‘Night Vision’ of the human eye is an interesting phenomenon.
The dynamic range of our eyes is remarkable. The eye can adapt to an extremely wide range of light (brightness) conditions from very bright to extreme low light conditions.
Human Night Vision Facts & Tips
The eye automatically adjusts to changes in brightness. Remarkably the human eye can function effectively within an extreme range in brightness from very bright to very dim. I have read that this dynamic range is “well over 1,000,000:1” !
There are two primary mechanisms (receptors) in the eye’s retina which collect and send light impulses to the brain. Rods and Cones.
Without going into lots of technical detail, the basics are this:
Rods are receptive to brightness (intensity of light).
Cones are sensitive for color perception and visual acuity (focus).
While both Rods and Cones in the eye function throughout a very wide range of illumination, below the intensity of moonlight the Cones cease to function.
The dimmest light in which the Cones will somewhat effectively function is similar to 50% moonlight. The Rods can ‘see’ all the way down to an overcast night with no moonlight – however with greatly reduced focus and color perception due to the Cone’s inability to function well in extreme low light.
Night Vision Adaptation Of The Eye
Rods and Cones differ greatly in their rate of dark adaptation (the speed at which they adapt to changing illumination conditions).Rods (the receptors for greatest night vision) require 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 (Night Vision) is about 80% complete within 30 minutes , but it may take hours, or even days, to acquire total dark adaptation.
Human Night Vision ‘Blind Spot’
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 (approx. 50% moonlight), a blind spot will present itself in the central field of vision .
The Rods reach a maximum concentration at a peripheral around 17 degrees of your central vision.
How To See In The Dark
While attempting to ‘see’ in low ambient light dimmer than moonlight (relying solely on your eye’s Rods to ‘see’)Look approximately 15-20 degrees to one side, above, or below the target in order to place the target on the part of the retina that has 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.
Poor Color Sensitivity When It’s Dark
Rods and Cones have different sensitivities to visible wavelengths of light (colors).
Under very low light conditions, color vision is lost (The Cones “don’t work”).
Rods are mostly sensitive to blue – green while less sensitive to the red portion of the visible spectrum. In other words while using human night vision, if looking off-angle at a target (15-20 degrees), Green will appear brighter than the Red .
Best Color Light For Night Vision Retention
Therefore, if a light must be used at night (e.g. map reading) and you are concerned about retaining your human night vision, it’s 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 under low light conditions is not good. Focus 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 brightness between objects and their background (contrast).
Dark adaptation of the Rods develop slowly over a period of 20 to 30 minutes to approximately 80% ‘night vision’.
However human night vision 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 to retain your night vision.
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