Vision and Visibility. Human Eye. Eye Components. Cones and Rods. Typical Vision Impairments. CVEN 457 & 696 Lecture #3 Gene Hawkins

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1 Vision and Visibility CVEN 457 & 696 Lecture #3 Gene Hawkins From Allen Chapter 2 Human Eye Eye Components Cornea & lens focuses the image Lens loses ability to focus on close objects with age (presbyopia) Iris muscle that controls pupil Becomes less flexible with age limits ability to admit more light Pupil controls light into eye Retina processes the image Cones for color vision (near fovea) Rods for low light vision (no color) (on periphery) Fovea center of eye sharpest focus Cones and Rods Typical Vision Impairments Rods Differences in eye shape affect where the image is focused Cones Source: Huchingson: New Horizons for Human Factors in Design 1

2 Driver Vision Aspects Visual acuity Contrast sensitivity Light-dark adaption Effects of glare Peripheral vision Movement in depth Related measurements Luminance Contrast Field/cone of vision Visual Acuity Snellen visual acuity 20/20 is normal 20/40 is typical min for drivers license Angle subtended 20/20 = 5 angle 1 for critical detail 20/40 = 10 angle Legibility index Old 50 ft/in of letter ht. Current 30 ft/in Calculating LI FHWA Letter Series Angle subtended ( ) D Example: Legibility distance for 4 inch letters for someone with 20/40 vision SW Ht H2 tan 2 D 2inch tan0 5', D ft D Legibility Distance ft LI ft Letter Height 4 in in Series Height/Stroke Width Legibility Index* B 8:1 33 C 7:1 43 D 6.5:1 50 E 6:1 57 E(m) 6:1 60 F 5:1 60 *For 20/20 vision Based on 1950 research Clearview Size is Important More on Clearview in the sign design lecture 2

3 Acuity Comparisons Dynamic Acuity Among the vision variables studied, dynamic visual acuity shows the strongest and most consistent relationships with driving record. There is substantial, but not conclusive, evidence that static visual acuity, glare recovery, and visual fields are also related to driving record. From Allen, Forensic Aspects of Vision and Highway Safety Luminance Luminance is the brightness of an object Blue italic numbers are in candela/m 2 Range of Luminance for Eye Photopic Vision Min sign luminance Mesopic Vision Scotopic Vision Relationship between Acuity and Adaptation Luminance Contrast CONTRAST 3

4 Types of Contrast Positive contrast Target is brighter than background Negative contrast Target is darker than background Question What type of contrast exists with freeway guide signs? Original image Negative contrast Brightness adjusted Contrast Example Contrast Contrast Sensitivity Good acuity and normal contrast sensitivity Poor contrast sensitivity Source: Vistech From Allen, Forensic Aspects of Vision and Highway Safety Measuring Contrast Sensitivity Foveal Vision 20/20 visual acuity achieved only in the foveal region of the eye ~2 of visual field Width of thumb at arms length From New 4

5 Foveal (Sharpest) Vision Cone of Vision (10 20 ) What Makes Objects Visible The following are factors that an engineer can control that have the greatest impact on visibility of an object Contrast Object must have contrast with background to be visible Luminance Object must be bright enough to see Size Object must be big enough to see Location Object must be located within field of view Driver Vision Concerns Major concerns Minimum acuity level Older driver vision Color blindness Older Driver Vision Visual acuity worsens as drivers age Contrast sensitivity worsens with age Drivers need more light to see as they age 5

6 Effect of Aging Older Driver Luminance Age 20 Age 60 Age 70 Starting at age 20, the amount of light needed to see doubles every 13 years x x x 8 4 1x 2x 4x Elements & Photometrics Photometric Terminology Target Illuminance Luminance (brightness) Source Receptor Luminous intensity Photometric Term Concept Units Luminous Intensity (light source) candela (cd) Illuminance (light falling on a lux source) Luminance (light coming from a surface) cd/m 2 Luminance is brightness From Vision Lecture Color Code Color Blindness More common in males Various types of color blindness Variations of red-green most common 5% of males Challenge for traffic signals Blue tint in green indication 6