visual acuities

Question 1

what are the different types of acuities

Answer 1

• Detection acuity
• Hyperacuities
• Resolution acuity
• Recognition (letter acuity)

Question 2

what are the two equivalent approaches to testing VA

Answer 2

• keep viewing distance fixed but vary target size

OR

• vary viewing distance and keep the target size fixed

Question 3

detection acuity

Answer 3

• What is the thinnest dark line that can be detected against a white background? (or vice-versa)
• ΔI is the dip in intensity in the retinal image caused by the dark line
  I is the intensity due to the white background
• If ΔI is too small, line is not detectable.
  When ΔI gets bigger, line becomes detectable
• The value of ΔI depends on I, so that ΔI /I is a constant [i.e. ΔI needs to be bigger if I is bigger]

Question 4

Hyperacuities

Answer 4

• Hyperacuity tasks involves judging the location of one target element relative to the rest of the target (‘localisation’ acuity)
• Best know of these is Vernier acuity
• Vernier acuity is so precise it’s exploited in measurement scales when precision is required: ‘Vernier callipers
• When the break in the line is just detectable, the angular separation between the lines (ω) is a measure of the Vernier acuity

Question 5

Resolution acuity

Answer 5

• Targets for a resolution task can be dark objects on a white background or vice versa (e.g. stars against the night sky)
• They are moved closer together or further apart until two points are just visible (i.e. they are ‘resolvable’)

Question 6

Rayleigh’s resolution criterion

Answer 6

• states that two points will be just resolable by an optical system if they are seperated by not less than:
• (1.22 x wavelength of the light) / pupil
• put everything in metres
  mm to m divide by 1000
  nm to m divide by 10^11
• answer is in radians so convert to degrees
  radians x (180/π) = degrees
  degrees to minutes of arc = x60
  minutes of arc to seconds of arc = x60

Question 7

recognition acuity

Answer 7

• involve the presentation of letters
• Recognition acuity tests resolution ability but it also tests literacy and past experience (e.g. how would you do reading letters of the Greek alphabet?)
• When there’s a language barrier, or if the Px can’t speak or is illiterate, we can instead use a Resolution Acuity test
• This is normally a Landolt C/Ring or Tumbling E test