Vision and Visual Illusions

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Chapter 5
Vision and Visual Illusions
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• Light
• Characteristics and Measurement
• Light is a radiant energy which is capable of
  exciting the retina of the eye and producing a
  visual sensation.
• Light waves exist in a narrow band near the
  middle of the total radiant energy spectrum.

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• Visible light waves are the only electromagnetic
  waves we can see.
• We see these waves as the colors of the rainbow.
  
• Each color has a different wavelength.
• Red has the longest wavelength and violet has
  the shortest wavelength.
• When all the waves are seen together, they make
  white light.

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When white light shines through a prism or
through water vapor like this rainbow, the white
light is broken apart into the colors of the
visible light spectrum.
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How do we "see" using Visible Light? Our eyes
are receivers for these tiny visible light waves.
The Sun is a natural source for visible light
waves and our eyes see the reflection of this
sunlight off the objects around us. The color
of an object that we see is the color of light
reflected. All other colors are absorbed.
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Photometry The measurement of light Different
units used for measurement causes confusion (US
vs. SI) Old standard was the candle. A candle
equal the intensity of one candle at one
foot. Current standard is the candela. Related
to the light emitted by platinum at 1773 C.
Inverse square law. 10 lux 1 candle.
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Reflectance After light strikes a surface some
of the energy will be absorbed each different
surface will absorb a certain combination of
wavelengths. The reflectance of a surface is the
ratio of the amount of reflected light
(brightness) to the amount of light striking it
(illumination). Illumination Different lamps
of the same wattage provide different levels of
light output. It is therefore important to
carefully match lamp characteristics to workplace
requirements.
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Contrast The relationship between the
brightness or luminance of an object to its
background or surrounding, A contrast of 100
means the background brightness is twice that of
the object. Refraction When light passes from
one medium to another of a different density it
is bent or refracted. Each medium has its own
refractive index. The angle of refraction is
also a function of the wavelength of the light.
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Color Variations in wavelength of energy within
the visible spectrum give rise to the perception
of color. White light A light source which
includes all wavelengths in about equal
proportions. Color Temperature Related to the
color of light emitted by a heated piece of
iron. Colorblindness 6 of males have a
notably reduced sensitivity to color. Most
common form is red/green blindness.
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Pseudoisochromatic Plates
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Pseudoisochromatic Plates
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Pseudoisochromatic Plates
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Pseudoisochromatic Plates
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Pseudoisochromatic Plates
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Pseudoisochromatic Plates
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Pseudoisochromatic Plates
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Anatomy of the Eye
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Retina The interior lining on the rear surface
of the eye. Consists of a complex layer of light
sensitive nerve cells. The optic nerve connects
the retina to the brain. At the point where the
optic nerve connects to each retina there is a
blind spot, devoid of any light receptors.
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Rods and Cones The light sensitive cells of the
retina consist of two kinds of receptors called
rods and cones, each having different
characteristics. Rods Sensitive to dim light.
No color sensitivity. Scotopic Cones Good
light and color sensitivity. Photopic
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Rods and Cones (Contd)
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Fovea The center of the retina. Area of dense
cone concentration. Periphery Area
surrounding the fovea. Cone density decreases
rapidly outside the fovea. Periphery consists
primarily of rods.
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Visual System at Work - Adaptation
Accommodation Adaptation Process by which the
eye adjusts to changes in amount of light present
in the environment. Coarse adjustment Pupil.
Varies the amount of light entering the eye by
approximately 30 times. Rods Cones Sensing
passes between these two receptors depending on
the amount of light present. Visual purple
(rhodopsin) Chemical compound which increases
the sensitivity of the rods. Bleached by bright
light. Requires time to be reconstituted. (fig.
5.4)
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Red light was used until about 1950 to speed
adaptation (30-40 min.) White light now the
standard Has fatigue/drowsiness benefits.
Increased age increased visual acuity with
increased brightness.
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Accommodation Focusing on an object. In the
human eye, accommodation is accomplished by the
ciliary muscles which change the shape of the
lens. Binocular vergence Changing the
convergence of the eyes. Six extrinsic muscles
control eye movement under the brains control for
depth perception (stereopsis).
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Visual Acuity Factors affecting visual
acuity Brightness Contrast Time to view Glare
Source of discomfort, a makes people tense and
restless, visual performance is
reduced. Sensitivity to glare increases with age
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Accommodation Convergence To see clearly at
different distances, two basic adjustments of the
eye are needed. These adjustments change the
refractive power of the lens and are called
accommodation and convergence. Accomplished by
the ciliary and the six extrinsic muscles
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Dark Focus When visual cues are weak (empty
field) the muscles controlling these adjustments
take up an intermediate position or resting
state. This resting state is called the empty
field or dark focus. The empty field or dark
focus distance is just less than one meter
although it varies widely among individuals.
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Mandelbaum Effect This is highly significant in
searching for distant targets when visual cues
are weak, as the eye will not be adjusted to
detect them. The condition is aggravated by
objects close to the eye which tend to draw
focusing closer to the observer. This is known
as the Mandelbaum effect and is more severe the
closer the other objects are to the dark focus
distance. Windshield/rain Visual acuity is
optimal at the dark focus
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Individual Differences Conventional eye tests are
generally adequate for predicting visual
performance under normal conditions. Yet, two
individuals who have 20/20 ratings may differ
widely in their visual performance when it comes
to actual visual tasks. (Traffic scanning)
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Visual Acuity and Aging Pilots Visual acuity is
a real problem for aging pilots. Visual acuity
at age 65 is half that of age 40 Difficulty
accommodating - Near, Intermediate, Far Use of
Trifocals
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The Brain Perception Visual perception involves
the eyes, the balancing mechanism of the ear
(vestibular apparatus) and the brain.
Perception is forward looking and predictive.
When prediction is not possible, response
suffers a delay.
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Perception is greatly influenced by what we have
come to believe about the properties of the
object we are viewing and what we expect the
properties to be by the time we have to respond
to them. Uncertainty and ambiguity only occur
when sensed information is processed by the
brain. The visual input is then combined with
other information based on learning, experience,
and expectation. Emotional factors also play a
role in perception.
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Perception Exercises

• What Do You See?

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Barriers to Perception There are many
physiological and psychological barriers to
perception. Several which are germane to aviation
are Blind Spots Fascination Depth
Perception Hypoxia and Smoking
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Blind Spots As mentioned earlier, a blind spot
exists in each eye. The missing information from
the blind spot of one eye is filled in by the
other eye. This has relevance to visual
acquisition of traffic during flight.
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• Fascination (set) Failure to respond to
  stimulation within the visual field.
• Set usually is the result of one of two
  conditions
• Extreme concentration on a visual task
• (tunnel vision)
• Sensory disconnect Deep thought temporarily
  overrides visual information
• Both of these conditions may be aggravated by
  fatigue or anxiety

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• Depth and Distance Assessment A number of cues
  are involved in accurate depth perception
• Binocular vision
• Perspective
• Apparent Motion
• Superposition
• Relative size
• Texture gradient
• Height of an object in a plane

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Hypoxia and Smoking Vision is the highest
order sense. The visual system requires a highly
oxygenated environment for proper function.
Smoking increases carbon monoxide levels in the
bloodstream which impairs oxygen transport to the
body's cells.
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• Smoking has the following negative effects on
  vision
• Reduction in night vision
• Reduction in visual acuity
• Increased brightness threshold
• Increased reaction time to visual stimuli

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Visual Illusions Brightness Contrast
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Illusions from Stationary Stimulus Autokinesis -
An isolated stationary light in an otherwise dark
visual field may appear to wander. This
phenomenon is is typically encountered in flight.
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Illusions from Moving Stimulus Stereokinesis and
Kinetic Depth Perception Common with rotating
objects. (Propellers)
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Illusions of Movement - Vertigo Somatogravic
illusions False perception of attitude relative
to the gravitational vertical. Somatogyral
False sense of turning Oculogryral False
sense of turning Oculogravic False Visual
component of the altered perception following a
change in force vector. Cross-coupled
stimulation Sensory illusion generated within
the semi-circular canals of the ear.
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Induced movement False sensation of movement
caused by the movement of an adjacent
object. Jetway Adjacent aircraft
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Vision and Visual Illusions in Air
Transport Flightdeck Geometry
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Design Eye Point - (cockpit keyhole, or reference
eye position) The Design eye point ensures
adequate visibility outside the cockpit and of
the important displays on the flightdeck.
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Effect of Sitting Too Low - A pilot sitting too
low will lose some visibility of displays located
behind the control column and wheel (if so
equipped) and also some of the visual segment in
low visibility approaches.
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The design eye point is located to allow the
pilot to see a length of approach or touch-down
zone lights which would be covered in three
seconds at final approach speed. This represents
a distance of approximately 600 750 feet along
the flight path. For each inch the eye is
located below the design eye point, approximately
120 feet of ground visibility will be lost (along
with precious seconds of reaction time).
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Eye Point Locator
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Flightdeck Design
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Glareshield Upper surface should provide a
lateral horizontal reference and should coincide
with the forward downward vision angle.
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Windshield Glass - Minimize optical
distortion. Heating elements Reduce light
transmission Delamination Bubbling obstructs
visibility Posts Minimize to the extent
practical Shape Minimize discontinuities Promote
attitude awareness.
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General Flight Illusions
Non-precision Approaches The transition from
instrument to visual control is the most critical
point a low visibility approach. It is at this
point that pilots are most susceptible to visual
illusions.
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Autokinetic effect Stationary light in a dark
field appears to move. Stroboscopic effect
Some people react badly to lights flashing at
certain frequencies. While this is not an
illusion in the common sense, it does reflect an
interaction between the eyes and parts of the
brain.
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Rain and the Windshield/Wipers Distraction -
Pulls eye to the windshield (dark
focus) Windshield wipers Similar to
flicker Operate at slowest possible
speed Avoid fixation Distortion Adverse
affects on light transmission Interpretation of
visual cues will be more difficult Available
reaction time will be reduced
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Judgement of Distance in Low Visibility Mist
and fog change the aerial perspective by dimming
the image and reducing color contrast and
texture. This reduces perceptual distance cues
and causes pilots to tend to overestimate
distances in poor visibility.
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Illusions During Taxiing Movement of adjacent
aircraft/jetways Blowing snow Pilot eye height
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Illusions During Takeoff Pitch up illusion
High rates acceleration can cause a pitch up
illusion (Less common in civil air transport
aircraft) Blowing snow Can cause directional
control difficulty during takeoff False horizon
Nose up attitudes reduce or eliminate the
natural horizon and create opportunities for
perception of false horizons.
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Objects which may create false horizons
include Surface lights Stars Sloping
clouds Boats on water (at night) Sloping terrain
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Illusions in Cruise Autokinesis Overreaction
to lights at night Relative altitude of
traffic/terrain - TCAS Blind spot and Dark Focus
Reduction in effective traffic acquisition
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Illusions in Approach and Landing Runway slope
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Sloping Terrain -
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Runway width Visual illusion of distance and
height. Black hole phenomenon Night approaches
over dark landscape. Creates illusion of
height Windshield location - Windshield location
of an object may cause height misjudgment in
different confirgurations. Relevant to visual
flying at night around airports where the
surrounding terrain has no lights. Undulations
in runways Appearance that insufficient runways
remains Glassy surfaces Difficult to judge
height. Important in seaplane operations.
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Protective Measures Recognition Understanding
situations in which visual illusions are likely
to be encountered Supplement visual cues with
information from other sources. Known locations
associated with visual illusions should have
notes on approach plates, etc. Design
Adequate visibility, angles, optical quality
glass in flightdeck design