Visual Acuity, Direct Glare, and Visual Comfort Probability

1
Visual Acuity, Direct Glare, and Visual Comfort
Probability
2
Visual Acuity

• Def The measure of a persons ability to
  distinguish fine details
• Details are perceived most keenly when the image
  is focused on the fovea, an area in the center of
  the retina where there is the densest
  concentration of cones.

3
Visual Acuity
4
Visual Acuity

• Visual acuity continually decreases with age
• Four factors that affect visual acuity
• Size
• Luminance
• Contrast
• Time

5
Visual Acuity

• Size
• The effect of size on acuity does not mean
  physical size
• Size is concerned with the visual angle of the
  objects detail with respect to the eye
• Ex Snellen Eye Chart

6
Visual Acuity

• Luminance
• Def The intensity of the source or surface in
  the direction of the observer divided by the area
  of the source or surface seen by the observer
• If a surface is perfectly diffusing, its
  luminance is constant from every viewing angle

7
Visual Acuity

• Contrast
• Def the magnitude of the luminance difference
  between an object (print) and its background
  (paper) divided by the background luminance
  (paper)
• Visual Task that is specular or glossy
  increasing the light level may decrease the
  persons ability to see critical detail. The
  extra light may reduce the contrasts
• Research has found that an increase in background
  luminance will cause an increase in a persons
  sensitivity to contrasts

8
Visual Acuity

• Time
• There is a time lag in the photochemical
  processes in the retina
• Given sufficient time, even small details can be
  seen under relatively low levels of light

9
Direct Glare

• Def Bright light that can interfere with visual
  perception
• Influenced by brightness conditions within the
  entire field of vision

10
Direct Glare

• Three visually disturbing results
• The eye adapts to a higher luminance level,
  making it harder to see what is being looked at
• The eye is drawn simultaneously in two
  directions involuntarily towards the light
  source, and voluntarily towards what is being
  looked at
• The adaptation level is continously varying as
  the eye is drawn towards the glare source and
  then away again

11
Direct Glare
12
Direct Glare

• Useful rules of thumb for light sources
• Luminances of large sources should not exceed
  2500 cd/m² ( a blue sky)
• Small sources should not exceed 7500 cd/m² ( a
  fluorescent lamp)

13
Visual Comfort Probability

• Rating scheme for arrangements of fixtures in
  terms of the percentage of viewers who would find
  direct glare conditions tolerable at a specified
  location and direction of view

14
VCP

• Factors in determining VCP 1000-lux illuminance,
  room dimensions, fixture height, observer
  position, and field of view limited to 53 degrees
  above and forward from the observer

15
VCP