human factors
Tomm.
HFchapter4.ppt
Stimulus-Response
- Stimulus: Something that causes a change or a reaction
- Receptor: Any of various devices that receive information, signals, etc.
- Response: Something that is done as a reaction to something else
- Stimulus – Response Examples:
- Bang a drum – vibration in each creates a sound
- Introduce electricity – feeling of shock
- Most prominent stimulus = Light
- We are able to perceive things visually because of light
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Properties of Light
Wavelength
Amplitude
Light is an electromagnetic wave
Amplitude perceived as brightness
Wavelength (nm) perceived as hue
Anatomy of the Eye
Accommodation
Cornea – outer covering of the eye where light is first focused
Lens – does the fine tuning
Retina – area in back of eye containing photoreceptors
Accommodation – changing the shape of the lens to focus images on the retina caused by contracting/relaxing ciliary muscles.
- lens flattens (muscles relax) when focusing on distant objects and becomes rounder for focusing on close objects (muscles contract causing fatigue).
Myopia – nearsightedness caused by inability to flatten the lens enough to focus image on retina (may be due to elongated eye)
Presbyopia – farsightedness that occurs naturally with age as the lens becomes less flexible.
The Lens: Accommodation
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Problems with the Lens
- Presbyopia: Farsightedness
Myopia: Nearsightedness
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Photoreceptors
Rods – sensitive to dim light (night vision)
- found mostly in periphery
- why can see a dim star better if look a few degrees to one side
Peripheral vision
Cones – sensitive to color (daylight vision)
- concentrated in fovea
- provides fine detail
For Focal vision
light
Cones vs. Rods
| Location | Acuity | Sensitivity | Color Sensitivity | Adaptation | Wavelength Sensitivity | |
| Cones | Fovea (middle 2°) Periphery (lower concentration) | Fine detail best resolved by cones | Low sensitivity – difficulty picking up dim light | Can discriminate color | Little affected by light stimulation | Sensitive to all wavelengths |
| Rods | Periphery | High sensitivity to motion | Very sensitive to low light | Color blind | Slow adaptation (hard to see when go from light to dark and vice versa) | Insensitive to long wavelengths (red) |
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Brightness
It’s Complicated.
- Luminous Flux = amount of light/energy leaving the source
- Luminous Intensity = amount of light/energy in a given direction
- Illuminance = amount of light that strikes a surface
- Function of the distance from the source (declines with the square of the distance)
- Luminance = amount of light leaving/reflected from the surface
- Brightness is visual intensity experienced
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Color Vision
Types of color deficiencies and color blindness
Protanomaly (1% of males) – low sensitivity to red
Deuteranomaly (6% of males) – low sensitivity to green
Protanopia (<1% of males) – see in shades of blue and yellow
Deuteranopia (<1% of males) – see in shades of blue and yellow
Tritanomaly/Tritanopia (very rare in both sexes) – blue-yellow deficiency/blindness
Monochromacy (extremely rare) – inability to distinguish any colors
Purkinje Shift
- Purkinje shift – as we switch from cone to rod vision (day to night), shorter wavelengths appear brighter (green objects appear brighter, red objects appear darker).
- (Human Factors application: How does the Purkinje shift play a role in the design of airport and aircraft lights?)
Dark Adaptation
Human Factors application: why are red lights used in cockpits and darkrooms?
- because rods are insensitive to longer wavelengths (red) the eyes “think” they are in the dark already allowing user to dark adapt more quickly)
- Takes about 30 minutes to dark adapt, but just a few minutes to light adapt
Night Vision
- Poorly illuminated viewing conditions
- Can reduce contrast sensitivity
- Can inhibit perception of print or detection and recognition of objects
- Glare
- Irrelevant light of high intensity
- Temporarily destroys rods sensitivity to low spatial frequencies
- Impedes ability to spot dimly illuminated road hazards
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Visual Acuity
- The amount of fine detail that an be resolved
- Types
- Minimum separable acuity
- General measurement of smallest detail detectable
- Vernier Acuity
- Are two lines parallel?
- Landolt Acuity
- Is the gap in a ring detectable
- Snellen Acuity (Eye Chart at the Dr.)
- Detail resolved at 20 ft relative to distance a normal observer can view it (e.g., 20/20)
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Visual Acuity
20/20 Vision – a person can see from 20 feet what a person with “normal” vision can see from 20 feet
20/40 Vision – a person can see from 20 feet what a person with “normal” vision can see from 40 feet
Snellen Eye Chart
Contrast Sensitivity
- The ability to distinguish bright and dim components of a static image
- Necessary to detect shapes from backgrounds
- Contrast is a function of the difference between the Luminance of the light surface (L) and the dark surface (D)
c = (L – D) / (L + D)
- The higher the contrast sensitivity (CS) an individual posses, the smaller the contrast that can be detected (CM)
CS = 1 / CM
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Contrast Sensitivity Cont.
- Grating – alternating patterns of dark and light
- Are they discriminable or does it look like smooth bar?
- Influencing Factors:
- Spatial Frequency (cycles/degree) – how close they are together
- # of dark/light pairs that occupy 1 degree of visual angle
- Inversely related to width of bars
- Humans most sensitive to 3 cycle/degree
- Contrast
- Lower contrast less easily discerned
- Best contrast: black letters on white background (negative contrast) - best; white on black(positive contrast)
- Illumination
- Lower illumination reduces sensitivity
- More severe for high spatial frequencies (rely on cones)
- Resolution of eye – sensitivity reduces with age
- Dynamic characteristics of viewing conditions
- Harder to discriminate when stimulus moving
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Spatial Frequency
Spatial Frequency
Low High
100 %
0.5%
Contrast
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In this sinusoidal spatial frequency grating, tThe luminance of peaks and troughs remains constant along a given horizontal path through the image. If the detection of contrast was dictated solely by image contrast, the alternating bright and dark bars should appear to have equal height everywhere in the image. However, the bars seem to be taller in the middle of the image.
Campbell, F. W. and Robson, J. G. (1968) Application of Fourier Analysis to the Visibility of Gratings. Journal of Physiology (London) Image Courtesy of Izumi Ohzawa, Ph.D. University of California School of Optometry
Reading Text
Contrast, Spatial Frequency (Font Size) and Font Style must be considered in the display of text for optimum legibility
Human Factors Application: While the background provides good contrast and the font size is highly legible, the multiple font styles are distracting.
Reading Print
- S p a t i a l F r e q u e n c y
- Can make print hard to read if too high or too low.
- Best is at 3 cycles/degree
- Maximize Contrast
- Black letters on white background
- Use typical fonts
- UPPER CASE MORE READABLE
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Monocular Depth Cues
Linear Perspective
- converging parallel lines
Relative Size
- if 2 objects are known to be similar size, the smaller one appears further away
Ames room illusion fools us into thinking that the distance is the same to both people in the photo, therefore their size must be different.
Monocular Depth Cues
Interposition
- Nearer objects obscure the view of further objects
Light & Shading
- 3-D objects cast shadows and shade on opposite side of illumination source, and reflections on same side
Monocular Depth Cues
Textual Gradients
- Texture appears more blurry with increasing distance
Aerial Perspective
- Objects in the distance appear hazy or bluish
Monocular Depth Cues
Motion Parallax
- As perceiver moves, objects in foreground appear to move by faster than objects in background
Binocular Cues
Convergence
- Eyes rotate inward as object gets closer
Retinal Disparity
- Slightly different image to each eye, brain combines them
Depth Perception
- Allows us to navigate the 3D world and gauge the distance between objects
- 3 Cues which help us perceive depth and distance
- Accommodation
- Accommodative response signals brain how much accommodation was accomplished and hence how far away object is
- Convergence
- Cue based on the amount of inward rotation eye must accomplish to bring image to rest on corresponding parts of two eyes
- Binocular Disparity (aka Stereopsis)
- Eyes have two different (disparate) views of an object
- The closer to the eye, the greater the disparity
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Retinal Disparity (3D T-Rex)
Depth Perception
Identify the monocular cues to depth perception in this photo.
Problems with Depth Perception
- Degraded cues can distort depth perception, due to, for instance:
- Darkness
- Poor weather
- Distortions
- Smaller cars perceived as farther away – lead to more accidents
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What is Visual Search?
- The task of looking for something in a cluttered visual environment; finding a target among distracters
- Goal: To detect an object or event; but detection is a different phenomenon
- Extremely prevalent in our lives
- Examples:
- Looking for an open parking spot on campus
- Looking for a contact on your smartphone
- Looking for your friend in a crowd at the ball game
- Looking for a relevant link in a Google search results list
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Eye Movements
- Necessary for visual search
- Two Types:
- Pursuit eye movement
- Constant velocity; Following moving target
- Example: watching a bird fly overhead
- https://www.youtube.com/watch?v=dD-bbjx79Y0
- Saccadic eye movements
- Abrupt, discrete movements from one location to the next.
- Example: At a stop sign looking from one side of the street to the other
- https://www.youtube.com/watch?v=_bfvOK37bk8&list=PL6hslGPUQLHbtyftrfIXQOdcDDtccGydX&index=22
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Saccadic Eye Movements
- Main Features:
- Saccades – eye movement to/from object
- Fixations – gaze upon an object
- e.g., When gaze doesn’t leave a 2 degrees of visual angle with 200ms
- Other Characteristics/terminology
- Initiation latency – time from presentation of target until initiation of eye movement
- Movement time (or Speed)
- Destination (aka, Dwell, Area of Interest (AOI))
- Dwell Duration (aka fixation duration)
- Useful Field of View (UFOV)
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Eye tracking Technology
- Most common method: Corneal reflection
- Components
- IR Light
- Camera
- Different Types
- Remote
- Head Mounted
- Measures
- Fixation locations, durations, intersections with AOIs
- Pupil diameter
- Blink Frequency
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Visual Search
How many O’s are there?
Get ready…
Q Q Q Q Q Q Q Q
Q Q O Q Q Q Q Q
Q Q Q Q Q Q O Q
Q Q Q Q Q Q Q Q
Q Q Q O Q Q Q Q
Q Q Q Q O Q Q Q
Visual Search – Serial Search
Q Q Q Q Q Q Q Q
Q Q O Q Q Q Q Q
Q Q Q Q Q Q O Q
Q Q Q Q Q Q Q Q
Q Q Q O Q Q Q Q
Q Q Q Q O Q Q Q
Search time: T = (N x I) / 2
I = inspection time per object
N = total number of objects
Visual Search
How many X’s are there?
Get ready…
O O O O O O O O
O O O O O O X O
O X O O O O O O
O O O O X O O O
O O O O O O O X
O X O O O O O O
Visual Search
How many X’s are there?
Get ready…
O O O O O O O O
O O X O O O O O
O O O O O O O O
O O O O O O X O
O O X O O O O O
O O O O O O O O
Visual Search – Parallel Search
O O O O O O O O
O O X O O O O O
O O O O O O O O
O O O O O O X O
O O X O O O O O
O O O O O O O O
Pop-out Effect
- conspicuities
Attributes which Affect Type of Search
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Detection
- Detection: to confirm it is the item of interest
- May or may not be preceded by a search
- Different types of signals
- Visual
- Auditory
- Haptic
- Proprioceptive
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Signal Detection Theory (SDT)
- Framework for evaluating likelihood of correctly identifying a signal in a noisy environment
- Examples: baggage screening, auditory alarm in airplane
- 4 possible results
- Hit (DETECTED signal when it was PRESENT)
- Miss (Did NOT DETECT signal when it was PRESENT)
- False Alarm (DETECTED signal when it was NOT PRESENT)
- Correct Rejection (Did NOT DETECT signal when it was NOT PRESENT)
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Signal Detection Theory
d’ (sensitivity)
Signal Detection Theory
Receiver Operating Characteristic Curve
1.0
0
0
1.0
P(FA)
P(H)
Zero Sensitivity (d’ = 0)
Moderate Sensitivity (d’ = 2)
High Sensitivity (d’ = 4)
