Measurement Of Vision

Question 1

What is Vision?

Answer 1

Perception of differences in the external world resulting from stimulation of the retina by light
Can include light changes, colour, shape, position etc

Question 2

Why assess vision?

Answer 2

• allow us to quantify what people can and can’t see
• changes in vision in health and disease
• OCCUPATIONAL - visual requirements for some professions
• Statuatory - registration as severely sight impaired

Question 3

What are Herman Snellens innovations?

Answer 3

He designed special characters, optotypes (testypes)
Arranged a letter chart to test distance vision
Define a standard for comparison - the standard observer

Question 4

What is the standard observer?

Answer 4

Has a visual acuity that just allows him/her to identify a letter with a stroke width subtending 1’ arc

Question 5

What is form vision?

Answer 5

Ability to recognise an object from its ‘form’ - shape

Question 6

What is resolution?

Answer 6

The ability to detect fine detail

- recognition of letters as they get smaller requires both form vision and resolution

Question 7

What is Visual acuity? BVCA (best correct visual acuity)

Answer 7

Smallest line of letters read with spectacles or contact lenses worn - provided they correct all defects of focus

Question 8

What is unaided vision (UVCA)

Answer 8

Smallest line of letters read with the unaided eye

Can be both eyes (binocular) or one eye (monocular)

Question 9

What is the Snellen Fraction, V?

Answer 9

For small angles, V=d/D
d= the testing distance - 6m is standard in uk
D= Distance at which limbs of the last line of the letters correctly identified subtend 1’ arc. Equivalently, the angular height of the last line of letters correctly identified is 5’ arc.

Question 10

Minimum Angle of Resolution, MAR

Answer 10

Angular separation of 2 points or lines at the resolution acuity limit. Angle A is the smallest angle needed to recognise a letter.

Question 11

Decimal V

Answer 11

Snellen fraction can be written as a decimal viz.

Decimal V= 1/MAR (MAR in ‘ of arc)

Question 12

Radians to Degrees

Answer 12

Angles in degrees = 360/2Pi x angle in radians

Question 13

Degrees to Radians

Answer 13

Angles in Radians = 2Pi/360 x angle in degrees

Question 14

Size of letters on a Snellen Chart

Answer 14

Number by each line is the letter size in ‘metres’
Distance at which a letter height subtends 5’ arc or the gap/limb width subtends 1’ arc
This is the DENOMINATOR of the snellen fraction

Question 15

Snellen chart Letter Styles

Answer 15

5x4 sans - serif (British) - these letters are more difficult to identify in the presence of a focusing error
5x5 serif (snellen)
5x5 non serif (sloan)
serifs help readability in large sections of text

Question 16

Snellen Chart Letter Legibility

Answer 16

Should have a recommended selection of letters for reading charts excluding easy letters and very hard letters
increases reliability of test

Question 17

Snellen Chart Reliability of test

Answer 17

Include one or more difficult letter on each line
Include letter pairs commonly confused on each line (commonly confusion pairs) eg UV
Details in the letters must be resolved to distinguish them

Question 18

Snellen Chart Progression of Letter Size

Answer 18

Recommended letter sizes (m): 3,4,5,6,9,12,18,24,36,60

There is no common ratio and hence no geometric progression of letter sizes

Question 19

Snellen Chart Letter and Line Spacing

Answer 19

On British charts the letter width is 4 units
For 60m to 18m lines the letter soacing is 3-5 units
For 12m - 3m line the spacing is 4-6 units
Spacing has to be specified as people suffer from crowding
20mm bet successive lines on a full chart

Question 20

What happens when VA is below 6/60

Answer 20

• Reduce distance to chart until px can read top letter, If at 3m then record 3/60
  1) Counting fingers - Used at less than 1m. Record CF at 25cm if px can count fingers at 25cm
  2) Hand Movements- Record HM if px can detect your hand moving
  3) Perecption of light - Record PL if px can detect your pen torch. (can also record if they can accurately detect which quadrant light is projected into)
  4) No Perception of Light - Record NPL if px cant detect light

Question 21

Advantages of Snellen Chart

Answer 21

Quick 
Reliable 
Sensitive to Ametropia 
Narrow (physical width)
Less affected by guessing compared to grating acuity, Landolt C or Iliiterate E charts 
Letters familiar to most 
Enshrined in legislation

Question 22

Limitations of Snellen Chart

Answer 22

Unequal Number of letters per line -task is not equivalent for differing levels of VA
Non uniform change in letter size for successive lines
Line by line scoring system
Fixed Row spacing
Unequal Letter Spacing - crowding affects change

Question 23

Bailey - Lovie LogMAR Chart

Answer 23

Same number of letters per line
5 chosen
Letters of similar legibility chosen from BS 4272:2003
Approx equivalent task at each level of VA

Question 24

LogMAR chart - Lines

Answer 24

Has a fixed ratio between lines
0.1 log unit per line
so 0.02 logMar units for each letter
Size doubles/halves every 3 lines - MAR doubles/halves

Question 25

LogMar Chart - Letter Spacing

Answer 25

Letter spacing on each line = letter width

Equivalent to 4.1 limb widths since 5x4 letters on Bailer - Lovie Chart

Question 26

LogMar Chart Line Spacing

Answer 26

Height of letters in the smaller row ~ width of letters in the larger row

Question 27

Snellen v LogMAR

Answer 27

• Snellen has unequal no of letters per line, logMAR has 5 letters per line
• Snellen has a non uniform change in letter size for successive lines, logMAR has 0.1 log unit change in letter size per line
• Snellen has a line by line scoring system but LogMAR has a letter scoring system (0.02 per letter)
• Snellen has fixed row spacing but LogMAR is the height of letters in the smaller row