‘Night vision’ of the human eye is an interesting phenomenon – a sensation, a uniqueness that many do not experience in today’s modern (bright) world. There are times though when our human night vision if properly adapted, could become a powerful nighttime asset, and with regards to survival & preparedness, this might be one’s own security (for example).
Our eyes are a remarkable thing in how well they can adapt over an extremely wide range of light (brightness) conditions.
Here are a few facts and tips regarding your human ‘night vision’ while using your own unassisted eyes…
As the sensitivity of the eye automatically adjusts to changes in illumination, the human eye can function effectively to changes in brightness as much as 1,000,000,000 times! More importantly (with regards to ‘night vision’) there are two primary mechanisms (receptors) in your eye which collect and send light impulses to the brain. Rods and Cones.
Without going into lots of peripheral technical detail, the thing to know is that this dual-receptor system (rods and cones) each have their own attributes for ‘seeing’ or collecting light…
Rods are used during night vision.
Cones are primarily useful for day vision, visual acuity (focus), and color perception.
While both rods and cones function throughout a wide range of illumination, below the intensity of moonlight the cones cease to function. The dimmest light in which the cones can somewhat effectively function is similar to 50% moonlight, while the rods can ‘see’ all the way down to an overcast night with no moonlight (with greatly reduced focus and color perception).
Dark Adaptation Of The Eye
Rods and cones differ greatly in their rate of dark adaptation.
Rods (the receptors for greatest night vision) require 30-45 minutes or longer of absolute darkness to attain maximum sensitivity after exposure to bright light.
The cones do not achieve the same level of night vision sensitivity as the rods. The rods slowly adapt to dim illumination, but eventually achieve a much greater sensitivity than the cones.
Dark adaptation is about 80% complete within 30 minutes, but it may take hours, or even days, to acquire total dark adaptation.
Blind Spot Of The Eye During Night Vision
The center of the eye’s visual feed is loaded with cones, and is completely absent of rods. Therefore if the ambient light is below the cone threshold (apprx. 50% moonlight), a blind spot exists in the central field of vision.
The rods in your eyes reach a maximum concentration at a point around 17 degrees of your central vision. Therefore while attempting to ‘see’ in ambient light dimmer than moonlight (relying solely on the rods to ‘see’), to best detect small targets under such conditions you must look approximately 15-20 degrees to one side, above, or below the target to place the target on the part of the retina that possesses the highest density of rods.
You can also fixate to one side of a target to avoid the central blind spot, as well as to scan, utilizing the most sensitive part of the retina to improve target detection at night.
Color Sensitivity Of Rods
Rods and cones posses different sensitivities to visible wavelengths of light (colors).
Under very low light conditions, color vision is lost.
Rods (night vision) are mostly sensitive to blue – green while less sensitive to the red portion of the visible spectrum. In other words, during night vision, if looking off-angle at a target (15-20 degrees), green light will appear brighter than the red. Therefore, if light must be used at night (e.g. map reading) it is best to use dim red light (some flashlights integrate with adaptable red filters).
Visual Acuity (Focus) At Night
Your eye’s ‘focus’ or visual acuity, is reduced at night under low illumination and 20/20 vision cannot be sustained below a level of deep twilight. It may be reduced to 20/200 or less.
Visual acuity at night is derived from small differences in the brightness between objects and their background (contrast).
Dark adaptation of the rods develops slowly over a period of 20 to 30 minutes (apprx. 80% ‘night vision’), it can be lost in a few seconds of exposure to bright light.
Dark adaptation is independent in each eye. Even though bright light may shine into one eye, the other eye will retain its dark adaptation if it is protected from the light.
You can prevent flash blindness and preserve dark adaptation in one eye by simply closing or covering it. For example, if you must momentarily turn on a flashlight, you might close one eye first.
Let’s hear from you –
Your own experiences or tips regarding human eye ‘night vision’ ?