The eyes are amazingly sophisticated devices capable of detecting and interpreting visual data under a wide range of conditions. The sensitivity of the eye automatically adjusts to an impressively large range of ambient light levels. The optics of the eye can concentrate light energy on the retina by a factor of 100,000 times, which is one of the reasons the eye can be susceptible to damage from sun exposure, but is also the key to understanding one of the most fascinating features of the eye: its ability to adapt to conditions of low light.
In simple terms, everything we see is the result of complex processes that take place within the anatomy of the eye. Electro-chemical signals are sent to the brain by tiny, light sensitive receptor cells covering the retina. There are two distinctive types of receptor cells: rods, which are the most common and are found predominately in the periphery of the retina, and cones, which are located mostly in the center and near periphery of the retina. Although there are about seventeen rods to every cone, it is the cones, concentrated in the center of the retina, that are responsible for the resolution of fine detail and color discrimination. Unfortunately, the cones only function in good illumination. While the rods are unable to distinguish colors and have poor resolution, they have a much higher sensitivity to light than the cones and are the reason we are able to see in dimly lit conditions. The dimmest light in which the rods can function is equivalent to an overcast night with no moonlight, while the cones cease to function in anything less than a night with 50% moonlight. Thus a white light barely bright enough to be seen by the rods must be increased in brightness 1,000 times before it becomes visible to the cones.
Because the central portion of the retina, called the foveola, possesses a high degree of cones but is completely devoid of rods, everyone has a blind spot in the center of their visual field at night. A person attempting to see in low-light conditions has to depend entirely on the rods at the edges of the retina. Hence it is usually most effective to look approximately 15-20 degrees to one side, above, or below an object in order to place its image on the part of the retina that possesses the highest density of rods. Individuals requiring keen night vision, like pilots, are taught to fixate to one side of an object to avoid the central blind spot or to scan, utilizing the most sensitive parts of the retina.
Despite the fact that the rods in the retina can detect extremely low levels of ambient light, they have significantly reduced visual acuity. Specifically, it is often difficult for them to distinguish objects at night if they are not either lighter or darker than their background, as they lack the finely focused detail and color detection present in cones. This limitation means that night vision relies on small differences in the brightness between objects and their background, and so any transparent medium between the eye and the object should be kept spotlessly clean. Light reflected from dirty windshields, visors, spectacles, or even fog or haze may significantly reduce the eye’s ability to discriminate between levels of contrast, which is one of the reasons that those who wear glasses frequently complain of poor night vision. Additionally, those with astigmatism, a condition when the cornea, or lens of the eye, is irregularly curved and light rays refract and create blurred vision, often suffer from poor night vision. The dilation of the pupil to let in more light, a natural response to dimming conditions, engages more of the distorted cornea, exaggerating the refractive error. Finally, glaucoma, drug toxicity and numerous hereditary disorders can also significantly reduce night vision acuity, resulting in night blindness, as can long-term vitamin A deficiency resulting from chronic starvation, alcoholism, deficient fat absorption, and diseases of the liver.
If you have any questions about how the eyes work or are interested in any of the many services offered at Georgia Eye Physicians and Surgeons, please contact us today. Be sure to follow Dr. William Segal and Dr. Marc Lay on Facebook, Twitter, and Google+ for more tips for healthy eyes.