Flying at night can be a very pleasant experience. The air is generally cooler and smoother, resulting in better helicopter performance and a more comfortable flight. You generally also experience less traffic and less radio congestion.  Before discussing night operations, it is important you understand how your vision is affected at night and how to counteract the visual illusions, which you might encounter. Vision is by far the most important sense that you have, and flying is obviously impossible without it. Most of the things you perceive while flying are visual or heavily supplemented by vision. The visual sense is especially important in collision avoidance and depth perception. Your vision sensors are your eyes, even though they are not perfect in the way they function or see objects. Since your eyes are not always able to see all things at all times, illusions and blindspots occur. The more you understand the eye and how it functions, the easier it is to compensate for these illusions and blindspots. The eye works in much the same way as a camera. Both have an aperture, lens, method of focusing, and a sur- face for registering images. [Figure 13-1].  Directly behind the lens, on the retina, is a small, notched area called the fovea. This area contains only a high concentration of cone receptors. When you look directly at an object, the image is focused mainly on the fovea. The cones, however, do not Vision is primarily the result of light striking a photo- sensitive layer, called the retina, at the back of the eye. The retina is composed of light-sensitive cones and rods. The cones in your eye perceive an image best when the light is bright, while the rods work best in low light. The pattern of light that strikes the cones and rods is transmitted as electrical impulses by the optic nerve to the brain where these signals are interpreted as an image. The area where the optic nerve meets the retina contains no cones or rods, creating a blind spot in vision. Normally, each eye compensates for the other’s blind spot. [Figure 13-2] Cones are concentrated around the center of the retina. They gradually diminish in number as the dis- tance from the center increases. Cones allow you to perceive color by sensing red, blue, and green light. 13-1 Figure 13-2. This illustration provides a dramatic example of the eye’s blind spot. Cover your right eye and hold this page at arm’s length. Focus your left eye on the X in the right side of the visual, and notice what happens to the aircraft as you slowly bring the page closer to your eye. Figure 13-1. A camera is able to focus on near and far objects by changing the distance between the lens and the film. You can see objects clearly at various distances because the shape of your eye’s lens is changed automatically by small muscles. NIGHT FLIGHT PHYSIOLOGY VISION IN FLIGHT THE EYE CONES The rods and cones (film) of the retina are the receptors, which record the  image and transmit it through the optic  nerve to the brain for interpretation.  Light passes  through the cornea  (the transparent window on  the front of the eye) and then through  the lens to focus on the retina. The pupil (aperture) is the  opening at the center of  the iris. The size of the  pupil is adjusted to  control the amount  of light entering  the eye. Lens Iris Pupil Cornea Optic Nerve Retina Rods and GLYPH<.notdef> Cones FoveaGLYPH<.notdef> (All Cones) Rod GLYPH<.notdef> Concentration { { { { {