John Grigg
Senior Lecturer
Save Sight Institute, University of Sydney
Sydney Eye Hospital & Children's Hospital at Westmead
johng@eye.usyd.edu.au
- The Snellen chart is designed to measure visual acuity in angular terms
- Accepted convention does not specify acuity in angular units
- Uses notation in which the numerator is the testing distance (in feet or meter, d)
- Denominator is the distance (in feet or meter, D) at which a letter subtends the
standard visual angle of 5 minutes of arc (Snellen visual acuity, V = d/D)
- Resolution limit is between 30 secs of arc and 1 min of arc
- 6/6 vision allows resolution of 1 min of arc
- All letters in alphabet are not equally legible
- Test depends on verbal communication
- Dutch Ophthalmologist, born 19th Feb 1834, Zeist (Utrecht); died 18th Jan 1908
The smallest readable line gives the patient's visual acuity as a ratio of the testing distance to the size of the smallest line read.
Settled most of the disadvantages of the Snellen-type chart
- Five letters are presented on each line
- Spaces between letters and lines have been standardized
- space between letters is one letter wide
- space between lines is equal in height to the letters of the next lower line - Letter sizes range from 58.18 mm to 2.92 mm
- Providing a visual acuity equivalent of 20/200 to 20/10 or, at a distance of
4 m, 4/40 to 4/2, for which this letter chart was designed
- The progression of letter height from line to line is geometric
- Letters on each line are 1.2589 times the height of the letters on the next lower line
- The multiplier is the tenth root of 10, or 0.1 log unit
- Ten Sloan letters had been chosen for the ETDRS chart
- S, D, K, H, N, O, C, V, R, and Z
- The difficulty score of each letter is approximately equal to each other and to a Landolt C ring
- Infancy
- grating acuity most easily measured
- Toddlers and young children
- recognition acuity assessment possible by using letter or picture matching
- for example, Lea Symbols, Kay pictures, Sheridan Gardner or Tumbling E - Older children, recognition acuity
- measured by standard letter charts
- Grating acuity is the spatial threshold for resolving dark and light stripes
- The behavioural methods use the infant's inherent preference for looking at
high-contrast patterns to determine the smallest stripes that evoke a detectable response
- Advantages
- relative ease of both experimenter training and infant testing
- low cost
- ready availability
- existence of established norms for large populations over a wide age range
- Emmetropia (no refractive error)
- every object point brought to sharp focus point on retina
- Refractive error
- blur circle formed on retina instead of a point
- size of blur circle directly proportional to size of subject's pupil
- Pupil made smaller > blur circle smaller
- Pinhole aperture acts as artificial pupil
