Chapter 1 Introduction to Night Vision Equipment

CHAPTER 1 INTRODUCTION TO NIGHT VISION EQUIPMENT Throughout the history of wars, the cover of darkness has provided a tactical advantage for one force or the other. In this chapter, we will begin with a brief explanation of how the human eye “sees” during daytime and nighttime. We will then introduce and explain the various devices used in today’s military to enhance the night vision capabilities of military personnel. THE HUMAN EYE The primary components of the eye that control day vision and night vision are the rods and the cones, found in the retina. The retina translates light energy, absorbed by the rods and cones, into nerve impulses to be carried to the brain by the optic nerve. The brain then converts the nerve impulses into images. The eyes of some animals that hunt at night and sleep in the daytime (such as the opossum) have retinas composed almost entirely of rods; other animals that sleep in the nighttime (such as most birds) have retinas composed almost entirely of cones. The human eye has a retina composed of both rods and cones. CONES The primary factor in both color vision and acute daytime vision is the cones. During the day, the eye tends to rotate, to center the image nearer to the area of the retina where the cones are most concentrated. The center three degrees of the retina is made up entirely of cones. This area is called the fovea. RODS The rods, being very sensitive to low illumination levels, control night vision. At night, the eye switches from cone vision to rod vision and becomes color blind. Since only rods can adjust to the low-light levels available at night, the center three degrees of solid cones becomes a blind spot in the center of the field of view. 1-1 The average field of view for the human eye is about 80 degrees vertical by 170 degrees horizontal. OFF-CENTER VISION To compensate for the blind spot in the center of the field of view, an observer should use off-center vision. Off-center vision involves looking off to the side of the object of interest, scanning its periphery using short three-second movements. Three seconds allows the rods adequate stimulation time. In the dark, extra stimulation time is needed for the rods to gather sufficient information to form visual images. DARK ADAPTATION The cones and rods are turned on and off by the secretion of photosensitive pigments called iodopsin and rhodopsin. Cones adapt (secrete iodopsin) very quickly to high-light levels. However, the rods require several minutes to adjust (secrete rhodopsin) to low-light levels. In extremely dark situations, the normal eye may require up to 40 minutes to become fully dark-adapted. Now that we have discussed how the human eye reacts to different light levels, let’s move on to electro-optical devices. HISTORICAL BACKGROUND Early efforts to remove the cover of darkness included the use of torches, brush fires, flares, and rockets. During World War I, formal research work was begun in the realm of night vision, but those efforts were limited to developing and refining searchlight illumination. Research by an electronics company in the 1930’s led to development of an image tube that could be used to convert infrared images to visible displays. The military significance of this quickly became apparent. Further development led to the production of a small arms unit—the “Sniperscope.” Use of the sniperscope involved pointing the device toward some sound heard in the darkness,