Good vision + knowledge of how to use the eyes for maximum effectiveness are important for?
- -take-off and landing,
- -collision avoidance,
- -visual target acquisition,
- -identifying colours of signal flares and beacons and
- -night flying to name but a few.
What is the anatomical blind spot?
- -optic disc
- - optic nerve enters the eye
- - absence of photoreceptors
- What is the Fovea?
- -responsible for high VA in bright light conditions
- -the point where the image is focused.
- -high concentraion of cones
What the cones ?
- -photoreceptive cells concentrated in the fovea
- -diminishes progressively to the periphery corresponding degradation in VA
What are the rods?
- - photoreceptive cells outside the fovea
- -night time vision sensitivity to low light levels
- -do not allow for colour discrimination and have a corresponding low visual acuity
- -rods reach a maximum concentration at a point some 17 degrees from the fovea
Eye chambers -gel-like fluid Anterior?
- -filled with aqueous humour
- - that is replenished every 90 minutes or so.
Eye chambers -gel-like fluid Posterior?
- filled with vitreous humour that is permanent in nature.
- -exerts a pressure known as the intra?ocular pressure
- -maintains the spherical shape of the eye,
- -the pressure is normally within the range of 16?20mmHg.
- -the ciliary muscles contract
- -curvature of the lens to increase.
- -is the ability to change the focal length of the eye when,
- -for instance, changing from reading instruments to looking out to the horizon.
- -Focal length can vary from about 7.0cm to infinity.
- - lens becomes less flexible
- -reduction in accommodation will normally occur.
- -presbyopia and causes difficulty with near visual tasks like reading small print.
- -Many people will require reading glasses above the age of forty years even though their visual acuity may be 6/6 (20/20) or better for distance.
- -cones and rods.
- -a finite period of time (about 0.15 sec) after the light rays fall on the retina to trigger a chemical reaction in the photoreceptors giving rise to a nerve impulse.
- - sensitive to high levels of illumination
- - day vision.
- -dimmest light in which the cones can function is roughly equivalent to a night with 50% moonlight.
- - periphery of the retina,
- -low levels of illumination
- -night vision.
- - monochromatic and are most sensitive to light emitted in the blue/green wavelengths.
- -Many rods (6?600) are connected to each nerve
- -VA 30 degrees eccentric to the fovea =6/60
- -The dimmest light in which rods can function is equivalent to ambient conditions of an overcast moonless night.
- -A white light which can just be seen by the rods must be increased in brightness 1,000 times before it becomes visible to the cones.
Peripheral vision ?
- -sensitive to movement and flashes of light (such as a strobe or sunlight reflecting off a mirror),
- - hence the value of a heliograph in survivor detection.
- -visual acuity decreases significantly once light falls outside the fovea.
- -Considering acuity at the fovea as unity, by 10o
- off?centre acuity has decreased to 25%, and by 30o, acuity has decreased to as little as 10%.
Describe the ambient Visual System ?
- -is based upon peripheral vision,
- -consequently visual acuity is poor and monochromatic,
- -motion detection is good.
- -provides information about large structures in the immediate vicinity
- - is sensitive to lines and boundaries.
- -also unconsciously provides information to the brain about the position of the horizon.
- As the processing of information from the ambient system is largely an unconscious process, it can be subject to errors and illusions in flight.
What is Photopic Vision?
- - provides the capability for seeing colour and resolving fine detail (6/6 or better on the Snellen chart),
- - but only in illumination brighter than or equivalent to full moon.
What is Scotopic Vision?
- -poorer quality; it is limited by reduced resolution (usually 6/60 or less) and
- - provides the ability to discriminate only between shades of black and white.
- -sun 30,000 times that of the moon,
- -dimmest light in which rods can function is equivalent to ambient conditions of an overcast moonless night.
- -dimmest light in which the cones can function is roughly equivalent to a night with 50% moonlight.
- -A white light which can just be seen by the rods must be increased in brightness 1,000 times before it becomes visible to the cones.
What is Mesopic Vision?
- -transition zone between photopic and scotopic vision (at dusk-equivalent illumination) is called mesopic vision.
- -Mesopic vision is of primary importance to military aircrew because a small amount of light is often present during night operations.
- -The image brightness of an NVG display falls within the mesopic visual range.
What is Stereopsis?
- -is the brain?s ability to judge distance based on the slightly different position of an image on each retina.
- - As our eyes are set about 5?6cm apart, they both have a slightly different view of the same object with this difference being resolved by the brain to present a single image and judge distance.
Monocular cues include?
- a Retinal Image Size: Observing an object of known size will form an image on the surface of the retina that will be directly proportional to the distance from the observer.
- b) Motion Parallax: Targets that are separated by distance appear to move in opposite directions when the head is moved. Additionally, near objects appear to move past more quickly than do distant objects when the observer is moving.
- c) Aerial Perspective: If an object is close, resolution of detail is high. As the object becomes more distant, an increase in the number of air molecules and dust particles between the eye and the object will result in a reduction in clarity, the object will also appear to have a bluish hue.
- d) Linear Perspective: Parallel lines appear to converge in the distance and meet at the horizon.
- e) Overlap: One object in front of another will obscure the farther object. This depends on the perceived shape of the objects.
- f) Position in Visual Field: Objects which are closer to the middle of the visual field seem to be closer to the horizon, therefore appear further away.
What is SACCADIC MOVEMENTS?
When moving or rotating, the eyes are only able to fix on and follow a small part of the visual scene for a short time then they must flick ahead quickly to the next object. Whilst the eyes are in motion, they are unable to discriminate any detail and all that is detected by the eyes is a blur. The brain is able to edit this information so that a clear and coherent image is seen. An image must fall on the retina for at least ? of a second for it to register, and it has been shown that image quality does not improve after about 1 second. When scanning an empty sky, the eyes will move in this saccadic fashion from one point of fixation to the next. With this in mind, a search area should be covered in a series of short fixations of about ? to 1 second (Figure 8?5).
THRESHOLD OF ACUITY?
- The resolving power of the eye is the smallest angle between two points that would allow the formation of two discernable images,
- which is normally considered to be 1 minute of arc.
- Two lines would have to be 4 ?m apart on the retina to be detected as distinct objects,
- and as photoreceptors are approximately 1.5 ?m in diameter,
- it is assumed that there are two stimulated photoreceptors separated by an unstimulated receptor,
- that allows this discrimination.
- Separations smaller than this would be perceived as one homogenous mass.
Using central vision with normal eyesight,
- a 2-metre diameter disc should be visible at a distance of about 11km.
- Once outside an arc 10 degrees either side of the central vision, the same disc would be visible at a range of just over 1 km.
- This highlights the importance of using central vision to detect objects and
- has important implications for mid-air collision avoidance and visual searches.
- Too little illumination and an object will not be visible;
- too much and the ?glare? may affect vision.
- Glare effectively reduces contrast by causing internal reflections of light within the eye.
- This occurs at high altitude where there is less haze and a brighter sky,
- -the increase in glare reduces contrast affecting visibility.
- To minimise the glare, tinted windows or sunglasses/visors may be used.
- -normally come from above and cast shadows downward (sunlight/moonlight).
- - flying environment, illusions may occur with light coming from below (for example reflected off cloud),
- or from the sun at a low angle in the sky when flying in cloud (?lean on the sun? illusion).
- -Terrain features may also be masked by illumination casting shadows at varying angles, creating a risk of flying into unperceived terrain when flying at low altitudes.
ANATOMICAL BLIND SPOT
- -where the optic nerve leaves the eye, this is known as the ?anatomical blind spot?.
- -With binocular vision there is overlap of the visual fields of the two eyes
- -so an object whose image falls on the blind spot of one eye will be seen by the other.
- If however, vision is blocked in one eye (by a window pillar for example), an object may be obscured from view.
- -subtends an angle of about 5-6 degrees, making an 18metre object invisible at a distance of 200 metres.
Stare at the black spot in Figure 8-6 below with your left eye closed. Move the paper towards your eye and the helicopter will disappear when it is focused on the blind spot of the right eye (when the page is approximately 20 cm from your eye).
FIGURE 8?6. FINDING THE ANATOMICAL BLIND SPOT
- IT TAKES A FINITE AMOUNT OF TIME FOR AN OBJECT TO BE DETECTED, RECOGNISED, A DECISION MADE ON AN ACTION AND THEN FOR THAT REACTION TO BE INITIATED.
- -5.5 seconds for the process to be completed
EMPTY FIELD MYOPIA?
- With nothing to focus on, the eyes naturally come to rest with a focal length of 1 to 2 metres.
- This results in objects beyond 2 metres being blurred, effectively causing short?sightedness (myopia).
- Occurs in conditions of poor visual cues such as
- -at night,
- -high altitude,
- -instrument meteorological conditions (IMC) or
- -over water or snow.
- -to overcome this, it is best to push your focus outward periodically by fixing the eyes on a wing tip or distant object.
- - occur when something very close in the visual field draws the focus of the eyes inwards causing blurring of objects in the distance.
- -This effect may be seen while driving with a stone chip in the windscreen
- - or when trying to look through a fly?screen door.
- It is important to make sure that aircraft windscreens and visors are clean and free from anything that might cause focal traps.
- -Sometimes even necessary equipment can cause this problem, such as windshield wipers and head?up displays.
bottom?up processing bottom-up processing?
- -Visual perception relies on experience and the brain interpreting electrical impulses from the eye.
- -Unfortunately, it is not infallible and there is an unavoidable link between the psychology of perception and visual function.
- -Visual information is processed almost immediately
- -relay the information from the retina to the brain via the optic nerve.
- -These nerve cells lie in front of the photoreceptors that connect to them and also interconnect with each other.
- -The impulses therefore start to be organised even at this early stage of transmission,
- -a process referred to as ?bottom?up processing?.
- -What is perceived however, is not necessarily reality.
bottom?up processing Top down processing?
- -occurs in higher centres of the brain
- -the internal model of the world is compared to the incoming information.
- -Sense is then made of the information based on previous perceptions and experiences.
- -The illusion of ?a bird in the the hand? (Figure 8-7) demonstrates this well.
- - visual cues lost due to dust.
- -- dust storms.
- -fixed wing aircraft land onto a dusty strip.
- -This is exacerbated by reverse pitch used for braking.
- -The dust cloud around the cockpit can lead to disorientation and a loss of directional control on the runway.
- -White out occurs for the same reasons in snow.
- -In white?out conditions all visual references are lost.
- -Whiteout may also occur due to atmospheric conditions eg. between cloud layers.
- -Flying between cloud layers in these conditions has been likened to flying inside a milk?bottle.
- -Sector white out conditions were thought to have contributed to the loss of Air New Zealand Flight 901 on Mt Erebus, Antarctica, in 1979.
There are illusions arising from both the focal and the ambient visual systems?
- -occur when expectation imposes an interpretation that does not exist in reality, as seen in the examples above.
- -These can be experienced when visual cues are ambiguous, confusing or degraded.
- -Monocular cues of depth perception discussed previously rely on the focal system
- -caused by the orientation mechanism of the peripheral vision
- -false horizons,
- -vection illusions and
- streaming cues,
- -sloping cloudbank
- -top of the cloud-tendency to align the wings with the sloping cloud tops as if they were the true horizon.
- -misperception ground lights can look like stars and create the false impression of a nose-high pitch or wing-low attitude.
- -overcast conditions,unlit areas of terrain may blend with the dark overcast to create the illusion that the unlit terrain
- is part of the sky.
- -Movement of objects in the peripheral visual field can create a sense of false motion.
- -While stopped at traffic lights and distracted inside the vehicle, it is not an uncommon experience to suddenly find yourself standing hard on the brake pedal when the row of cars to the right moves forward.
- -The strong illusion of backward motion occurs when the ambient system detects the forward motion of the cars in the adjacent lane and feeds that information directly into the orientation mechanism causing a reflex ?emergency stop?. Helicopter pilots trying to hover over water or a field of long grass experience a similar illusion. Down?wash from the rotors pushes the water or grass outwards in apparent waves, which when viewed by the pilot?s ambient system, causes a feeling of sideways drift in the opposite direction.
- -Flashes of light - disorientation due to an ambient visual illusion.
- -flickering shadow(eg rotor disk)-conflict between the vestibular + visual system.
- -irritation, distraction, nausea lke motion sickness.
- -epileptic seizures
- Causes of Flicker Vertigo?
- -helicopters or propeller aircraft -
- -Flying in cloud with rotating beacons or strobe lights
- -wearing sunglasses with polarised lenses when looking through windshield heating elements
- -vibration may all cause this phenomenon.
To prevent flicker vertigo?
- -turn away from the light source,
- -turn off offending beacons/strobes,
- -alter the RPM to change the frequency and
- -avoid physiological risk factors such as hypoxia, hyperventilation and hypoglycaemia.
- The streaming of objects such as cultural features,
- vegetation, and terrain in the peripheral vision, are important cues for height and speed during approach and landing.
- They may be open to misinterpretation as they depend on the expected size and nature of the objects,
- which may differ from previous experience.
- Runway slope: Figure 8?10 illustrates the problem of landing on sloping runways.
- The correct approach path is judged by the image of the runway on the retina.
- On a sloping runway, in order to get the same visual picture as in
- (a), the eye must be much lower
- b (2) with an increased probability of an undershoot.
THE EFFECT OF RUNWAY SLOPE?
- Sloping Terrain: Down-sloping terrain before the threshold can cause a similar illusion to runway slope that may result in landing long. Up?sloping terrain gives a false impression of excess height leading to a low approach and the possibility of landing short.
- Runway Dimensions: If operating from an airfield with a runway of 5000ft x 150ft, the correct approach angle for that field is judged by the runway aspect on finals. When making an approach to a larger runway (e.g., 10,000 ft x 300ft) the retinal image at 1 mile is the same as 1/2 mile for the shorter runway (Figure 8?11) resulting in a tendency to flare high on the larger runway as it appears closer. Conversely, if the runway is narrower than normal, there is a tendency to undershoot and flare too low.
At night, if runway lights?
- viewed from an approaching aircraft are displaced laterally,
- the pilot may have the impression that the runway is closer than it really is because it appears wider.
- This false perception may result in a tendency to land short.
- In addition to the physical dimensions, the relative intensity of coloured runway lights may appear to vary,
- which may lead to a false perception of altitude.
- -the pilot may feel further away from the runway (or higher) than is actually the case which may lead to an overshoot.
- - may also occur at night if using low intensity runway and approach lighting.
- -Viewing conflicting traffic in dust and haze will also give a false impression of distance.
Featureless Approach Path?
- - When approaching a runway at night or when there is a featureless approach path (e.g., over water),
- there is a tendency for pilots to feel higher than they actually are leading to a low approach.
- Pilots get accustomed to the size of the trees or vegetation during the approach to land phase giving them a sense of height above the ground.
- If they then operate into an unfamiliar airfield with markedly different tree or vegetation sizes to that normally experienced,
- a high or low approach may result.
- Some of the problems associated with avoiding mid-air collisions have already been discussed, f
- importance of
- -central vision,
- -blind spots,
- -clean windscreens, glasses or visors,
- -correct search techniques and
- -empty field myopia.
- Other factors include the most likely search area, constant relative bearing, and perception and reaction time.
Constant Relative Bearing?
- -While maintaining a constant heading,
- the angle of closure remains constant which means that the conflicting traffic will be ?stationary? in the peripheral visual field.
- As the ambient visual system is good at detecting moving targets,
- one that is stationary may not stimulate the brain and detection will be more difficult
PERCEPTION AND REACTION TIME?
-IT TAKES A FINITE PERIOD OF TIME TO PERCEIVE AND RECOGNISE A COLLISION RISK, DECIDE WHAT ACTION TO TAKE AND TAKE AVOIDING ACTION
FACTORS AFFECTING NIGHT VISION?
- - a process during which each eye independently adjusts from a high-luminance environment to low-luminance.
- -exact mechanisms are unclear, include:
- a) Biochemical,
- b) Physical,
- c) Neural aspects.
- Both rods and cones contain light?sensitive chemicals called photopigments.
- photopigment in the rods
- -is called rhodopsin,
- -which consists of vitamin A and the protein opsin.
- -three different types
- -red, blue, and green-sensitive contain opsins similar to rhodopsin.
- -On exposure to light, the photopigments undergo a chemical reaction that converts light energy to electrical activity, initiating visual impulses in the retina that are conducted by nerve fibers from the eyes to the visual cortex in the brain.
- -In the initial photochemical reaction the photopigments are decomposed.
- -Intense light will decompose the photoreceptor pigments rapidly and completely, reducing retinal sensitivity to dim light.
- -Regeneration of the photopigments occurs during dark adaptation.
dark adapted eye?
- -photopigment regeneration is complete and retinal sensitivity is restored to its maximal level.
- -Rods and cones have significant differences in their rates of dark adaptation.
- - Cones attain maximum sensitivity in 5-7 minutes,
- -while rods require 30?45 minutes (or longer) of absolute darkness to achieve maximum sensitivity after exposure to bright light.
- -The time course of dark adaptation after exposure to bright light is illustrated in Figure 8?14.
- -very low when darkness is first encountered,
- - but the sensitivity increases 10 times within 1 minute
- ? the retina can respond to light of 1/10th intensity.
- -After 20 minutes, sensitivity has increased 6,000 times,
- - and after 40 minutes, sensitivity has increased 25,000 times.
- -the rods adapt more slowly but eventually achieve a much greater sensitivity than the cones.
- -dark adaptation is usually about 80% complete within 30 minutes,
- -but it may take hours to become maximally dark adapted.
- -Retinal adaptation can be affected by physical changes in the size of the pupil.
- -The amount of light entering the eye is proportional to the area of the pupil.
- -The diameter of the pupil can contract to 1.5 mm in bright light and expand to 8 mm in dim light, equivalent to a 30?fold range in the quantity of light entering the eye.
- -mediated by retinal neurons at successive stages of the visual chain in the retina.
- -When light intensity first decreases, the intensities of the signals transmitted by the retinal neurons are very low. -However, the intensities of these signals increase rapidly over time.
- -this change in "neural gain" occurs in seconds and can improve night vision by a factor of 10 or more.
- The inherently greater sensitivity of rod dark adaptation is also a result of retinal summation.
- As many as 100 rods can converge onto a single nerve?fiber in the retina, greatly increasing retinal sensitivity;
- - if the rods are slightly stimulated, the summation of several low?level stimuli might be enough to initiate a light signal to the brain.
- Cones have a much less extensive neural network.
- - The neural dark adaptation mechanisms occur instantaneously, unlike the photoreceptor chemical changes.
VISIBILITY AND SCANNING AT NIGHT?
- -poor acuity of the rods, fine details will not be seen at night and the size of objects becomes paramount.
- -An unlit aircraft should be visible at a range of about 1,000 feet if dark-adapted and correct off?centre vision is used.
- - A similar scanning technique as in day vision is required, however it requires the use of ?off?centre viewing?.
- To aid in seeing outside the cockpit at night, instrument/internal lighting should be kept to a minimum level allowing the pupils to fully dilate.
NIGHT BLIND SPOT
- The location on the retina responsible for the highest visual acuity is the fovea,
- which is located in the center of the visual field.
- The center of the fovea
- -the foveola
- -possesses a high density of cones
- -but has a complete absence of rods.
- -are present in increasing numbers away from the central 1 degree foveola.
- -maximum concentration at a point some 17 degrees from the fovea.
- - have a lower light stimulation threshold than the cones,
- -are much more sensitive to light.
- -A person attempting to see in scotopic illumination (light dimmer than moonlight) has to depend entirely on rods.
A small object held in the central visual field ?
- -cannot be seen in low ambient light conditions,
- - because at luminance levels below dim starlight, which is below cone threshold,
- -a blind spot exists in the central 1 degree of the visual field.
Central blind spot corresponds to?
- -the foveola,
- -purely populated by cones;
- -being rod-free it cannot function in diminished illumination.
- -night blind spot is approximately the size of a thumb held at arm?s length
To best detect small targets with the rods under such circumstances?
- -the individual must look approximately 15-20 deg to one side,
- -above, or below an object to place the object of regard on the part of the retina that possesses the highest density of rods.
- -Individuals can be taught to fixate to one side of an object to avoid the central blind spot and to scan, utilising the most sensitive part of the retina to improve target detection at night.
- An appropriate scan would incorporate a diamond shape scanning pattern around an object of interest, obtaining a 10?degree overlap and fixating for 2 seconds at each point on the diamond.
- -produce temporary but cumulative after?effects on dark adaptation and night vision.
- - diminished rod performance after prolonged sunlight exposure at, for example, the beach.
- -2-3hrs of bright sunlight exposure has been shown to delay the onset of rod dark adaptation by 10 minutes or more, - decrease the final threshold so that full night vision sensitivity could not be reached for hours.
- -After 10 consecutive days of sunlight exposure, the losses in night vision sensitivity were reported to cause a
- -50% loss of visual acuity, visibility range, and contrast discrimination.
- Repeated daily exposures to sunlight
- - prolong the time to reach normal scotopic sensitivity,
- -normal rod sensitivity may not be reached, even overnight.
Means for providing eye protection during the day to conserve night vision include?
- a) Remain in a darkened environment, and
- b) Use of sunglasses.
HYPOXIA AND NIGHT VISION?
- -retinal cells require a plentiful oxygen supply.
- -small reductions will cause a deterioration of visual acuity, brightness and dark adaptation.
- -at 10,000 feet, night vision decreases about 20% compared to MSL.
- -carbon monoxide bound to haemoglobin
- -level in the blood is about 1%,
- -smokers however may have levels as high as 6?7% which will compromise night vision.
Without supplemental oxygen, scotopic vision has been reported to decrease by?
- a) 5% at 3,500 ft,
- b) 20% at 10,000 ft, and
- c) 35% at 13,000 ft.
The use of oxygen?
- even at low pressure?altitudes,
- can be very important in the preservation or maximisation of night vision.
NUTRITION AND NIGHT BLINDNESS?
- -can be caused by long?term vitamin A deficiency
- - Retinal conditions which cause night blindness include
- -progressive cone/rod dystrophies (e.g., retinitis pigmentosa), and
- -drug toxicity (e.g. chloroquine retinopathy).
There is a strong inverse correlation between night vision and age,
related to the loss of flexibility in the ageing lens.
- Approximately 15% of aircrew have substandard night visual acuity despite excellent (6/6 or better) daytime acuity.
- Aircrew who do not normally wear spectacles may have a small myopic shift under reduced illumination and spectacle?wearing aircrew may notice inadequate correction during night operations.
DISADVANTAGES OF RED COCKPIT LIGHTING
- Although red lighting is desirable for preservation of dark adaptation,
- -the disadvantages generally outweigh the advantages
- - low level white illumination is the preferred option in modern aircraft.
- -Red objects become invisible or difficult to see with red lighting.
- This includes red writing on maps and instrument markings, normally things critical to flight safety.
- Red light is focused slightly behind the retina and images will appear out of focus and more difficult to read, especially for older long?sighted aircrew.
VISUAL ILLUSIONS AT NIGHT
- - phenomenon of illusory movement exhibited by a static light viewed in the dark.
- - the dim light will appear to move, even though it is known to be stationary.
- - Without knowing whether the light source is static or moving, it could be quite confusing to interpret.
- cause of autokinesis is not known,
- -it is thought to be related to microfixational movements of the eye and the loss of surrounding references which normally stabilise visual perception. T
- -his illusion will normally disappear when multiple lights are visible.
By misperceiving a static light as ?moving?, pilots on night missions have mistaken stars or ground lights for other aircraft and have become disoriented, sometimes with fatal consequences.
The autokinetic effect can be reduced by maintaining good visual scanning techniques rather than staring at a solitary light source.
BLACK HOLE ILLUSION
- - occur on a dark night over water
- -un?illuminated terrain where the horizon is not easily discernible.
- -worst case occurs when the only visual cues are the runway lights, and there are no surrounding visible references.
Without peripheral visual cues for orientation,
- -the pilot tends to perceive that his aircraft is stable,
- - the runway itself is malpositioned, usually down?sloping.
- makes the landing approach dangerous
-often results in a landing far short of the runway.
A particularly hazardous type of black hole approach occurs when the earth appears to be totally dark except for the runway and the lights of a city on rising terrain beyond the runway.
By maintaining a constant vertical visual angle on the distant city lights, the pilot?s approach may fall below the intended glide?slope as the aircraft gets closer to the runway, possibly leading to controlled flight into the undershoot terrain.
- Flying over terrain with no cultural lighting or features at night will provide little in the way of visual height cues.
- Due to the importance of these cues in the approach and landing, the approach to land may be difficult to judge.
As runway lighting is spaced a little wider than the actual runway, the runway may appear wider than it actually is at night.
- This can lead to an early flare as the pilot will feel too low.
- -Intensity of runway lights can vary due to the lights themselves being brighter or atmospheric conditions between the aircraft and the runway. Bright lights seem closer at night, and dim lights further away.
DISTANCE ILLUSION PHENOMENON
- - a dangerous illusion that can
- - occur when one aircraft is trailing another in a dark environment with few peripheral vision cues.
A likely scenario occurs when the trailing aircraft pilot, for position reference, places the image of the lead aircraft at a certain spot on his windscreen.
- The pilot orients the lead aircraft in exactly the same spot on the windscreen but, over time, inadvertently falls back to a greater separation distance.
- If the trailing aircraft started out 2 nautical miles behind and 300 ft below the lead aircraft and then fell back to 4 nautical miles, that would mean that the trailing aircraft was now 600 ft below the lead.
- If the lead aircraft gradually descends to 600 ft or less, the trailing aircraft may impact the ground.
The potential mishap exists only if the pilot
- -maintains the lead aircraft in exactly the same spot on the windscreen,
- -but fails to monitor the actual altitude
- -or fails to realise that the separation distance has increased;
- - this is an ideal scenario in which a HUD (Head?up?Display) would be of benefit.
- Aircrew sunglasses
- -have frames with a wide field of view
- -fitted with CR 39 ? ND15 lenses with the following properties:
- a) CR 39 lenses are plastic and are distortion free. Glass lenses are unsuitable because they may shatter
- causing splinters to enter the eyes.
- b) ND15 means neutral density (absorbs all colours equally providing faithful colour reproduction) filtering out
- all but 15% of the light transmission.
- c) The CR 39 lenses absorb harmful ultra?violet radiation.
- d) Polarised and photo?chromatic lenses are unsuitable. Polarised lenses create interference lines
- with windshield heating and restrict vision. Photo?chromatic lenses react too slowly to light intensity changes
- and may be overwhelmed by very bright light.
HELMETS AND VISORS
- - required for bird strike, wind?blast (ejection), lasers and nuclear flash.
- Flying aircraft that require a flying helmet,
- -either one or both visors should be down at all times.
For ejection, it is essential that both visors be down.
Where considered necessary,
- -goggles or visors with the requisite density at the laser wavelength will protect against eye damage.
- -Special goggles (PLZT) have also been developed to protect from nuclear flash.
MAXIMISING NIGHT VISION
- The following are some ways for aircrew to protect, improve, or maintain their operational night vision:
- a) Complete a training course that emphasises the inherent limitations of night vision;
- b) Keep spectacles, visors, and windscreens clean;
- c) Have adequate visual correction;
- d) When practical, dark?adapt before flying;
- e) Avoid bright lights, or at least protect one eye;
- f) Do not fixate centrally, but scan and look 15?20 deg to the side of the visual target;
g) Wear sunglasses (neutral density, 15% transmission) on days of night missions;
h) Eat an adequate diet that includes vitamin A;
j) Consider the use of 100% oxygen at night if available, even at low altitudes (subject to local SOPs)