Flight is particularly demanding for our visual sense, as it is not adapted to the high speeds and three-dimensional movements involved in flying.
It is the limitations of human vision that make it susceptible to illusions, particularly at night, in poor light or in conditions with few external visual cues.
This briefing note provides an overview of
- How our visual system works
- Aspects of vision playing an important role in day and night flying
- The main refractive eye disorders
2 Image formation
Incoming light undergoes a series of optical transformations entering the eye before striking the retina. The two main optical lenses (refracting systems) are:
- The cornea, which is the first and most powerful refracting system
- The crystaline lens, which provides us with the abilty to accommodate our vision to objects at different distances, by contracting or relaxing the ciliary muscles surrounding the lens that change its curvature.
Dioptre is the unit used to express the optical power of a lens. A dioptre is defined as the reciprocal of the focal point distance in meters (D = 1/m). The optical power of the human eye at rest is approximately 60 dioptres.
Figure 1: The essential features of the eye. The pupil. The iris, a colored, circular muscle that gives us our eye's color and controls the size of the pupil so that more or less light, depending on conditions, is allowed to enter the eye. The sclera is commonly known as "the white of the eye” and serves as the eye's protective outer coat. The crystalline lens is behind the iris and suspended by ligaments attached to the anterior portion of the ciliary body.
Light is absorbed in the pigments of the retinal receptors. Then a chemical reaction transforms this energy into an electrical pulse that is transmitted via the optic nerve (and tract) to the occipital (at the back of the skull) lobes of the brain.
The retina is composed of two types of neural receptors: rods and cones. Cones are associated with high visual acuity and color vision, but require sufficient light. Rods are associated with colorless vision, poor acuity, but higher sensitivity than cones in dim light conditions.
The distribution of these neural receptors is inhomogeneous. The concentration of cones is maximal at the center point of image focus, the macula, with the fovea at its center, and decreases rapidly towards the peripheral areas. The concentration of rods peaks at about 20 degrees from the fovea, which is devoid of rods.
Figure 2: Angular Distribution of Rods and Cones on the Retina
The retina's sensory receptor cells are absent where the optic nerve departs the eye. Because of this, everyone has a blind spot. This is normally not noticeable because vision of both eyes overlaps.
3 Visual Modes
The visual system can best be described in terms of three operating modes:
- Day vision or photopic vision
- Night or scotopic vision
- Mesopic vision
In daylight illumination, cone sensitivity is high. The rods become saturated because nearly all rod photopigments are bleached by daylight. Rod outputs are then inhibited by cone outputs. Photopic vision is thus mainly the result of cones activation, and is therefore characterized by high acuity, color discrimination and good depth perception.
Pure scotopic vision is vision that occurs at light intensities below which cones are no longer sensitive. Scotopic vision is therefore characterized by poor acuity and lack of colour discrimination, but greatly enhanced sensitivity to light thanks to the mechanism of dark adaptation.
At intermediate intensity, rods and cones outputs are combined in mesopic vision. Visual performance in the mesopic range will depend on the luminance and whether the objects are in the central or peripheral vision. A typical effect that can be experienced is a color-shift to blue. Most outdoor night activities (e.g. driving) occur in the mesopic range.
The human eye uses pure scotopic vision in the range below 0.034 cd/m2, and pure photopic vision in the range above 3.4 cd/m2. The following figure displays the range of each operating mode.
4 Visual function in the flight
Landing an aircraft is an intensive visual task, requiring both focal vision and peripheral vision to help build an accurate model of the aircraft's flight path, so that the flare and touchdown can be accurately judged.
This chapter treats the various aspects of day and night vision that are required to perform these complex visual tasks and shows their limitations in peculiar conditions.
Day vision (photopic vision)
4.1 Colour perception
Rods include only one type of pigment and enable perception of gray in dim light conditions.
There are three kinds of cones, depending on their type of pigment. Each type can be more sensitive to either short (400-500nm), medium (450-630 nm), or long (500-700 nm) wavelengths of light, and are therefore usually called S-, M-, and L-cones. The visual system combines the information from each type of receptor to give different perceptions of different wavelengths of light.
Figure 4: Frequency response of rods (R), and S-,M- and L- cones
4.2 Accommodation time
Accommodation is the automatic adjustment of the eye’s focal length to the distance of the observed object.
Large accommodations are required between objects in the external environment and instruments in a cockpit. This requires considerable time.
Eg. To move the eye from scanning the external environment to reading an instrument and returning to clear distant vision takes around 2.39 seconds. At cruise speed, the aircraft would travel about 0.3 nm.
The time of accommodation increases with age and depends on light intensity. It is an important factor for pilots of high-speed aircraft.
To reduce accommodation and scanning pattern times, instruments should be designed, installed and illuminated to be easily and rapidly readable.
4.3 Visual Acuity
Visual acuity is the spatial resolving capacity of the visual system. It is the ability to distinguish small contrasts or small objects at long distances and is essential in flight.
Due to the densely packed cones at the fovea, visual acuity is the greatest at the center of fixation. This central part of the visual field within which alphanumeric data can be read, is about a two-degree angle to the eye, and it corresponds at the instrument panel distance to a circle of about 3 cm (1.17 in) in diameter. At a distance of 5 minutes of arc from the center of fixation (paracentral visual field), there is a measurable loss in visual acuity but it is still capable of rapidly attracting the eye. At 10 minutes of arc (1/6 of a degree) from fixation, there is a 25 percent loss of visual acuity. In the peripherical part of the visual field, only large moving or flashing objects are seen.
Figure 5: Visual Acuity along the Visual Field
The main factors impairing visual acuity are refractive errors, aberrations, illumination and contrast.
Dynamic visual acuity is the ability to resolve the details of an object while there is relative motion between the object and the observer. The underlaying mechanism is ocular pursuit, which is capable of maintaining a steady fixation if the angular velocity does not exceed a value of about 30 degrees/s. Our vision is more sensitive for targets crossing our visual field than targets approaching head on.
4.4 Depth perception
Principles of depth perception
Depth perception is the visual ability to judge the relative distance of objects and the spatial relationship of objects at different distances.
As the three-dimensional world projects onto a two-dimentional retina, this projection on its own cannot provide depth information. The brain has to combine various monocular and binocular cues given by the eyes to recover the depth, distance and three-dimensional shape of objects.
The two main binocular cues are convergence and stereopsis.
Vergence (convergence) refers to the rotation of the two eyes to place the images of an object on the foveas. The degree of convergence varies with the distance of the object and gives a binocular cue to distance and depth: the closer the object is to the observer, the greater the degree of convergence.
Figure 6: Vergence
Stereopsis is considered to be the most important cue for depth perception in daily life. It is the consequence of the interpupillary separation between the two eyes (6 cm, 2.34 in), which causes each eye to have a slightly different view of the same scene. This separation is called retinal disparity. If two targets are at different distances from the observer, each target will have a different binocular angle (binocular parallax) depending on its distance and the interpupillary separation. The retinal disparity increases with the distance between an object or between several objects. The brain is able to combine these two views into a single 3D image, a process called stereopsis.
Figure 7: Principles of Stereopsis
Binocular cues are most efficient at short distances. Monocular cues in flight are the biggest contributers to depth perception; other contributors include motion parallax, size constancy, accommodation and pictorial cues.
Motion parallax is the apparent motion of two stationary targets at different distances due to a change in observer position. If the observer is moving or if his head is moving from side to side, targets will move in opposite directions relative to each other. The target that is closer will appear to move more quickly and in reverse direction to the observer’s movement, while the one farther way will appear to move slowly in the same direction.
Figure 8: Motion Parallax
A particular case of motion parallax is the optic flow perceived by our peripheral vision. It is based only on the relative speed of flow through our visual field, which provides an indication of the distance and speed of objects.
Figure 9: Optic Flow in Peripheral Vision
Size constancy, or the projected retinal image size of familiar objects, is an indication of its relative distance and is most efficient at greater distances, for example during approach. The retinal image shape contributes to perception of orientation relative to the object.
The degree of activation of the ciliary muscles during accommodation provides an extra cue for depth perception, but it is more efficient at short distances.
Finally, there are also some pictorial cues to depth and shape :
- Aerial or atmospheric perspective: particles and vapor in the atmosphere cause a scattering of light that makes objects at greater distances appear hazy. Blue light is scattered more than other colors. Therefore, distant objects appear hazy and bluish in color. Thus distant mountains appear blue. Conversely, mountains are perceived to be closer when the atmosphere is clear.
Figure 10: Aerial or Atmospheric Perspective
- Occlusion or interposition is the overlapping of one object by another. Near surfaces overlap distant surfaces. This gives information about depth rather than distance
Figure 11: Occlusion or Interposition
- Linear perspective, which is based on the fact that distant objects appear smaller and closer to the horizon, and that all object outlines converge to vanishing points. For example: runway outlines and position relative to the horizon.
- Highlights and shadows can provide information about an object's dimensions and depth. Humans are used to typical light infal (e.g. from top to bottom), which drives their interpretation of shadows.
Figure 12: Influence of Shadows on Depth Perception
Depth perception during Approach and Landing
Aircraft landing studies have highlighted that binocular cues were only effective in the last seconds of landing (Dorfel 1982). The most important cue for spatial orientation during final approach are at large distance, size constancy, i.e. retinal runway image size (reflected in distance and height judgment) and shape (which affects slope and judgment). At medium distance, motion parallax (which gives the time history of these factors) becomes the preferable way to perceive depth, with the binocular cues taking over at faily close distance. (Buffett 1986).
4.5 Empty field myopia
Empty field myopia is the result of the eye’s reflex to accommodate in featureless environments to their rest position, which corresponds to a focus between 1 and 2m away. The pilot is then functionally short-sighted and would find it difficult to detect a target at the normal far point, since objects at infinity are blurred.
This short-sightededness is usually of relatively minor importance since the eye rapidly readjusts when a target becomes visible. To reduce the accommodation time, pilots should look periodically and deliberately at objects at virtual infinity such as the wing tip.
Night vision (scotopic vision)
4.6 Dark adaptation
Dark adaptation, which is necessary for night vision, is the process by which eyes increase their sensitivity to low levels of illumination.
Both rods and cones contain photopigments which, on exposure to light, undergo a chemical change that initiates visual impulses in the retina. Dark adaptation is based on the reverse process, when there is regeneration of photopigments. In the fully dark-adapted eye, photopigment regeneration is complete and retinal sensitivity is at its maximum level.
Rods and cones differ in their rate of dark adaptation. Rods require 20 to 30 minutes or longer, in absolute darkness, to attain their maximum sensitivity after exposure to bright light. Cones attain maximum sensitivity in about 5 to 7 minutes. This explains why dark adaptation is not linear.
Figure 13: Dark Adaptation Time of Rods and Cones
An exposure to a bright light after dark adaptation will temporarily impair retinal sensitivity. The degree of impairment depends on the intensity and duration of the exposure. Brief flashes from high-intensity, white strobe lights, which are commonly used as aircraft anticollision lights, have a smaller effect on night vision because their energy pulses are of short duration (milliseconds). In contrast, an exposure to a flare or a searchlight for longer than one second can seriously impair night vision.
Maximum dark adaptation is necessary to be able to detect dim objects outside the aircraft. The valuable feature of rod vision is its abilitiy to detect movement as an image crosses the visual field. The use of red light (wavelength greater than 650 nanometers) for illumination of the cockpit is desirable because it does not affect dark adaptation. Indeed, rods are not sensitive to wavelengths of light greater than about 650 nanometers. However, red light does deteriorate dark adaptation of cones. Therefore, the use of red light maintains the greatest rod sensitivity, while still providing some illumination for central foveal vision. A drawback of red cockpit lighting is that it creates some near-vision problems and a distortion of color of objects. Low-intensity white cockpit lighting is now used to solve those problems. It enables a more natural visual environment within the aircraft without degrading the color of objects.
4.7 Night Blind Spot
Due to the distribution of rods and cones on the retina, if the ambient light is below cone threshold light intensity, a blind spot 5 to 10 degrees wide develops in the center of the visual field. As a result, an object viewed directly at night may not be detected because of the night blind spot and, if it is detected, may fade away.
However, since rods are much more sensitive to light than cones, objects can be detected by an exentrical fixation, i.e. looking at the objects under an angle of 17-20 degrees to one side, above or below. Proper education and training are, therefore, essential if maximum effectiveness of night vision is to be achieved.
5 Main refractive eye disorders
A myopic individual does not see distant objects clearly without corrective glasses or contact lenses. A slightly myopic or marginally corrected pilot, who may see fairly well during the day, will experience blurred vision at night when viewing blue-green light. In addition, as luminance levels decrease, the focusing mechanism of the eye may move toward a resting position and increase myopia (dark focus). These are important factors when pilots look outside the cockpit during night flying. Special corrective lenses may then have to be prescribed.
In hyperopia, nearby objects are focused behind the retinal plane. Objects that are nearby are not seen clearly; only more distant objects are in focus. Corrective lenses must be worn to read instruments and documentation.
Astigmatism is an irregularity in the shape of the cornea that may cause an out-of-focus condition.
Presbyopia is part of the normal ageing process as individuals gradually lose accommodation capability for nearby objects. When individuals are about 40 years old, their eyes lose the ability to focus on nearby objects. As presbyopia worsens, instruments, maps and checklists become more difficult to read, especially with red illumination. This difficulty can be corrected with certain types of bifocal spectacles.
6 Factors Affecting Vision (From FAA)
The greater the object’s size, ambient illumination, contrast, viewing time and atmospheric clarity, the better its visibility.
During the day, objects can be identified at a great distance with good detail resolution. At night, the identification range of dim objects is limited, with poor detail resolution.
Surface references or the horizon may become obscured by smoke, fog, smog, haze, dust, ice particles or other phenomena, although visibility may be above Visual Flight Rule (VFR) minimums. This is especially true at airports located adjacent to large bodies of water or sparsely populated areas where few, if any, surface references are available. Lack of horizon or surface reference is common on overwater flights, at night and in low-visibility conditions.
Excessive ambient illumination, especially from light reflected off the canopy, surfaces inside the aircraft, clouds, water, snow or desert terrain can produce glare that may cause uncomfortable squinting, eye tearing, temporary blindness and reduced visual acuity. Sunglasses or visors can provide protection against external glare and facilitate the view of flight instruments. Sunglasses should have a luminous transmittance of the order of 15 percent (within ISO standards) and a field of view as wide as possible.
Refractive eye disorders include as myopia (nearsightedness — impaired focusing of distant objects), hyperopia (farsightedness — impaired focusing of near objects), astigmatism (impaired focusing of objects in different meridians), or presbyopia (age-related impaired focusing of near objects).
Self-imposed stresses such as self-medication, alcohol consumption (including hangover effects), tobacco use (including withdrawal), hypoglycemia and sleep deprivation/fatigue can seriously impair vision.
Inflight exposure to low barometric pressure without the use of supplemental oxygen (above 10,000 ft during the day and above 5,000 ft at night) can result in hypoxia, which impairs visual performance.
Other factors that may adversely affect visual performance include:
- windscreen haze,
- improper illumination of the cockpit and/or instruments,
- scratched and/or dirty instrumentation, which reduces the luminance contrast
- use of cockpit red lighting,
- inadequate cockpit environmental control (temperature and humidity),
- inappropriate sunglasses and/or prescription glasses/contact lenses, and
- sustained visual workload during flight.
7 Visual Illusions during Approach and Landing
The previous chapters have provide the necessay background (mechanisms and limitations) to understand the origin of the visual illusions that can be encountered in approach and landing. The Visual Illusion Awareness briefing note provides an overview of typical visual illusions during approach and landing and proposes prevention strategies to reduce their effects.
8 Key points
Although all parts of the eye are important for obtaining a good image, the most vital layer for vision is the retina. The retina is essentially a piece of brain tissue that gets direct stimulation from the outside world's lights and images.
Depth perception gives the pilot an accurate sense of where objects are in relation to one another and his or her position in relation to those same objects. The brain has to combine various cues given by the eyes to recover the depth, distance and three-dimensional shape of objects
There are two types of sensory receptors in the retina -- rods and cones. According to the widely accepted duplicity theory of vision, rods are responsible for vision under very dim levels of illumination (scotopic vision), and cones function at higher illumination levels (photopic vision). Only cones are responsible for color vision. Therefore, at very low levels of luminance, a blind spot develops in the center of the visual field because ambient light is below cone threshold.
Scotopic vision is characterized by poor acuity, lack of color discrimination and difficulty in judging distance and depth. Night flying is very different from day flying and requires more attention. The most noticeable difference is the limited availability of outside visual references. Therefore, flight instruments should be used to a greater degree in controlling the airplane. This is particularly true for night takeoffs and climbs.
During daylight, one can best see an object looking directly at it, but at night a scanning procedure to permit off-center viewing is more effective.
The eye’s adaptation to darkness is another important aspect of night vision. An exposure to bright light after dark adaptation will temporarily impair retinal sensitivity.
9 Additional OGHFA Materials
10 Other References
MEASURING PERFORMANCE IN A TRACKING TASK BETWEEN PANEL-MOUNTED DISPLAYS AND HELMET-MOUNTED DISPLAYS,James Blanchard (firstname.lastname@example.org) Mitre Corp, Corde Lane (email@example.com) Space Systems Laboratory, Univ. of Maryland, Glen Lee (firstname.lastname@example.org) Dept. of Computer Science, Univ. of Maryland, Marko Teittinen (email@example.com) Human-Computer Interaction Laboratory, Univ. of Maryland
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