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.

Visible Light

• 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.

White Light

When all the waves are present together in approximately the same proportions, they make white light.

When white light shines through a prism or through water vapor like this rainbow, the white light is broken apart (refracted) into the colors of the visible light spectrum.

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.

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.

Terminology

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.

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.

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.

The Human Eye

Anatomy of the Eye

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.

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

As depicted in the graph above, rods have a much lower sensitivity threshold to light than do cones. 

Rod and cone sensitivity also varies with wavelength.  Cones tend to have better visibility in the red and orange wavelengths while rods are more sensitive to the blue violet wavelengths.  This explains why the reddish orange colors are the first to disappear as light levels decrease.

Foveal and Peripheral Vision

Fovea – An area of dense cone concentration located at the focal point on the retina. The fovea captures the highest quality visual image produced by the lens. When we read, concentrate on an image, drive a car, or fly an airplane we are predominantly relying on foveal vision to acquire information from our surroundings.

Periphery – Area surrounding the fovea. Cone density decreases rapidly outside the fovea. The periphery consists primarily of rods.  The periphery does not provide detailed vision.  Peripheral vision supplements foveal vision and provides important sensory input related to movement. Peripheral vision is a key element in determining orientation, speed, height, and warning of approaching objects.

Visual System at Work - Adaptation & Accommodation

Adaptation – The 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. Rhodopsin is bleached out of the rods by bright light. When light levels decrease, rhodopsin begins to reconstitute in the rods increasing their sensitivity. 

The time required for rhodopsin to reconstitute varies in individuals and increases with age.

Red light was used until about 1950 to speed adaptation (30-40 min.)

White light is now the standard – The use of white light reduces fatigue and drowsiness. White light is also better tolerated by older pilots whose visual acuity is highly dependent on brightness levels.

Accommodation – The ability to focus the eyes at varying distances is called accommodation. 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 (cross eyed). Six extrinsic muscles control eye movement under the brains control for depth perception (stereopsis).

Visual Acuity

Factors affecting visual acuity:

• Brightness
• Contrast
• Time to view

Glare – Glare is unwanted reflected light. Glare is a source of discomfort and makes people tense and restless.  Glare can significantly reduce visual performance. 

The angular difference between the source of glare and the object being viewed will determine the reduction in visual performance.  The greater the angle, the less glare will be a factor.

Sensitivity to glare increases with age

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

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.

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. Dirt, dead insects, or rain on a windshield can create this effect.  It is therefore very important to keep windshields as clean as possible.

Visual acuity is optimal at the dark focus

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)

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

The Brain and Perception

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.

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.

Perception Exercises

Click HERE to download a PowerPoint presentation on perception

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

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. Since windshield posts may prevent one eye from acquiring a target in the other eye's blind spot, it is important that pilots understand this phenomenon and compensate with effective visual scanning techniques.

This has relevance to visual acquisition of traffic during flight.

You may use the image above to locate your blind spot.  Close your left eye. Concentrate your right eye on the crosshairs on the left side of the image.  Move your head towards/away from the image until the aircraft on the right disappears in your blind spot.

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

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

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.

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

Visual Illusions

Brightness Contrast Illusions - 

Bright background or surroundings can make the center area seem lighter. You can experience this phenomenon by viewing the image below.

Stare at the small black square in the in the image below for 30 - 60 seconds.  Then shift your eyes to the small white square.  You should see dim white squares appear over the white crosses.  Additionally the white squares should appear to move about slightly.

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.

Rotosnake

The image above is not moving. If you hold your eyes very still the apparent motion will stop.

Click on the image to download a full-sized version of the Rotosnake

Illusions from Moving Stimulus

Stereokinesis and Kinetic Depth Perception – Common with rotating objects such as propellers.  After viewing the rotating object to a few seconds the object will appear to rotate in the opposite direction.

Illusions of Movement - Vertigo

The following are all examples of the spatial disorientation phenomenon commonly called 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.

Induced movement – False sensation of movement caused by the movement of an adjacent object.

Induced movement may be caused by the relative motion of:

• Jetways
• Adjacent aircraft

Flightdeck Geometry

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.

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).

Eye Point Locator

Most transport category aircraft are equipped with eye point locators to assist pilots in adjusting their seat to obtain the design eye point.  

The most common type of eye point locator is the triple ball (see photo below).  

The triple ball eye point locator is found on the center window post.  Pilots adjust their seats so that the white ball in the center of the locator completely obscures the orange ball behind it. When this has been accomplished their eye is located very near the design eye point.

Slightly aft of design eye point	Positioned correctly	Slightly forward of design eye point

Vision Related Aspects of Flightdeck Design

The following are examples of elements which should be incorporated into flight deck design to enhance vision and reduce the possibility of visual illusions.

General - The flightdeck should contain non-reflective surfaces to reduce glare and should utilize colors which reduce eye strain where appropriate.

Glareshield – The glareshield surrounds the upper portion of the forward instrument panel.

• Upper surface should provide a lateral horizontal reference.
• Forward angle should coincide with the forward downward vision angle.
• Should enhance general illumination of the forward instrument panel.

Windshield – The forward windshields should provide a maximum of visibility within design limitations.

• Glass should minimize optical distortion.
• Heating elements – Must minimize reduction in light transmission
• Posts – Minimize to the extent practical
• Shape -

  • Promote attitude awareness

  • Minimize discontinuities between window panels

Vision and Visual Illusions in Air Transport

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.  Non-precision approaches frequently require the aircraft to circle, or maneuver laterally, for runway alignment and to descend at unusually high rates.  These maneuvers, combined with darkness and/or reduced visibility, create ideal conditions for visual illusions to occur.

Autokinetic effect – A 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.

Rain and the Windshield/Wipers - Rain on a windshield can be visually distracting since the windshield is located near the point of dark focus. 

Windshield wipers create an additional distraction due to their motion at this critical focal length and also due to the noise they generate. Windshield wipers also create stroboscopic effects.

Considerations when rain is present on the windshield:

• Distortion – Adverse affects on light transmission
• Interpretation of visual cues will be more difficult
• Available reaction time will be reduced

General rules for windshield wiper operation:

• Never operate on a dry windshield
• Always operate at slowest possible speed
• Avoid fixation

Rain repellent - Chemical rain repellent is available in some aircraft. Chemical repellent coats the windshield and facilitates rain removal by slipstream, bleed blast, and wipers. Use of rain repellent on a dry windshield or in light rain will cause streaking and blurring of the windshield and can significantly reduce visibility. Use of this agent is being phased out by most operators.  Chemical rain repellent should only be used as a last resort in heavy rain.

Judgment 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.

Illusions During Taxiing - The following are example of common visual illusions encountered during taxi operations.

• Movement of adjacent aircraft/jetways - Create appearance that stationary aircraft is moving.
• Blowing snow - Create illusion of lateral movement across ramp, taxiway, or runway
• Pilot eye height - As pilot eye height increases there is a reduced sensation of speed

Illusions During Takeoff

Pitch up illusion – High rates of 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.

Objects which may create false horizons include:

• Surface lights
• Stars
• Sloping clouds
• Boats on water (at night)
• Sloping terrain

Illusions in Cruise

Autokinesis –  It is difficult to judge distances at night.  This combined with the effects of autokinesis may cause pilots to misidentify distant objects as traffic threats.  There are numerous cases where pilots have taken aggressive evasive action to avoid perceived traffic threats only to discover that the threat was a bright star or planet.

Relative altitude of traffic/terrain - It is difficult to judge the altitude of distant objects in relation to one's own altitude.  This illusion becomes even greater the nearer the altitudes are to one another. TCAS has helped alleviate this problem by providing pilots with altitude information on converging traffic.

Blind spots and Dark Focus – Blind spots and the dark focus (Mandelbaum) effect cause a reduction in effective traffic acquisition.  It is important that pilots understand these phenomenon and adjust their scanning techniques accordingly.

Illusions in Approach and Landing

Runway gradient and/or sloping terrain can create the illusion that the aircraft is either too high or too low. Identifying airports with these runway/terrain features is an important aspect of pre-flight planning. 

Runway slope –

Sloping Terrain

Runway width – Unusually wide or narrow runways may create illusions of distance and/or height.

Black hole phenomenon – Night approaches over dark landscapes, or water, creates the illusion of excessive height. It is important to supplement visual cues with other sources of glide path information (ILS/VASI/PAPI) during this type of approach.

Windshield location - The windshield location of an object may cause height misjudgment in different aircraft configurations. This type of illusion is particularly relevant to visual flying at night around airports where the surrounding terrain has no lights.

Undulations in runways – Humps an dips is runways may create the appearance that insufficient runways remains and may lead to excessive braking on landing.

Glassy surfaces – It is exceptionally difficult to judge height above glassy water. this phenomenon dictates special procedures and the use of great care in seaplane operations.

Protective Measures

The following protective measures will reduce the likelihood of reacting inappropriately upon encountering a visual illusion.  Remember - Visual illusions cannot be eliminated.

• Recognition - Visual illusions are a natural phenomenon and occur as a result of the human perception process.
• Understanding - Develop an understanding of situations in which visual illusions are likely to be encountered
• Supplement - Supplement visual cues with information from other sources.
• Known locations - Airports associated with visual illusions should be approached with caution and planning.
• Design – Design flightdecks with careful consideration for adequate visibility, glare and  blind spot reduction, etc.