VISUAL ACUITY THEORY

     John Grigg
     Senior Lecturer
     Save Sight Institute, University of Sydney
     Sydney Eye Hospital & Children's Hospital at Westmead
     johng@eye.usyd.edu.au
 
Visual Acuity
 

    Definition

    A measure of the resolving power of the eye and refers to the spatial limit of visual discrimination

 
Factors affecting eye and reducing it from an 'ideal' optical instrument
 

  1. Diffraction
    - limitation of aperture causes a spread of light even in a fully focused system

  2. Aberrations
    - peripheral rays may not converge on geometric image point
    - contributing to spread of light beyond that caused by diffraction

  3. Scatter
    - due to microscopic structures in the ocular media

  4. Absorption
    - shorter wavelengths the greater the absorption

  5. Focus factors
    - accommodation stance may not always match stimulus distance

  6. Retinal Anatomy
    - diameter of retinal receptors a physical limitation to partitioning spatial information
 
Visual Acuity: Types
 

  • Minimum visible

  • Minimum resolvable

  • Minimum discriminable

  • Stereo acuity

 
Minimum Visible
 

  • The minimum visible is a light discrimination function

  • It includes brightness sensitivity and brightness discrimination

  • Minimum visible acuity is the ability to detect a small difference in the brightness of two light sources

  • It determines the presence or absence of a target against a background

  • The physiologic term is local brightness difference threshold (Dl)

 
  Practical Example
  • Measures the minimum width of a telephone wire that can be seen against a uniformly coloured sky

  • Although object changes in a spatial dimension, detection is purely a brightness change

  • Threshold value is of the order of 1 second of arc

  • Represents a very small fraction of the diameter of a retinal photoreceptor

 
Minimum Resolvable (ordinary visual acuity)
 

  • Visual acuity as measured clinically also called:

    - minimum resolvable or minimum separable

  • Measure of the resolving power of the eye

  • A spatial discrimination function representing the smallest visual angle at which two separate objects can be discriminated

  • Resolution limit is called the minimum angle of resolution (MAR)

  • Normal minimum angle of resolution 30sec to 1 min of arc

  • Clinically, use such systems to assess visual acuity

    - spatial judgment eg P or F, B or R, C or O
    - examples of minimum resolvable include the Landolt C and Snellen E

  • Concordance between minimum angle of resolution and resolving capacity of eye's optics

  • Elements for detecting doubleness

    1. underlying retinal image pattern with two peaks by a trough
    2. retinal illuminance difference between peaks and troughs that is within the ΔI adaptation
    3. separate localisations of the differentially stimulated region
 
Minimum Discriminable
 

  • Minimum discriminable (hyperacuity, Vernier acuity) is another example of spatial discrimination

  • Determines the relative location of two or more visible features with respect to each other

  • Eye is capable of subtle discrimination in spatial localization

  • Detect misalignment of two line segments in a frontal plane if these segments are separated by as little as 3 to 5 seconds of arc

  • Considerably less than the diameter of a single foveal cone

  • Mechanism subserving hyperacuity is still being explored

 
Minimum Discernable (Verner Acuity)
 

  • Depends on integration of information by the visual cortex

  • Vernier acuity is defined by the ability to detect small changes in alignment or position

  • Vernier acuity matures extremely rapidly

    - over the first 6 postnatal months
    - followed by a slower rate of improvement that lasts many years

 
Minimum Discriminable
 

  • Types of misalignment detected by this function. (A and B)

  • The more conventionally used minimum resolvable distance between two objects (C)

 
Stereo acuity
 

  • Form of binocular function critical for depth perception

  • Stereopsis is based on binocular disparity

    - that is, the principle that an image is observed at slightly different angles by the two eyes
    - depends on comparison of information from the two eyes within the visual cortex

  • Stereoacuity is a measure of the minimum detectable binocular disparity

    - generally measured with pairs of stimuli consisting of patterns of randomly distributed dots
    - so that binocular disparity is the only cue available for judging depth

 
Classification of visual acuity according to criteria
 
 
Factors influencing visual acuity
 
  • Refractive error

    - defocused optical system
    - point spread function widens
    - two points need to be further apart to detect
    - pupil : depth of focus increase with reduction in size

  • Retinal eccentricity

    - centre of fovea site maximum acuity
    - one degree from centre 60% of maximal acuity
    - due to neural connectivity with increased summation areas rather than optical blur or receptor spacing

  • Luminance

    - visual acuity remains constant over a wide range of photopic luminances
    - very high luminances > unexplained decrease in VA

  • Pupil Size

    - VA remains constant in range 2.5 - 6.0mm
    - beyond range aberrations begin to widen the point spread function
    - < 2.5mm (pupil) point spread function becomes progressively wider

  • Exposure duration

    - ↓ VA with ↓ exposure when in millisecond region
    - even when ↓ luminance is taken into account

  • Target and Eye movement

    - ↓ VA during saccades
        - significant movement of the retinal image
        - small movements do not ↓ VA

  • Meridional variations in acuity

    - can be difference in VA
    - difference rarely exceeds 15%

  • Interaction affects

    - ↓ VA when targets too close together
    - Max ↓ VA at 2-5 mins of arc separation
    - not purely optical

  • Developmental aspects

    - several months after birth for development of pursuit

  • Aging

    - acuity ↓ with age