensitivity to 507nm radiation. In between the above two situations, when a person is in a darker but not very dark situation, such as in the "vast twilight", their vision is called intermediate vision. When the brightness is high, colors can still be distinguished, but when the brightness is low, there is only a sense of brightness.