Rods and cones are part of the cell structure of your retina, and they function as light receptors.
Your eyes have several parts that are involved in helping your brain interpret what they see. Sight, by nature, is dependent on light, so the human eye has special parts to ensure that light you see is received and processed correctly. In addition to your pupil widening and shrinking to adjust to different levels of brightness, your eyes have rod and cone cells:
Scotopic vision
There are more rods than cones in your eye – typically hundreds of millions – and these cells are responsible for helping you see in dark conditions, a state known as scotopic vision. Not only do your eyes have a greater number of rods, but these cells are also more sensitive than cones. However, their adaptation process falls short, as it takes about 30 minutes for your eyes to become completely adjusted to dark conditions. This is why it takes some time to see objects in your bedroom after you’ve shut off your bedside lamps to rest for the night.
The sensitivity of rods also helps you see dim objects out of the corner of your eye, as they account for more of your peripheral vision. Additionally, rods excel with regard to motion and blue wavelength detection.
One disadvantage of scotopic vision, however, is that rods cannot process color, which is why colorful objects appear gray under dim conditions.
Photopic vision
Conversely, cones specialize in processing color and high resolution vision, a state called photopic vision. There are millions of these cells and three types: red cones, green cones and blue cones – named for their sensitivity to red, blue and green wavelengths. Although they fall short in overall sensitivity, cones are much faster at adapting to changing daylight conditions. As a result, you can come inside during a cookout on a sunny day and it takes less than a second for your eyes to adjust.
Cones are located in and around your eye’s fovea centralis. If you’ve ever noticed that your eyes are continually moving, this is because they are shifting to direct light from the object you’re looking at to the cones in this part of your eye.
Mesopic vision
Light levels between scotopic vision and photopic vision are known as mesopic vision. This specifically refers to a range between 0.003 candela per square meter and 3 candela per square meter. Levels beneath this range are scotopic and those above are photopic.
When using mesopic vision, both your rods and cones are at work, though neither group of cells is operating at maximum efficiency.
Central blind spot
Given that cones are located in the center of your eye – an area with a dense concentration of these cells and no rods – you’ll have trouble focusing on small objects with your central vision under low light. Thus, if you look directly at a word on a page under low illumination, you may not be able to see it. However, objects outside of your central field of vision could appear clearer.
Associated problems
Issues can arise with your scotopic vision, and you may develop Irlen syndrome, also known as scotopic vision syndrome, Meares-Irlen syndrome or visual stress. When this condition occurs, your brain has trouble processing visual information. According to the Irlen Institute, there are various side effects of Irlen syndrome: headaches, fatigue, distorted vision, slow or inefficient reading, poor comprehension and other problems.
The main symptom of this condition is light sensitivity. If, for instance, your eyes feel strained by light from your floor reading lamps, glare or sunlight, you may be experiencing visual stress. Additionally, Irlen syndrome can lead to other symptoms, such as dizziness, irritability or mood changes.
Irlen syndrome is treated with colored overlays for reading or special lenses and contacts if problems extend beyond the task of skimming a novel or cookbook.