There are two types of sensors in the eye: rods and cones.
Cones function well in brightly lit surroundings, perceive colour and much detail.
Rods are far more sensitive than cones, giving night vision with the pupils wide open. Rods do not distinguish colours or see fine detail.
Both rods and cones respond to a wide range of colours, but rods are very insensitive to red light.
Rods respond to various colours, but give a sensation only of black, grey or white.
Rods and cones are distributed non-uniformly across the retina.
Cones are only in the center. While in the periphery, the rods dominate.
Thus, the best visual acuity in good illumination is obtained for that part of the image focused on the center (fovea). The eye is most sensitive in conditions of weak illumination to images focused off-center (on the periphery of the retina).
Rods are de-sensitized by light (very quickly). It takes 20-30 minutes to re-sensitize (or dark adapt).
If light is needed, yet it is important to keep eyes dark-adapted, then use a red filter on the light. Rods are not sensitive to red light.
Night Observation Procedures
- look for shapes, shadows, contrast and movement
- since the center of the pupil is insensitive, all looking should be toward a side
- Eyes should be kept in motion – to keep the rods at their peak of sensitivity
- don=t stare directly at a sighted object
- to avoid eye fatigue, every few minutes the observer should close their eyes for a few seconds to allow them to rest
- allow eyes to become dark-adapted
- colours are not discernable
- details cannot be seen
- use red light only
Using Binoculars at night
One measure of the effectiveness of binoculars for night use is exit pupil. This is the diameter of the bundle of light rays exiting toward the eye from the eye-piece of the binoculars.
This number is found by dividing the objective lens diameter by the power (7×35 = 5mm exit pupil, or, 7×50 = 7mm exit pupil). The pupil of the eye is about 7mm when fully dialated. If the exit pupil is less than 7mm, then the eye isn’t fully used.
(For day use, a small exit pupil is satisfactory, since the eye pupil is only about 2mm in diameter. However, the large exit pupil is advantageous because of ease of centering the eye behind the eye-piece.)
Binoculars should be held aimed straight forward and the eyes turned off-center – to avoid using the insensitive center of the retina. This requires practice, but is very effective.