What is Scotopic?
Scotopic vision refers to your eyes’ ability to see in low light levels. Scotopic vision is used when sitting in a dark movie theater or driving at night. This capability is controlled by photoreceptors known as rods, which are visual cells that are sensitive to very small amounts of light.
While rod photoreceptors do not detect color or fine detail, they are necessary for forming images in dark viewing conditions. When vision occurs through the use of rods only, the lighting condition is defined as scotopic.
- Scotopic vision is used to see in very low light levels or in the dark.
- Scotopic vision uses one type of photoreceptor, called rods, to detect light.
- Rods are more sensitive to light, integrate information from larger areas, and make up the majority of photoreceptors in the retina. However, rods do not provide color vision or fine detail.
Understanding Scotopic Vision
Scotopic vision is the vision you use to see in the dark or in low lighting conditions. Rods control your scotopic vision. Rods are a type of photoreceptor highly sensitive to tiny, incremental changes in light. They are the predominant photoreceptor you use to see in the dark. The other type of photoreceptor, called cones, are largely inactive in the dark.
Scotopic and Photopic Vision
Scotopic vision allows you to see in low lighting conditions, such as a dark room, with the use of rod-shaped photoreceptors. Photopic vision allows you to see in well-lit lighting conditions, such as normal daylight, with cone-shaped photoreceptors.
Eyes contain two types of photoreceptors: rods and cones. Rods allow the eyes to see under dark luminance levels, and cones detect colors in bright luminance levels. The photoreceptors are located in a layer of the back of the eye called the retina. When light hits the retina, these photoreceptors become excited and spread electronic charge that the brain eventually interprets as an image. The human retina contains approximately 120 million rods and 6 million cones.
Other key differences between scotopic and photopic vision include:
- Scotopic vision is almost entirely mediated by rods, which are most sensitive to the light wavelengths on the electromagnetic spectrum of 498 nm, which correspond to the blue-green bands of color.
- Photopic vision normally prevails during normal lighting conditions, such as during the day. It is based on three types of cones that are sensitive to short, medium, and long wavelength ranges, which appear blue, green, and red to the human eye, respectively.
- Scotopic vision is very sensitive to small amounts of light. It is most sensitive at twenty degrees from the fovea, slightly distant to the area responsible for central vision.
- Photopic vision is not activated by small amounts of light. It is most sensitive near the fovea.
- Scotopic vision gives black and white vision.
- Photopic vision provides detailed color vision.
- Scotopic vision is low resolution and cannot create clear images.
- Photopic vision creates high resolution, finely detailed images.
- Adaptation Time
- Scotopic vision is slow to activate, and it takes approximately fourteen minutes for the eyes to fully adapt to the dark.
- Photopic vision activates quickly and as illumination levels increase, we can rapidly adapt to the lights being turned on.
Scotopic Vision Related to Night Vision
Rods have only one type of pigment, called rhodopsin. It is not possible for this one rhodopsin pigment to detect all the colors of the rainbow. On the other hand, cones have many types of pigment that can see different wavelengths of light and color. The many types of cones allow for color vision. Because cones are inactive in low lighting conditions, humans have poor color vision in the dark.
Another unique phenomenon of scotopic vision is that humans can see with their peripheral vision better in the dark. The rod photoreceptors that perform in the dark are absent from the center of the vision. The fovea, the part of the retina that is responsible for central vision, only has cones that respond to bright light. This means that the central vision is ineffective in the dark. The side vision, or peripheral vision, has the most clear vision in dim conditions.
Besides the rod photoreceptors, the pupils also allow for clear night vision. In the dark, the pupils dilate and open wider, allowing for more light to enter the eye, allowing the rod photoreceptors to provide night vision and see in the dark. If the rods are defective due to an eye disease, it may reduce night vision.