The best part of learning to fly with Night Vision Goggles (NVG) is understanding the limitations of Depth perception and distance estimation. Taking advantage of several Monocular Cues and learning more effective scanning techniques can make flying with monocular vision so much easier.
Here is an excerpt from The US Army TC-204, Night Flight:
DISTANCE ESTIMATION AND DEPTH PERCEPTION (TC 1-204)
A knowledge of distance estimation and depth perception mechanisms and cues will assist crew members in judging distances at night. These cues may be monocular or
binocular. Monocular cues are more important for crew members than binocular cues.
a. Monocular Cues. The monocular cues that aid in distance estimation and depth perception include motion parallax, geometric perspective, retinal image size, and
aerial perspective. (GRAM)
(1) Geometric perspective. An object may appear to have a different shape when viewed at varying distances and from different angles. Geometric perspective cues
include linear perspective, apparent foreshortening, and vertical position in the field.
(a) Linear perspective. Parallel lines, such as runway lights, tend to converge as distance from the observer increases.
(b) Apparent foreshortening. The true shape of an object or a terrain feature appears elliptical when viewed from a distance.
(c) Vertical position in the field. Objects or terrain features farther away from the observer appear higher on the horizon than those closer to the observer.
(2) Retinal image size. The brain perceives the actual size of an object from the size of an image focused on the retina.
(a) Known size of objects. By experience, the brain learns to estimate the distance of familiar objects by the size of their retinal images.
(b) Increasing or decreasing size of objects. If the retinal image size of an object increases, the relative distance is decreasing. If the image size
decreases, the relative distance is increasing. If the image size is constant, the object is at a fixed relative distance.
(c) Terrestrial associations. Comparing an object, such as an airfield, with an object of known size, such as a helicopter, helps to determine the object's
size and apparent distance from the observer.
(d) Overlapping contours or interposition of objects. When objects overlap, the overlapped object is farther away.
(3) Aerial perspective. The clarity of an object and the shadow cast by it are perceived by the brain and are cues for estimating distance.
(a) Variations in color or shade. Subtle variations in color or shade are clearer the closer the observer is to an object. However, as distance increases, these
distinctions blur.
(b) Loss of detail or texture. As a person gets farther from an object, discrete details become less apparent.
(c) Position of light source and direction of shadow. Every object will cast a shadow from a light source. The direction in which the shadow is cast depends
on the position of the light source. If the shadow of an object is toward the observer, the object is closer than the light source is to the observer.
(4) Motion parallax. Motion parallax refers to the apparent motion of stationary objects as viewed by an observer moving across the landscape. When the crew
member looks outside the aircraft, perpendicular to the direction of travel, near objects appear to move backward, past, or opposite the path of motion. Far objects
seem to move in the direction of motion or remain fixed. The rate of apparent movement depends on the distance the observer is from the object.
I hope this helps. I use it every day, not just when flying!
