Visual Acuity Flashcards
Visual Acuity
Ability of the eye to see fine detail
A measure of the spatial resolving capability of the visual processing system.
What factors can affect Visual Acuity?
1) Diffraction
2) Aberrations: the failure of rays to converge at one focus because of a defect in a lens or mirror
3) Photoreceptor density
4) Refractive Error
5) Illumination
6) Contrast
7) Location of object in visual field
Unaided Vision:
Determined from the size of the smallest line of letters in a visual acuity chart that can be read by a person without the use of an optical device such as: spectacles or CLs.
Habitual Vision:
Determined from the size of the smallest line of letters in the test chart that can be read by the patient with their present spectacles or contact lens correction.
Visual Acuity:
Determined from the size of the smallest line of letters in a visual acuity chart that can be read with the aid of optimal, optical correction.
What is Visual Acuity used for?
- Adequacy of spectacle lens corrections
- Key indicator of ocular health and to monitor eye health conditions
- Fitness to drive
- Entry into professional registration (RAF, Police)
What is 3 principle measures of VA?
- Unaided VA (Visions)
- Habitual VA
- Optimal VA (Visual Acuity?
When should you record Unaided VA?
- When a patient does not wear spectacles
- Has lost or broken their spectacles so you cannot measure habitual VA
- Do not wear spectacles for the distance
- When a profession requires it i.e. a report is filled out
- When a practitioner suspects that a patient does not need to wear their spectacles for the distance.
Testing Distance
Reference Value –
6m in UK equates to 20FT in USA
In tests of Distance Vision, the testing distance should be…
large enough to not stimulate accommodation.
The distance between the person’s eyes and the testing chart is set to “optical infinity” to mimic the lens focusing ability.
Letter size -
The letter size is specified not by its height, but by the distance at which the complete letter subtends 5 minutes of arc.
Letter Size Info:
360’ in a circle
60 minutes of arc in 1 degree
60 seconds of arc in 1 minute
Snellen Chart
11 lines of letters in a progression of sizes
Each line designated by the distance at which the overall height of the letter subtends 5 minutes of arc.
High contrast - black letters on white background
Non-serif letters – sans serif typography
No extending features
Overall height of the 12m letter subtends 5 minutes at 12m
Overall height of the 6m letter subtends 5 minutes at 6m
Snellen Fraction
d/D
Testing distance ‘d’ in m
/
Distance at which the letter would subtend 5 minutes of arc ‘D’
so 6/60
Explain 6/6 Vision?
The ability to read the 6m letter at a distance of 6m.
Explain 6/9 Vision?
The ability to read the 9m letter at a distance of 6m
What is 20/20?
USA metric unit
Minimum Angle of Resolution, MAR -
For VA 6/6, one of the strokes of the letter subtends 1 minute of arc at the eye.
The angle at which two points of a grating are just perceived as separate.
Its logarithmic form (logMAR) is commonly used as a metric for visual acuity.
MAR can be used to specify VA
Taking the reciprocal of the Snellen fraction.
Decimal Acuity
Calculate the decimal of the Snellen Fraction e.g. 6/36 = 0.17
Advantages of Snellen -
- Widley available
- Snellen notation is universally understood
- Can be produced in a smaller format, on a projector, and as an addition to other targets
- The lower part of the chart has a similar number of letters to other charts
- Easily reproduced
- Quick and easy to use
Disavantages of Snellen -
- The scale is not linear i.e. not equal interval between the lines
- Some letters are easier to see than others especially when small
- The task is easier if you have poorer visual acuity
- Relative legibility of letters will depend on the magnitude and axis of any uncorrected astigmatism
- Many charts fail to adhere to the recommendations and standards relating to the selection of letters
What are the task differences with Snellen?
- Most charts have only one 6/60 letter and an increasing number of letters on lower lines.
- Patients with poor Acuity are required to read less letters than those with good Acuity.
- Letters on lower lines are more crowded than those towards the top of the chart.
- Crowding increases the task difficulty.
- There is no systematic relationshop between the spacing of each letter to its width or height; therefore, the visual demand changes down the chart.
- VA measured at a distance of less than 6m cannot easily be equated to a 6m equivalent.
- Small number of large letters means that it is not useful when meausring VA in low Vision Patients.
Name Alternative Optotypes
- Numerals
- Landolt Rings
- Illiterate E or Tumbling E