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Enhancing Visual Perception and Text Legibility

Explore human factors engineering principles, visual acuity, contrast sensitivity, and typography for optimized text readability and perception. Learn about factors affecting acuity, contrast sensitivity, color discrimination, reading patterns, and perception. Discover the importance of stroke width, font samples, and text reading guidelines.

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Enhancing Visual Perception and Text Legibility

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  1. Human Factors Engineering ECE 796/896 Text, Graphics, Symbols and Codes

  2. Process of Seeing

  3. Visual Capabilities • Accommodation: Ability of eye to focus light on the retina. • Visual Acuity:Ability to discriminate fine detail and it depends on the accommodation of the eye. • Contrast Sensitivity:

  4. Accommodation • Near Point, Far Point: • Normal vision: Far point = infinity, near point = 14 inches. • Dark Focus: Arm’s length = 1m Generally measured in diopters (D). 1D = 1m, 2D = 0.5m, 3D = 0.33m

  5. Accommodation • Nearsightedness / farsightedness • With age (40+) --> farsightedness

  6. Visual Acuity • The ability to discriminate fine detail. • Common measure = minimum separable acuity • Measured as the reciprocal of the VA subtended at the eye by the smallest detail that can be distinguished.

  7. Visual acuity • VA (minutes) = (3438 * H)/ D (H=ht.,D=dist.) • Normal acuity = 1.0 the higher the number the better the vision. Snellen acuity: 20 ft. or 6 m from the chart 20/20 normal … 20/30 -> a normal person could read at 30 what the subject barely reads at 20.

  8. Acuity Testing

  9. Types of Acuity • Vernier Acuity: differential one line offset from another. • Min. Perceptible acuity: Detect a spot from its background. • Steroscopic Acuity: differentiate the two images received by the two eyes. Differ most close, least further away.

  10. Contrast Sensitivity • Spatial frequency: The number bars per unit distance. • 1 cycle = 1 dark + 1 light bar • The waveform of a grating’s luminance. • Square wave and sine wave versions.

  11. Acuity Grating

  12. Contrast Sensitivity • Modulation contrast or Michelson Contrast: C= (Lmax - Lmin)/(Lmax + Lmin) values between 0 and 1

  13. Contrast Sensitivity • Threshold Contrast: The point at which you see the bars from the background. • Contrast Sensitivity = 1/ threshold • Most sensitive => 2 and 4 cycles per degree

  14. Contrast Sensitivity • More detailed the target = high spatial frequency • Visual acuity tests use high contrast objects. Tells little about how a person sees in a low contrast environment

  15. Factors Effecting Acuity and Contrast Sensitivity • Luminance Level: • Contrast: • Exposure Time: • Target Motion: • Age: • Training:

  16. Adaptation • Changes to light sensitivity • Two phases, cones and rods

  17. Color Discrimination • Normal color vision: Trichromats - 100’s of colors • Complete lack of color vision: monochromats • Color Deficiency: dichromats or anomalous trichromats

  18. Reading • Complex eye movements: • Combination of (saccades)”fast movements”, and stationary periods (fixations) • Also regressions

  19. Perception • Visual Displays: 1. Must be seen clearly 2. Help to viewer to correctly perceive the meaning 3. Horizontal-vertical illusion

  20. Text: Hardcopy • Visibility: separation from surroundings • Legibility: (discriminability) • Readability: character grouping/ formating

  21. Typography • Most type faces are very readable • 4 types of circumstances for “preferred” • Viewing conditions are unfavorable • Information is important or critical • Viewing is at a distance • People with poor vision use displays

  22. Stroke Width • Ratio of thickness to height • Irradiation: white on black • Rule: black on white should be thicker

  23. Font Samples

  24. Stroke Width • With good illumination: B-W (1:6,1:8) W-B (1:8 to 1:10) • With reduced illumination: thick letters become more readable than thin ones. • Low levels and low contrast -> boldface type w/ low stroke w-to-h (1:5)

  25. Stroke Width • Highly luminous letters (1:12-1:20) • For black letters on luminous background need to very thick strokes

  26. Width to Height Ratio

  27. Width-Height Ratio • Standard: 3:5 • Styles of Type: 30,000 • Roman: Most common, serifs • Gothic: uniform stroke width, sans serif • Script: simulate hand writing • Block: simple, German manuscript form

  28. Different W-H Ratios

  29. Size • Measured in points: 1 pt = 1/72 inch or 0.35 mm, this does not consider, ascenders and descenders. • Better measure 1 pt = 1/100 inch or 0.25 mm

  30. Mil spec Text

  31. Reading • 9-11 pt for newspapers / magazines • 22 to 27 min - viewing angle • If reading is critical or in low light than text height will need to be increased • Check table 4-2 for ranges

  32. Distance Reading • National Bureau of Standards • Ws = 1.45 * 10(-5) * S * D • Hl = Ws / R • Ws = stroke width, S = denominator (snellen), D = reading dist.,Hl letter height, R stroke W-H (decimal)

  33. Other Factors • Case: lower case in easier to read than upper case. A capital starting letter can make reading faster and more acurate. • Layout: interletter spacing, regular and dense spacing, dense is superior • interline spacing : spatial frequency may provoke eye strain, etc.

  34. Reading Ease • Type of Sentence • Order of Words • Indices of Readability

  35. Type of Sentence • The green button starts the motor. • The motor is started by the green button. • The red button does not start the motor. • Simple, affirmative,active are the easiest to understand.

  36. Order of Words • “push the green button after turning off the gas” • Better: “Turn of the gas before pushing the green button”

  37. Indices of Readability • Flesch Reading Ease Score • Based on an 100 word passage , the number of syllables(S) and average number of words per sentence (W). • Score = (0.846*S) - (1.015*W) • The book = 51 -> fairly difficult

  38. Text : VDT Screens • Not the same as hard copy - image quality • Text represented by pixels • Better the quality = closed to hard copy

  39. Typography • Dot matrix ( 5X7, 7X9, …15X24) • Pixel size and aspect ratio is a factor • Note figure 4-10 • Reading distance - greater than with hardcopy (24-36 --- 18-20) • Size: arc of 11-12 mins --> .06-.07 ANSI - .09 (.116,.128) max( .14)

  40. Hardware Considerations • Refresh rate,jitter,flicker,phospher persistence • Polarity: dark on light or light on dark • Color: use as few colors as possible, avoid using the extremes, avoid color pairs of saturated red/blue, maximize the contrast between text and background

  41. Screen Design Issues • Density: ( 24X80 available) percentage of spaces used. Most screens average about 25% with more than 40/50 rare. • Local Density: higher density in groupings

  42. Grouping • Data items viewed to form well define groups 1 If groups subtend less then 5 degrees of VA (12/14 W X 6/7 lines H) - search time was a function of the number of groups. 2 If the VA is > 5 degrees, search time was a function of the size. • Recommendation: min. number of groups, make them as close to 5 degrees as possible.

  43. Complexity • Arrangement (layout) complexity - Items placed on the screen follow a pattern. - Reduce complexity by lining up data into columns and/or rows.

  44. Complexity

  45. highlighting • Highlighting with validity leads to positive improvement. Higher the validity the higher the effectiveness of highlighting. (Validity is percentage of time that the target being searched is highlighted.) • Except for urgent critical information blinking should not be used.

  46. Graphic Representation • Instructional materials:Combine text with pictures for speed, long-term retention and accuracy. ( ex: airline safety instruction card) • Representing data graphically - distort results

  47. Distorting Data

  48. Symbols • Symbol Vs. Verbal signs • Symbol is preferred generally Ex: deer crossing, rest rooms, etc.

  49. Objectives of Symbolic Coding systems • Strongest connection between the symbol and the referent • Ease of learning the association - “recognizability”

  50. Criteria for Selecting Symbols • Experimental testing may be required • Factors: Recogntion, Matching, Preferences and Opinions

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