**2.1 Vision:**

Basic anatomy and physiology of the visual system

Types of vision (photopic, mesopic, scotopic)

Factors affecting vision inflight (object size, ambient illumination,
cockpit illumination and use of red lighting, object contrast, object
viewing time, atmospheric clarity, refractive eye disorders,
self-imposed stresses, hypoxia, windscreen haze/scratches/dirt,
instruments haze/scratches/dirt, ambient temperature and humidity, etc.)

Hazards to vision inflight (including flash-blindness, bird-strike and
ejection related eye injuries, etc.)

The physiological blind spot

Acquisition and loss of dark adaptation/night vision adaptation, central
night blind spot, night myopia, cockpit illumination issues

Night vision equipment (NVGs, FLIR, etc.)

Visual scanning methods

Use of sunglasses (colored filters, neutral filters, reflecting filters,
polarizing filters, light transmission)

Use of goggles and visors

Use of spectacles (bifocals, trifocals, multifocals, photochromatics)

Use of contact lenses (soft and hard)

Use of monovision contact lenses

Monocular vision

Corneal Surgical Procedures

**[BASIC ANATOMY AND VISUAL PHYSIOLOGY]{.underline}**

Basic Anatomy of the Eye

Light enters the eye, passing first through the cornea, then aqueous of
the anterior chamber, the pupil, the lens and finally the vitreous body
before reaching the nerve receptors of the retina (rods and cones). The
human retina contains 100, million rods and 6 million to 7 million
cones. Most of the cones are in the macula and the fovea is devoid of
rods. The photosensitive pigments that initiate the visual response are
rodoposin in the cones and rhodopsin in the rods.

It is obvious that any obstacle to the transmission of an "image"
(corneal scar, cloudy aqueous or vitreous, cataract) will reduce visual
acuity. This is only the beginning of "normal" vision. When light
reaches the retina it is absorbed in the pigment and a chemical reaction
occurs (iodopsin or rhodopsin) transforming the light to a nerve impulse
transmitted to the occipital lobes of the brain (via the optic nerve and
tract). Again, one realizes that any impairment to transport of the
electrical impulse (tumor, stroke, macular degeneration) will produce
impairment to visual acuity.

Since the chemical reaction of light with the rods and cones to produce
a nerve impulse requires oxygen for the regeneration of rhodopsin
(creating a balance between bleaching and regeneration) it is
understandable that hypoxia will slow the regeneration process. Hypoxia
may produce a decrease in vision before any other symptoms of hypoxia
appear. (Night vision and ocular movements are affected early).
Interestingly, some studies have indicated that the visual deficit is
cortical rather then peripheral.

The blood supply to the eye is by way of the ophthalmic artery which
supplies branches to the choroid, ciliary body and iris, (these three
called the uveal tract) as well as being the primary vessel to the optic
nerve as it enters the eye.

There are three nerve supplies to the eye. These are called:

Sensory-arising from the ophthalmic division of the trigeminal nerve.

Sympathetic-coming from the carotid plexus.

Parasympathetic-from the oculomotor nerve.

The physiologic blind spot (devoid of rods and cones)

The "normal" area of blindness in an eye is produced at the entrance of
the optic nerve into the globe. The person with binocular vision does
not identify this area since the visual field defect produced in one eye
is covered by the functional retina of the other eye. The physiologic
blind spot is readily demonstrated with the tangent screen (one eye
occluded). At one meter the blind area lies between 13 and 18.5 degrees
temporal to the central macula (fixation point on the screen) and
measures some 4 degrees in width and 5 degrees vertically (using 2mm
white test target). Move the test subject 2 meters from the tangent
screen and the blind spot doubles in size. This is a concern when
evaluating the monocular pilot for certification because at several
meters the blind spot becomes quite large, and the subsequent visual
field defect is a potential danger to flying safety i.e., scanning for
air traffic.

Vision: Photopic, Mesopic, and Scotopic

Photopic vision, or light adaptation, begins with the bleaching of
rhodopsin which is the photosensitive pigment found in the rods. The so
called bleaching of rhodopsin is accomplished in several chemical steps
as the rhodopsin changes in color from red to yellow. The iodopsin,
photosensitive chemical in cones, is much less sensitive to light than
are the rods and as light increases the cones became receptive to the
longer rays of the visible spectrum. Daylight vision (photopic or cone
vision) stimulates three types of cones i.e., those of blue receptor
(shorter end of visible spectrum), green and red receptors. The chemical
reaction in the cones is similar to that of the rods but the recovery
phase is much more rapid. (3-5 minutes for the cone receptors).

In dark adaptation (scotopic vision) regeneration of the rhodopsin
occurs and maximal rod stimulation is present in 20-30 minutes. The
cones are non-functional in low intensity light and indeed there is a
foveal (central vision) blind spot. The highest concentration of rods is
approximately 7 degrees off the central fixation point and this is the
area of the retina that sees best in diminished light-i.e., form of
objects is seen in extremely low levels of illumination but there is low
visual acuity (poor discrimination of detail) and there is no color
perception with scotopic vision. The variation of light intensities
perceptible by the human eye are astounding, the threshold of light
detection to maximal operating range is one billion to one. Twilight
vision (dawn or dusk) is mediated by a combination of stimulated rods
and cones (also called mesopic vision).

SUNGLASSES

Sunglasses reduce discomfort from glare and light intensity. Glare is
defined as interference to vision by unwanted light, i.e., light in the
wrong place. **[Anti-reflective coating]{.underline}**: An
anti-reflective coating (typically with an ultra thin layer of magnesium
fluoride) can enhance the optical performance of all lenses. One benefit
is a reduction of disturbing multiple image reflections by on coming
headlights. If the coating wears off the lens (plastic or glass) with
repeated rubbing, while cleaning the lens, it can be reapplied. Normally
if the coating has been rubbed off, the lenses are so scratched that
they should be replaced rather than having the coating be reapplied.

Also, not all types of anti-reflective coating can be reapplied to
lenses. Some have the coating baked on and cannot be reapplied

When illumination levels are very high the light belonging in the
focused area of the retina spills over to the surrounding neurons and
produces a blurred image. High intensity light can also produce extended
miosis that is physically uncomfortable (headache) and absorptive lenses
may eliminate the problem.

In addition to transmission, **[absorption density]{.underline}** is
another term to be defined in discussing sunglasses. When light strikes
a homogeneous transparent substance, part of the light is transmitted
and part of it is absorbed (and reflected). Affecting light transmission
are two main factors, the tent of the medium and the density of the
material.

Thus sunglasses can be classified by percent transmission (50%= opacity
2:1) or by density; ^3^ most sunglasses reduce transmission in the
visible spectrum (400-700nm) by 60-80%. Infrared rays are reduced by 90%
and ultraviolet by 95-99%. Even in the unprotected eye ultraviolet rays
do not reach the retina but as much as 20% of infrared rays do. While
most of the UV is absorbed by the cornea and lens of the eye, it is
thought that some small amounts (1-2%) does get to the retina since it
is thought that UV may be a cause of macular degeneration. Also, we have
a good number of airmen who have had cataract surgery and, since their
natural lens has been removed they are more likely to receive the full
effect of UV on the retina without protection. Almost all pigment added
to glass lenses is spectrally selective i.e., light transmission varies
according to the wavelength of the incident light.

**[Specific Lens Tints]{.underline}**

**[Pink]{.underline}** (Rose or Cruxite) tints are popular. The rose
tint is often used in settings with fluorescent lighting, since it
reduces the flicker from these types of lights. While they filter out
most ultraviolet light, they do not decrease transmission of the visible
spectrum. However, the person who has been wearing these lenses to
reduce "glare" may find it virtually impossible to discontinue their
use. **[Neutral grey]{.underline}** tint is available in variable
densities that will reduce the visible spectrum of light up to 88% and
absorb 50% (glass) and 100% (CR-39) of UV. Since neutral gray tint does
not distort color but only reduces light intensity, it is probably the
best tint for general use. Other tints that offer the wearer other
options include **[green]{.underline}**, **[brown]{.underline}**, and
**[yellow]{.underline}**. The **[green tints]{.underline}** absorb 99%
of UV but they do not reproduce color as well as the grey and they
transmit more light (35% total) than the grey.

For higher power prescription lenses, CR-39 lenses may be better than
glass, since the tint will be even across the lens. In glass lenses, the
edges of the lenses will be much darker than the centers in a high minus
lens, and vice versa for a high plus lens.

**[Brown pigment]{.underline}** absorbs long and short wave lengths
similar to green and offer some cosmetic appeal but they distort color
more than green. **[Yellow tinted lenses]{.underline}** are sold as
outdoor or hunting lenses because they transmit about 80+% of the
visible spectrum while filtering out the blue range of the spectrum.
Atmospheric haze is largely produced by the shorter blue wavelengths and
the yellow lenses are good haze filters. While they absorb 100% of UV
wavelength they allow 100% transmission of infrared.

**[Polarized Lenses]{.underline}**

The wave theory of light transmission states that the transmission
vibrations are equal in all meridians. The light reflected from any
surface (except metal) tends to be all in one meridian and the light
rays are therefore partially polarized.

Polaroid lenses are useful in reducing reflected glare from surfaces
such as water, highways or ski slopes. Polarizing filters are made by
heating and stretching a thin sheet of polyvinyl alcohol to about four
times it's original length. The stretching aligns the molecular
structure into long parallel chains. Passing this sheet through a weak
iodine solution, the iodine molecules diffuse into the chains creating a
polarizing filter. The polarizing filter is then laminated between two
pieces of lens material (crown glass, CR-39) or suspended in the lens
when cast. Polarized lenses transmit only light in one plane and the
intensity of useful light transmitted is reduced by at least 50%.

**[Photochromic Lenses]{.underline}**

These lenses darken when exposed to sunlight (ultraviolet light
sensitive) and lighten when ultra violet is withdrawn. The lens is
typically grey or brown and may be combined with an absorption glass for
a further decrease in light transmission. The lenses typically darken
within a minute of being exposed to UV light but require much more time
indoors to allow increased light transmission. The lenses decrease light
transmission in the evening and at night and the FAA does not recommend
their use while flying. Incidentally, these lenses will work inside a
car (newer chemicals) even though the windshield glass filters out the
UV light. AME's are to counsel airmen that sunglasses are not acceptable
as the only means of correction to meet visual standards, but may be
used for backup purposes if they provide the necessary correction.

Worth mentioning is the incapacitation produced by the intensity of
reflected glare and illumination off of snow. "Snow blindness" is an
ultra violet burn to the corneal epithelium much like the so called
"welders burn." Photo Polar glasses are available that combine a
Polaroid filter and photochromic glass. These are one purpose lenses
because the light transmission they provide is too low for general wear.

There are many misconceptions regarding the wear of sunglasses. A few
include:

Sunglasses must be worn al all times out doors to protect the eyes.

Inexpensive sunglasses can harm the eyes.

Some confuse Polaroid with sunglasses and use Polaroid's as general
purpose lenses.

SPECTACLE CORRECTION OF VISUAL ACUITY

**[The Use of Spectacles]{.underline} (for improving visual acuity)**

Background: Three fourths of all adults older than 42 have a refractive
error greater than .5 diopter. More than 100 million Americans wear or
would benefit from wearing corrective lenses. 80 million Americans wear
spectacles and 29 million wear contact lenses. More than one million
Americans have undergone refractive surgery procedures.

The purpose of spectacles is two fold, the transmission, and refraction
of light

A refractive error occurs in an eye when parallel rays of light entering
a non-accommodating eye are not focused on the retina. The refractive
error may be simple, the rays come to a focus before reaching the retina
(simple myopia) or they reach the retina without being in focus
(hyperopia).

Refractive errors can be complex (common finding) and defined as
astigmatism which is produced by rays entering the eye with two focal
points due to an irregularly shaped cornea or lens or both (also
common). Thus, there can be present myopic astigmatism, hyperopic
astigmatism or mixed astigmatism (myopia is one plane and hyperopia in
the plane at right angles).

Correcting an abnormality in refraction is further complicated when the
person past the age of forty notes the physiologic change of presbyopia
(loss of elasticity of the lens which is the beginning, (usually about
age 42), of an inability of the eye to refocus an image from distance to
near reflexly.

The definition of "diopter" will make understanding presbyopia clear.
When an eye is fully corrected for distance vision (arbitrarily measured
a t 20 feet) the light rays entering the eye and focusing on the retina
are parallel rays of light. By definition a 1.0 diopter convex lens will
bring parallel light rays to focus, at one meter (approximately 40
inches). A 2.0 diopter lens will focus the light at 0.5 meter or about
20 inches and indeed a 2.5 diopter lens will focus the light at 14-16
inches-reading distance for the person with no ability to change the
shape of his or her optical lens (usually age 65 or older).

It is apparent that glasses wearers between the ages of 45 and 65 will
vary in the amount of "add" required for their near vision (based on
their residual lens malleability by the ciliary body) and indeed it is
not unusual for persons age 45-50 to require little or no additional
lens strength for near vision. If glasses are prescribed with a bifocal
of such magnitude that the object image is in focus at 14-16 inches, an
attempt to observe an object at, say arms length-28-32 inches will be
blurred. This is basically trifocal distance and requires an
intermediate lens in the glasses between the distance segment and the
near segment and mathematically is always ½ the strength of the bifocal
i.e., bifocal add is +2.00 diopter, trifocal strength will be +1.00
diopters.

In today's civilization, postponing the inevitable (face lifts, tummy
tucks, liposuction) aging factor, it is understandable that people have
wanted to avoid the "aging" stigma of bifocals and thus the innovation
of the multifocal (no line) bifocal. This lens is useful in sports,
computer use, reading the gauges in the cockpit or car because of the
progression of near vision as the eyes move downward.

There are three critical issues to consider when a person wears
progressive lenses:

Distortion: The optically pure corridor of the lens has a hour glass
shape, and bordering this corridor is a distinct area of distortion.

Small Frames: many of the so called stylish frames are small
particularly in the vertical height and this limits the scope of the
useful optics.

Computer views: Not so much with the newer progressives but some wearers
complain that they must tilt their head backward in order to see the
computer screen.

There are many bifocal styles that will solve problems of a specific
nature. For example, a painter who works on ceilings and walls above the
horizontal plane may have a bifocal type segment in the top of the lens.
For that matter, pharmacists who must retrieve medications from over
head shelves or postmen who place mail in overhead slots profit from
"occupational" glasses. The optician can assist in selecting the best
type correction for a specific need.

Today, most lenses are plastic, then polycarbonate, and then the newer
high index materials. Most of the newer materials will transmit about
92% of visual light (will increase to 99%, if an anti-reflective coating
is added to the lenses).

Surface and finish labs reported that less than 5% of their orders are
made with crown glass lenses. Crown glass does not absorb UV and must be
coated specifically for this purpose.

While plastic lenses are more susceptible to surface abrasions and
chemical damage they are 20% lighter than glass and have less tendency
to fog. Remember, half glasses **[are not]{.underline}** safety glasses,
contact lenses offer **[no]{.underline}** eye safety, and in order to
convert industrial thickness glasses into **[true]{.underline}** safety
glasses detachable side shields must be added.

**[Monocularity]{.underline}**

[The following is FAA policy regarding Monocular Vision:]{.underline}

**"Although it has been repeatedly demonstrated that binocular vision is
not a prerequisite for flying, some aspects of depth perception, either
by stereopsis or by monocular cues, are necessary. It take time for the
monocular airman to develop the techniques to interpret the monocular
cues that substitute for stereopsis; such as, the interposition of
objects, convergence, geometrical perspective, distribution of light and
shade, size of known objects, aerial perspective, and motion
parallax."**

**"In addition, it takes time for the monocular airman to compensate for
his or her decrease in effective visual field. A monocular airman's
effective visual field is reduced by as much as 30%. This is especially
important because of speed smear; i.e., the effect of speed diminishes
the effective visual field such that normal visual field is decreased
from 180 degrees to as narrow as 42 degrees or less as speed increases.
A monocular airman's already reduced effective visual field could be
reduced to even less than 42 degrees by speed smear."**

**"For the above reasons, a waiting period of six months is recommended
to permit an adequate adjustment period for learning techniques to
interpret monocular cues and accommodation to the reduction in the
effective visual field."**

**"An individual with one eye, effective visual acuity equivalent to
monocular (i.e., best corrected distant visual acuity in the poorer eye
is no better than 20/200), may be considered by the regional flight
surgeons or the AMCD for any class medical certification through the
special issuance provisions of FAR Part 67 if:**

**A six month period has elapsed to allow for adaptation to
monocularity.**

**A complete evaluation by an eye specialist as reported of FAA ForM
8500-7, Report of Eye Evaluation, reveals no pathology of either eye
which could affect the stability of the findings.**

**Uncorrected distant visual acuity in the better eye is 20/200 or
better and is corrected to 20/20 or better by lenses of no greater power
than plus or minus 3.5 diopters spherical equivalent.**

**Any applicant eligible for a medical certificate through special
issuance under these guidelines shall pass a medical flight test..."**

**AMEs may issue a certificate to monocular pilots with the limitation
of "Valid for student pilot purposes only," provided the applicant meets
the standards in the better eye.**

**[CONTACT LENSES AND AVIATION SAFETY]{.underline}**

It is not necessary that the pilot applicant leave contact lenses out of
eyes 24 hours prior to the exam. Both gas permeable (semi-rigid) and
soft contacts are permeated by oxygen in the tears and air so that
corneal epithelial edema (contact blur) rarely occurs. (It was common
with polymethyl methacrylate lenses, i.e. "hard" contacts). Keep in mind
that some deeply tinted lenses can reduce visual acuity particularly at
night.

Since the advent of laser corneal refractive surgery-many applicants
will have one eye corrected for distance and one for near. For six
months after this procedure, while flying, the airman must wear glasses
or contacts that correct both eyes for near and distance vision. When
not flying, the airman may remove the corrective lenses and get used to
the corrections made by the surgery.

During the six-month period, it is felt that the airman is learning to
use other visual cues to get his/her depth perception back. This is the
same process that occurs when a person loses an eye. The airman's AME
must keep the corrective lenses limitation on the medical certificate
for the six-month period. Then the airman may apply for a Statement of
Demonstrated Ability (SODA). If he/she passes the medical flight test,
the lens requirement on the medical certificate can be removed.

Bifocal contacts are more successful today than 10 years ago due to
better quality of lenses (better reproducibility) and better fitting
techniques. However, there are continued difficulties in positioning the
lenses for predictable near-distant visual acuity due to adequacy (or
inadequacy) of the eyes' tear film, allergies or eyelid abnormalities.
Therefore bifocal contacts are not waiverable for flying.

Contacts that correct for near only are not waiverable for the obvious
effect they have on distance visual acuity.

Orthokeratology is a management of myopia that involves placing a rigid
contact lens that is slightly flatter than the cornea's curvature on the
cornea to flatten it (i.e. make the globe "shorter"). The lens is
usually worn at night during sleep. The disadvantages of this technique
are that corneal abrasions are common and the correction is temporary
requiring continued use of the flattening technique. However, in some
cases, orthokeratology is accepted by the FAA, for treatment for myopia.

**[Refractive Surgery]{.underline}**

Introduction:

The FAA/AMCD policy is that most medically approved ophthalmologic
procedures are acceptable as long as they are not considered
investigational. Investigational procedures done at approved training
facilities will be evaluated on an individual basis. Radial Keratotomy
(RK), Photorefractive Keratectomy (PRK), and Laser Assisted In Situ
Keratomileusis (LASIK) are accepted procedures and the AME may certify
such applicant provided they meet the FAA criteria (see below and the
Guide for Aviation Medical Examiners). Of course, the applicant must
also meet all other applicable medical standards in FAA Part 67.

An AME may issue a medical certificate to a post-refractive surgery
applicant if the applicant meets the visual standards for the applied
class and the "Report of Eye Before granting a medical certificate to
the post refractive surgery applicant, the AME must obtain a complete
**ophthalmologic** evaluation with a written report demonstrating full
recovery with stable visual acuity and lack of deleterious sequelae. The
report must include field of vision, night-glare, and visual acuity
evaluations. There should be no other pathology of the effected eye(s).
The evaluation must be performed by an ***[eye care
specialist.]{.underline}*** Reports from optometrists and /or other
health care providers are not acceptable. In addition, as always, the
AME should document all pertinent medical data on Item 60 before
forwarding records to the FAA.

It is a common practice today for one eye to be corrected for near and
the other (dominate eye) for distance. If at the end of six months the
cornea is stable, with minimal glare and satisfactory visual acuity,
waivers are being granted on a case by case basis for the induced
monocularity. (SODA)

Aerospace Medicine Self Administered Course

Section II, 2.1. Vision

Factors affecting vision in-flight (object size, ambient illumination,
cockpit illumination and use of red lighting, object contrast, object
viewing time, atmospheric clarity, refractive eye disorders,
self-imposed stresses, hypoxia, windscreen haze/scratches/dirt,
instruments haze/scratches/dirt, ambient temperature and humidity, etc.)

Aviation operations at night are more easily conducted when ambient
light sources provide the greatest amount of illumination. Sources of
ambient light include the moon, solar light, background illumination,
and artificial light. However, meteorological conditions will affect the
level of light from the ambient light source.

The moon is the most important source of natural light at night. The
moon rises in the east and sets in the west and its angle changes about
15 degrees per hour (1 degree every four minutes). Ambient light levels
change as the moon angle changes and is the brightest when the moon is
at its highest point (zenith). The time at which the moon rises and sets
changes continually as the days of the year get longer and shorter.
There are four phases of moon, the first of which is the new moon phase.
During that phase the amount of the moon that is visible increases from
less than 2% to about 50% and takes about eight days. The second phase,
which takes about 7 days is the first quarter phase during which
illumination continues to increase until slightly less than 100% of the
moon is visible. The full moon phase or third phase begins when 100% of
the moon is illuminated and ends when about 50% of the mood is visible
(about seven days). The forth or last phase is the third quarter which
begins when about 50% of the moon is visible and ends when 2% or less is
visible (about seven days).

Ambient solar light is usable for a period following sunset and before
sunrise. After sunset, the amount of available solar light steadily
decreases until the level of light is not usable to the unaided eye when
sun is 12 degrees below the horizon (end of evening nautical twilight or
EENT). Before sunrise, solar light becomes usable when the rising sun is
12 degees below the horizon (beginning of morning nautical twilight or
BMNT). Sources of smaller amounts of artificial illumination include
lights from cities, automobiles, fires, and flares. However, lights of a
large metropolitan area will increase the light level around the city.
Light from these sources is most pronounced during overcast conditions
due to the reflected glare.

Ambient light levels cannot always be accurately predicted, because
meteorological conditions may deteriorate during the flight. Adverse
weather at night is difficult to detect and there will be a gradual loss
of the horizon from a decrease in visual acuity as weather conditions
worsen. Airmen should be constantly aware of changing conditions, as it
will assist in evaluating the available ambient light.

Clouds reduce illumination to some extent. However, the exact amount of
reflection or absorption of light energy by different cloud types is not
known. The amount of reduction will depend on the amount of cloud
coverage and the density or thickness of the clouds. A thick, overcast
layer of clouds will reduce the ambient light to a much greater degree
than a thin, broken layer of clouds and the combined effects of two or
more layers of clouds must also be considered. At night, airmen may fail
to detect a gradual increase in cloud coverage because of reduced night
vision, and may, without warning, inadvertently enter the clouds.
Therefore, airmen must be alert for indications that clouds are present.
These indications include a gradual reduction of the light level, loss
or reduction of visual acuity and terrain contrast, and the moon and
stars become obscured. The less visible the moon and stars, the heavier
the cloud coverage. Shadows obscuring the moon's illumination can be
detected by observing the varying levels of ambient light along the
flight route. However, lightning flashes are one meteorological
phenomenon that will increase illumination. They have an effect similar
to that of a bright flare and the intensity of the illumination will
depend on the proximity of the lightning. The airman's night vision may
be impaired if he is too close to lightning activity.

Visibility restrictions, such as ground fog, dust, haze, and/or smoke,
reduce illumination and are more pronounced at lower altitudes. The
probability of fog will increase as the temperature decreases and the
dew point spread approaches zero. An increase in moisture content in the
air will result in a decrease in the intensity of ground lights. A halo
effect around ground lights indicates that moisture is in the air and
that ground fog may be forming. Haze from pollutants is common around
large cities.

***Cockpit illumination and use of red lighting,***

The use of aircraft lights should be standardized to reduce the adverse
effects of these lights on night vision. During preflight night checks,
cockpit lights should be adjusted to the lowest intensity level that
will allow the instruments to be read. Interior lighting, supplemental
lighting (map light), or a flashlight with appropriate lens filter can
assist in illuminating the cockpit area. If a particular light is too
bright or causes reflection, it should be turned off or modified. All
lights not required for safe flight should be turned off. Also, as the
ambient light level decreases from twilight to darkness, the intensity
of the cockpit lights should be reduced to the lowest readable level.
This will reduce the reflection of the lights off the windscreen and
help minimize the interference with maximum dark adaptation for viewing
dim objects outside of the aircraft. In addition, all dirt, grease, and
bugs must be removed from the windscreen before each night flight.

Exterior lights are used to identify the aircraft. However, during aided
terrain flight, the illumination from these lights may degrade the
operation of the Night Vision Goggles (NVG\'s). To reduce the adverse
effect of exterior lights, the aviator should turn off all lights not
required by regulations. The remaining lights should be operated in the
dim mode or be properly taped or painted.

Since low ambient light levels stimulate retinal rods, use of red lights
(wavelengths from 650 to 670 nanometers) will not significantly impair
night vision if the proper techniques are used. Red cockpit lighting has
been used since World War II to maintain the greatest rod sensitivity
possible, while providing some illumination for central foveal vision.
However, red cockpit lighting can create some near vision problems for
the pre-presbyopic and presbyopic airman. With the increased use of
electronic and electro-optical devices for navigation and night vision,
the importance of the pilot\'s visual efficiency within the cockpit has
increased and new problems have been created. Low intensity, white
cockpit lighting is presently used to solve those problems. It affords a
more natural visual environment within the aircraft, without degrading
the color of objects. Blue-green cockpit lighting is used in aircraft in
which night-vision devices are used because, unlike the human eye, these
devices are not sensitive to light at that end of the visual spectrum.
In addition, blue-green light is the easiest for accommodative focus and
is seen by the rods more readily than any other color. It is not seen as
blue-green, however, but only as white light.

If an airman is unable to attain dark adaptation by staying in total
darkness, a more practical alternative may be to have him/her wear
goggles with a red filter. When a red filter with a cutoff at about 650
nanometers is worn, no light is transmitted to the eye than can
stimulate the rods. However, cones are sensitive to red light so there
is adequate visual acuity. Wearing the red filters for 30 minute will
result in the rods being almost fully dark-adapted. Although the cones
are not dark adapted, it will only take about 5 to 7 minutes after the
airman steps into the dark for the cones to adapt. Cone adaptation is
relatively unimportant, as these photoreceptors are incapable of
function in starlight illumination. There are some drawbacks to wearing
red filters, such as if the airman is reading a map that has markings in
red ink on a white background as they will be invisible. In addition,
red light creates or worsens near point blur in the pre-presbyopic and
presbyopic airman, as red light comes to a focus behind the retina and
requires more accommodation to bring into focus.

***Refractive eye disorders***

A nearsighted (myopic) individual does not see distant objects clearly
without corrective devices (e.g., glasses, contact lenses). A slightly
myopic airman who may see fairly well during the day, but may have
difficulty seeing at night. This is due to the blue wavelengths of the
visible portion of the spectrum, which are still visible at night. A
slightly myopic or marginally corrected airman will experience blurred
vision at night when viewing blue-green light. In addition, image
sharpness will decrease as pupil diameter increases, so vision of airmen
with mild refractive errors may become unacceptably blurred unless
corrective lenses are worn. Also, as luminance levels decrease, the
focusing mechanism of the eye may move toward a resting position and
making the eye more myopic (dark focus). These are important factors
when the airman looks outside the cockpit during unaided night flight.
Special corrective lenses may need to be prescribed to assist the airman
to see properly at night.

Farsightedness (hyperopia) is also caused by an error in refraction; the
lens of the eye does not focus an image directly on the retina, which
results is blurred vision. In hyperopia, however, the near image being
viewed is focused behind the retinal plane. Objects that are nearby are
not seen clearly; only more distant objects are in focus. In order for
the airman to be able to read instruments and flight materials,
corrective lenses must be worn.

Astigmatism is an irregularity in the shape of the cornea that may cause
an out-of-focus condition. For example, when an astigmatic airman
focuses on power poles (vertical), the wires (horizontal) may be out of
focus in most cases. An airman with astigmatism ≥ 1.00-diopter should be
individually evaluated before flying with NVG\'s that preclude the
wearing of eyeglasses.

Presbyopia is part of the normal aging process. Beginning in the early
teen years, individuals gradually lose accommodation (the ability to
focus on nearby objects). When individuals are about 40 years old, their
eyes are unable to reliably focus at the normal reading distance without
reading glasses. 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 that compensate for the inadequate accommodative
power of the crystalline lens.

Retinal Rivalry - Eyes may experience this problem when attempting to
simultaneously perceive two dissimilar objects independent of each
other. This phenomenon may occur when airmen are viewing objects through
a heads-up displays (HUDs). If one eye is viewing an image while the
other eye is viewing another, there may be a problem in total
perception. Quite often the dominant eye will override the nondominant
eye, possibly causing the information delivered to the nondominant eye
to be missed. Additionally, this rivalry may lead to ciliary muscle
spasms and eye pain. Mental conditioning appears to alleviate this
condition; therefore, retinal rivalry becomes less of a problem as
airmen gain experience with HUDs.

***Self-imposed stresses,***

Fatigue, a cold, vitamin deficiency, alcohol consumption, caffeine,
smoking and over-the-counter drugs can all contribute to diminished
night vision. A healthy person can experience vision deterioration at
altitudes as low as 5,000 feet due to diminished oxygen. A smoker's
threshold may be a great deal lower and sometimes will not improve even
with supplemental oxygen. Use of alcohol can have visual symptoms that
include eye muscle imbalance, which leads to double vision and
difficulty focusing.

Poor nutrition can turn a competent pilot into a sick, confused
passenger. The human fuel requirement may be one of the most frequently
overlooked elements in good pilot preflight planning. The only source of
energy for the brain or the central nervous system is blood sugar or
glucose, since neither can store glucose they require constant
refueling. Physiological responses to a lack of sufficient glucose can
include fatigue, mental confusion, faintness, headache, forgetfulness,
dizziness, blurred vision, coldness in the extremities, low blood
pressure, nervousness, depression and, of course, extreme hunger.\" A
pilot need to carefully consider whatever is ingested or not ingested,
as either can result in self-imposed stress.

***Hypoxia, windscreen haze/scratches/dirt,***

Hypoxia can cause several changes in vision. At a range from sea level
to 10,000 feet, ordinary daytime vision is not affected. There is,
however, a slight impairment of night vision. At altitudes from 10,000
to 16,000 feet, visual functions are impaired, but the airman should be
able to continue the flight. The following changes in vision occur,
becoming more marked with increasing altitude:

-   retinal vessels become dark and cyanotic

-   diameter of the arterioles increase 10 to 20 percent

-   retinal blood volume increases up to four times

-   retinal arteriolar pressure increases along with the systemic blood
    pressure

-   intraocular pressure increases somewhat with the increase in blood
    volume

-   pupil constricts

-   a loss (at 16,000 feet) of 40 percent in night vision ability

-   accommodation and convergence powers decrease

-   ability to overcome heterophorias diminishes.

All these changes return to normal by either the administration of
supplemental oxygen or the return to ground level. Up to 16,000 feet,
these effects remain latent, in the sense that physiologic compensation
enables the flyer to continue basic tasks, unless this altitude is
maintained for a long period.

Most general aviation airmen use supplemental oxygen if they are to be
at altitudes greater than 12,000 feet. However, a larger aircraft can
experience a rapid decompression at higher altitudes. If rapid
decompression occurs at altitudes from 16,000 to 25,000 feet, all of the
preceding ocular changes will become severe enough to produce
interfering visual difficulties. For example, visual reaction time is
slowed, motor response to visual stimuli is sluggish, mental processes
are slowed, heterophorias are no longer compensated by fusion and become
heterotropias with resulting double vision, accommodation is weakened
and convergence lost, with the instruments becoming blurred and doubled,
dilation of retinal vessels with the accompanying pressure changes
continues to increase until circulatory collapse intervenes. All these
changes are reversed by the use of oxygen or by returning to sea level.
If decompression were to occur at altitudes above 25,500 feet,
circulatory collapse occurs and there is a loss of both vision and
consciousness. As result of the death of neurons from severe hypoxia and
lack of circulation, the airman may suffer permanent damage to the
retina and brain.

Hazards to vision in-flight (including flash-blindness, bird-strike and
ejection related eye injuries, etc.)

**Flash-blindness,**

Flash-blindness is a visual interference effect, caused by a bright
light that persists after the light is terminated. Flash-blindness
continues to persists while an eye attempts to recover from an exposure
to the bright light. The ability of any given light source to induce
flash-blindness is directly related to the brightness of the light and
the level of dark adaptation in the eye at the time of the exposure. It
can be shown that the brighter the environmental luminance levels to
which an eye is adapted at the time of the exposure, the brighter the
light needed to induce flash-blindness. The corollary to this is that
the brighter the light in any given situation, the longer the ensuing
flash-blindness period. This directly relates to the ability of the eye
to recover from bleaching of the photosensitive pigments caused by the
new bright extrinsic light. During the period of recovery, the luminance
conditions of the object(s) being viewed as a primary task will also
determine how long it takes to functionally recover from the
flash-blindness. If the visual task being undertaken at the time of
exposure is well illuminated, recovery times will be shorter than
recovery from poorly illuminated visual tasks. These recovery times
reflect differences between the photochemical rejuvenation rate of rods
to that of cones.

Flash-blindness can last from several seconds to several minutes and has
been shown to be more prolonged in older individuals, largely based on
the speed and efficiency of the recovery mechanisms and richness of
vascular supply available in the target ocular tissue. Continuous wave
and pulsed laser beams are equally adept at inducing flash-blindness.

**Bird-strike**

Birds have been a potential hazard to aircraft since the beginning of
air travel. Despite their relatively small size, birds can cause
considerable damage to aircraft when the two collide, mainly due to the
high speed of the aircraft. Colliding at 130 knots, a 4-pound bird hits
an aircraft with more than 2 tons of force, concentrated in a small
area. At 260 knots, the same bird delivers a 9-ton punch. Resulting
damage most commonly involves turbofan engines, but may include bent or
broken rotor blades, blocked engine air intake screens, smashed
windshields and dented or torn tail structures.

During flight planning, an airman should check airport documentation and
NOTAMs for information about permanent or seasonal bird problems at both
departure and destination airports. The pilot should plan to fly as high
as possible. Only 1% of general aviation bird strikes occur above 2,500
ft. Avoid flying over bird and wildlife sanctuaries (Note: They may
contain a large number of birds that make regular flights at dawn and
dusk.), landfill sites, and fish packing facilities. Avoid flying along
rivers or shorelines, especially at low altitude. Birds, as well as
pilots, use these as navigational features. While most bird species are
active primarily during the day, there are many birds that fly at night,
as well as during dawn and dusk.

If there are two pilots, discuss emergency procedures before departure,
including those if cockpit communications are lost or if the windshield
is penetrated. Up to 80-90 knots, birds have time to get out of the way.
However, the higher the speed, the greater the chance of a strike.
Consider the use of goggles and helmet or other approved eye protection
during air racing or other high-speed low-altitude operations.

In the springtime, general aviation airmen should preflight their
aircraft thoroughly as birds can build a nest almost overnight. Any
signs of grass, leaves or twigs should be investigated further,
especially in hard-to-inspect areas. A nest under the cowling can catch
fire, or one in the tail area can restrict the flying controls. While
taxiing, watch for birds on the airport, and report all unusual bird
activity to the Air Traffic Control (ATC) or Flight Service Station
(FSS) and request that airport personnel disperse them before take off.
This is especially important for turboprop and jet powered aircraft at
airports that are mainly used by smaller, general aviation aircraft (the
birds may have gotten used to slow aircraft). Since birds on the ground
face into the wind and may not hear or see the aircraft coming never use
an aircraft to scare the birds away. Gulls have a gray or black back,
which makes them more difficult to see on concrete or tarmac runways are
the most frequently struck birds around airports.

If the aircraft has windshield heating, remember that its use, in
accordance with the Pilots Operating Handbook or Flight Manual, will
make the windshield more pliable and better able to withstand bird
impact. Since most bird strikes occur during takeoff, climb, descent,
approach and landing phases of flight it is recommended that the airman
use landing lights. Although there is no conclusive evidence that birds
see and avoid aircraft lights, they will make the aircraft more visible.
If an airman sees bird(s) ahead, attempt to pass above them as birds
usually break away downward when threatened. Be careful when near the
ground, and never do anything that will lead to an aircraft stall or
spin. If structural or control system damage is suspected (or the
windshield is holed) from a bird strike, consider the need for a
controllability check before attempting a landing.

If a bird strike has broken or cracked the windshield, slow the aircraft
to reduce wind blast and follow approved procedures, such as
depressurizing a pressurized aircraft. The airman should use sunglasses
or smoke goggles if available, to reduce the effect of wind,
precipitation, or debris. It is important that the airman remember to
fly the aircraft and to try not to be distracted by the blood, feathers,
smell, and wind blast in the aircraft. Small general aviation aircraft
and helicopter windshields are not required to withstand bird impact and
the propeller gives little protection against a frontal impact to the
aircraft. However, most aircraft between 5000 and 12,600 lbs. can
withstand a 2-lb. bird strike. Birds, such as gulls and pigeons, can
penetrate the windshield of a light aircraft. If dense bird
concentrations are expected, avoid high-speed descent and approach.
Reducing the speed by one-half result in one-quarter of the impact
energy. If flocks of birds are encountered during approach, go around
for a second attempt because the approach may then be clear.

**Acquisition and loss of dark adaptation/night vision adaptation,**

There are two types of sensory receptors in the retina, rods and cones.
Rods are responsible for vision under very dim levels of illumination
(scotopic vision), and cones function at higher illumination levels
(photopic vision) and are responsible for color vision. This receptor
system allows the human eye to function over an impressively large range
of ambient light levels. There is a common misconception that the rods
are used only at night and the cones only during the day. Actually, both
rods and cones function over a wide range of light intensity levels and,
at intermediate levels of illumination, they function simultaneously.

There is a transition zone between photopic and scotopic vision where
the level of illumination ranges from about 1 to 10.3 millilamberts.
Both the rods and cones are active in this range of light, and the
perception experienced is called mesopic vision. Although neither the
rods nor the cones operate at peak efficiency in this range, mesopic
vision may be of great importance to the aviator, because low level of
light is usually present at some instance during night operations. Below
the intensity of moonlight (10.3 millilamberts), the cones cease to
function and the rods alone are responsible for vision. Scotopic vision
is characterized by poor acuity resolution and a lack of color
discrimination, but greatly enhanced sensitivity to light. The dimmest
light in which the rods can function is about 10.6 millilamberts, which
is the rod threshold. This is equivalent to an overcast night with no
moonlight. The dimmest light in which the cones can function is about
10-3 millilamberts, the cone threshold, which is roughly equivalent to a
night with 50% moonlight. Thus, a white light which can just barely be
seen by the rods must be increased in brightness approximately 1,000
times before it becomes visible to the cones.

Dark adaptation is the process by which eyes increase their sensitivity
to low levels of illumination. Rhodopsin (visual purple) is the
photochemical substance in the rods responsible for light sensitivity.
The degree of dark adaptation increases as the amount of visual purple
in the rods increases through biochemical reactions. Each person adapts
to darkness in varying degrees and at different rates. However, the
lower the starting level of illumination, the more rapidly complete dark
adaptation is achieved.

Dark adaptation for optimal night vision acuity approaches its maximum
level (an increase in sensitivity of 10,000 fold) in approximately 30 to
45 minutes under minimal lighting conditions. 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 (xenon) strobe
lights, which are commonly used as anticollision lights on aircraft,
have a smaller effect on night vision since 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. The pilot's recovery of dark adaptation could take from 5 to 45
minutes depending on the brightness and duration of such an exposure.
Prolonged exposure to bright sunlight can also have a cumulative and
adverse effect on dark adaptation, which can be intensified from flying
over reflective surfaces, such as sand, snow, or water. Exposure to
intense sunlight for two to five hours can decrease post-exposure visual
sensitivity for up to five hours. In addition, the time it takes to
become dark adapted increases and there is a decrease in night visual
acuity. These cumulative effects may persist for several days. It is
recommended that, when exposed to bright sunlight, that neutral density
or equivalent sunglasses be worn to maximize the rate of dark adaptation
and improve night vision sensitivity.

Proper nutrition is essential for good night vision. A balanced diet
that includes such foods as eggs, butter, cheese, liver, carrots, and
most green vegetables help ensure the airman has an adequate intake of
Vitamin A, which is an essential element for the buildup of rhodopsin in
the rod photoreceptors. Dark adaptation may also be adversely affected
by illness, such as a fever and/or a feeling of unpleasantness that are
normally associated with illness. High body temperatures consume oxygen
at a higher-than-normal rate, resulting in an induced hypoxia and
potentially degrade night vision. The unpleasant feeling associated with
sickness may be distracting and restrict the airman\'s ability to
concentrate on flying.

Lack of oxygen to the rod cells will significantly reduce their
sensitivity, increase the dark adaptation time, and decrease the
airman's ability to see. Use of oxygen above 4,000 feet is recommended
when flying without correction (Note: Corrected vision is not
significantly affected.). Night vision devices (NVD) also affect dark
adaptation. An airman wearing a NVD, who was previously dark adapted and
removes the device in a darkened environment, can regain complete dark
adaptation level in about 2 to 3 minutes. However, no dark adaptation
period is necessary before using the NVD, since vision with NVD\'s is
primarily photopic. The low light levels produced by the NVD\'s do not
fully bleach out rhodopsin so no serious reduction in dark adaptation
occurs.

To maintain dark adaptation, airfield lighting should be reduced to the
lowest intensity. If possible, aircraft scheduled for night flights
should be positioned on the airfield where the least amount of light
exists, and departure and arrival routes should be selected to avoid
lighted highways and residential areas. In addition, hover lanes for
helicopters should be established and marked with minimal lighting to
permit hovering without the use of the landing or search lights. If the
airman expects a flash of high intensity light from a specific
direction, the aircraft should be turned away from the source. If flares
are in use, the airman should maneuver the aircraft to the periphery of
the illuminated area to minimize exposure. If the airman experiences an
unexpected flash of high intensity light, it is recommended that one eye
be closed. When the flash of light is over, the closed eye should
provide sufficient night vision for safe flight until the exposed eye
can recover from the illumination.

**Central night blind spot,**

The area of the retina responsible for sharpest visual acuity is the
fovea, which corresponds to the center of the visual field. The fovea
area is used constantly to fixate on objects. The fovea has no rods and
is composed entirely of cones. Therefore, at luminance levels below 10
millilamberts, a blind spot of 5 to 10 degrees wide develops in the
center of the visual field. This is due to the ambient luminance level
being below cone threshold. As a result, an object viewed directly at
night may not be detected because of the night blind spot, and, if it is
detected, it may fade away. It is important to remember that the night
blind spot size increases as the distance between the eyes and the
object increases, so larger objects, such as another aircraft, may be
hidden as the distance increases. An airman who wishes to see objects at
night must use retinal areas that are outside of the fovea. Rods are
present outside of the foveal area and gradually increase in number
further from the fovea. The maximum concentration of rods is at a point
approximately 7 degrees from the fovea. Since rods have a lower
threshold than the cones, they are much more sensitive to light. Thus,
an airman attempting to see in illumination dimmer than moonlight, has
to depend entirely on rods. To best utilize the rods under such
circumstances, the individual should look 7 degrees to one side, above,
or below any object to best see it. This is known as "eccentrically
fixating." Proper education and training is, therefore, essential for
maximum use of vision at night. Airmen should be taught to fixate
slightly above, below, or to either side of an image and to employ a
proper scanning technique.

**Night myopia,**

An airman who does not normally wear corrective lenses (i.e.,
emmetropia) may have a shift toward myopia under conditions of reduced
illumination. The exact cause of this "night myopia," although
controversial, suggests two components, ocular spherical aberration
produced by the widely dilated pupils and slight involuntary
accommodation. These components apparently vary in their importance with
different people, but most people will have about 0.75 diopters of night
myopia. This can also occur in spectacle wearers corrected to
emmetropia. No visually resolvable image is visible when night myopia
occurs and this usually is of relatively minor importance since when the
image does become visible, the eye rapidly readjusts. However, in
flight, those seconds that it takes the eyes to readjust may mean the
difference between a pilot being able to perform an evasive maneuver or
having a collision with another aircraft or object.

**Cockpit illumination issues**

See: Cockpit illumination and use of red lighting,

**Night vision equipment (NVGs, FLIR, etc.)**

In the past, night vision devices were used only by a small percentage
of the military aviation community. As technology improved these
devices, their use increased in all elements of the military. However
their use in civil aviation was not permitted until January 29, 1999. At
that time the FAA issued the first Supplemental Type Certificate (STC)
to permit use of night vision goggles (NVGs) by a civilian helicopter
emergency medical service (EMS) operator. Since that date, several more
STCs have been issued to other commercial operators. In addition,
rulemaking was initiated (but, is now temporarily on hold) for changes
to Federal Aviation Regulation (FAR) Part 91, that would permit use of
this technology by general aviation pilots.

Night vision devices include a variety of different technologies, such
as forward-looking infrared radar (FLIR) and NVGs. The simplest analogy
to explain how NVGs work is a video camera. The basic principle is the
same in that the user is not directly seeing what they look at, but
rather is viewing an electronic image of a scene. NVG equipment may be
monocular or binocular. In aviation, binocular, helmet-mounted equipment
is almost exclusively used. Like a video camera, electromagnetic energy
(i.e., light) enters the optical elements of the system and is directed
to an electronic processing unit, called an image intensifier or
photocathode. However, unlike the video camera, the NVG does not require
much light to produce an image. Light as faint as a starlight or
low-level moonlight may suffice. However, the efficiency of the NVG
equipment will be degraded in total darkness. The image intensifier
will, as its name implies, intensify what little light there is on
average 1,000 to 3,000 times. More sophisticated equipment is capable of
even greater intensification, some in the order of 35,000 times or more.
That amplified or intensified electromagnetic energy is projected onto a
phosphor screen that in turn creates an image, which the user sees
through the eyepieces. The NVG image typically is a monochrome, one
color in either green or amber, depending on the phosphor used. NVG
equipment currently lacks the ability to produce a multi-color
representation of a scene.

Aviation models of NVGs are helmet-mounted with electrical power
supplied by a battery pack on the back of the helmet. As with any
optical device, the user has a variety of ways of adjusting fit to the
head and focus of the oculars. The NVG oculars and mounting assembly are
cumbersome, weighing close to one pound. In addition, one must factor in
the weight of the helmet and battery pack.

The advantages of this night vision aid technology in aviation can be
summed as an increase in nighttime situational awareness for pilots.
This technology does not turn night into day, but it does permit the
user to see objects that normally would not be seen by the unaided eye
or would permit the user to visualize obstructions sooner. Many other
benefits exist, but the bottom line is this technology, when properly
used, has the potential to significantly increase nighttime flying
safety.

Unfortunately, this increase in safety comes with a price. Some of the
disadvantages of NVGs include:

decreased field of view, both in aided and unaided vision

decreased visual acuity

loss of depth perception

lack of color discrimination

neck strain and fatigue

required maintenance and ongoing training

Current NVGs provide approximately 40 to 60 degrees of aided nighttime
field of vision, although the user retains some unaided vision by being
able to look peripherally around or under the goggles. With a reduced
field of vision, effective scanning techniques are much more important
than with unaided vision alone. Because one is looking at an electronic
image, depth perception is lost, similar to watching a television
screen. This limits the ability of the pilot to determine precise
closure on terrain or other aircraft when they are first detected.

Low-light level operations inherently produces decreased visual
resolution and contrast, thereby making hazard detection more difficult.
Visual acuity from NVG devices provides a vast improvement over unaided
human night vision, which can be Snellen 20/200 or worse. With the
goggles at starlight or quarter moon illumination, one can have
nighttime visual acuity equivalent to 20/40 or maybe even 20/30. NVG
vision is enhanced proportionally to altitude and airspeed. With NVGs,
"lower and slower" improves visual acuity. Therefore, a helicopter pilot
would have some advantage over fixed-wing counterparts in determining
terrain features. In addition, newer generation equipment provides
greater contrast detection, thereby improving situational awareness. It
is important to note that NVG-aided acuity of 20/30 or 20/40 assumes
normal visual acuity of the pilot, perfect cockpit lighting, properly
focused goggles, and ideal environmental conditions.

NVGs do not provide full color vision, only a monochrome image. Because
the eye can differentiate more shades of green than other phosphor
colors, the night vision phosphor screen typically is green. This allows
the user to see more detail, but with an inability to detect differences
in color. Objects that appear light during the day and have a dull
surface may appear darker through NVGs than objects that are dark during
the day and have a highly reflective surface. For example, a shiny dark
colored jacket will appear brighter than a light colored jacket with a
dull surface. Changing illumination can affect visual acuity. Internal
or external incompatible light of the aircraft could result in "washout"
or halo effects, when using NVGs. This could result in glare,
flashblindness, and afterimage to the pilot. Ensuring aircraft and
cockpit lights are NVG-compatible is important and troublesome.
Incompatible lights make the outside scene less visible with NVGs.
Changing cockpit lights to be NVG compatible is very complicated and
expensive. NVGs are sensitive to light ranging from yellow-green to
near-infrared. FAA required aircraft position and anti-collision lights.
Such flickering or flashing lights that are not NVG-compatible, can
cause problems for goggle wearers. NVG's are also responsive to
reflective ambient light, such as from rain, clouds, snow, mist, dust,
smoke, and fog.

Any of these will tend to severely degrade the performance of the
equipment. A few black spots throughout the image area are also inherent
characteristics of all night vision technology. These spots will remain
constant and should not increase in size or number. However, these spots
may be troublesome to pilots in critical phases of flight.

Physical factors must be considered because of the weight of the unit,
in addition to the weight of the helmet. During prolonged use of
helmet-mounted NVG devices, the potential for neck discomfort and other
problems, such as increased general fatigue exists.

In order for NVGs to be used safely, comprehensive initial and recurrent
training are critical along with regular use to maintain proficiency.
With proper training on adjustment procedures, an airman can learn to
minimize the initial, blurry image and NVGs will become an invaluable
asset during night operations. NVGs will improve vision at night,
however, the airman will not have the sharp 20/20 acuity experienced
during daylight hours. Under optimum ambient illumination conditions,
NVGs can provide 20/35 or 20/40 visual acuity. When the ambient
illumination is degraded, NVG acuity will also be decreased. Regardless
of the degree of ambient illumination, only with proper training in NVG
adjustment will the airman obtain the best NVG acuity. The military
ensures that their pilots adjust their NVGs to maximum visual acuity
prior to flight. Their procedures use NVG test sets or special indoor
visual acuity charts when adjusting the NVG controls in a specific
sequence for the correct diopter setting. This may not be feasible for
civil aviation, as the acuity charts require a controlled light source
and the test sets may be too expensive. Civilian pilots may want to use
a high contrast object or a non-intense light source to adjust their
NVGs for maximum acuity. Proper procedures need to be established in
some sort of advisory material by the FAA. Training and practice on
proper adjustment procedures should eliminate poor visual acuity
settings and allow airmen to perform these adjustments quickly. Finally,
airmen using NVGs or other night vision technology must be aware that,
while there is the potential for greater safety enhancement for select
nighttime flight operations, these devices are also expensive to obtain,
implement, and maintain. Night vision goggles do not turn night into day
and, if not properly used, rather than preventing accidents they could
be the cause of one.

**Visual scanning methods**

Dark adaptation is only the first step toward increasing an airman's
ability to see at night. There are night vision techniques that enable
an airman to overcome many of the physiological limitations of their
eyes. The fovea centralis is automatically directed toward an object by
a visual fixation reflex. However, at night, scanning techniques are
required to properly identify objects. These scanning techniques require
considerable practice and effort on the part of the airman. To scan
effectively, the pilot must look from right to left or left to right.
They should begin scanning at the greatest distance an object can be
perceived (top) and move inward toward the position of the aircraft
(bottom). Because the light-sensitive elements of the retina cannot
perceive images that are in motion, a stop-turn-stop-turn motion should
be used. For each stop, an area approximately 30 degrees wide should be
scanned. This viewing angle will include an area approximately 250
meters wide at a distance of 500 meters. The duration of each stop is
based on the degree of detail that is required, but no stop should last
longer than 2 to 3 seconds. When moving from one viewing point to the
next, airmen should overlap the previous field of view by 10 degrees.
Other scanning techniques may be used if appropriate to the situation.

Viewing an object using central vision during daylight poses no
limitation. If this same technique is used at night, however, the object
may not be seen because of the night blind spot that exists during low
illumination. To compensate for this limitation, airmen must use
off-center vision. This technique requires that an object be viewed by
looking 7 degrees above, below, or to either side of the object. In this
manner, the peripheral vision can maintain contact with an object. The
technique of off-center vision applies only to the surveillance of
targets that are minimally illuminated or luminous. Under these
conditions, cone vision is not stimulated. Central vision is best used
when an object or a target is bright enough to stimulate the cones and
needs to be seen with considerable detail. In the dark, when the object
or target begins to fade, it should be redetected using off-center
vision and retained until central vision recovers sufficiently to permit
further observation. With off-center vision, the images of an object
viewed longer than 2 to 3 seconds will disappear. This occurs because
the rods reach a photochemical equilibrium that prevents any further
response until the scene changes. This produces a potentially unsafe
operating condition. To overcome this night vision limitation, airmen
must be aware of the phenomenon and avoid viewing an object for longer
than 2 or 3 seconds. The peripheral field of vision will continue to
view the object when the eyes are shifted from one off-center point to
another.

Because visual acuity is reduced at night, the image shape or silhouette
must be used to identify an object. To use this technique, the airman
must be familiar with the architectural design of structures in the area
covered by the flight. A silhouette of a building with a high roof and
steeple can easily be recognized as a church in the United States.
However, churches in other countries may have low-pitched roofs with no
distinguishing features. Features depicted on the aeronautical map will
also aid in recognizing the silhouettes.