1.4 - Using a snellen chart

Question 1

What is the standard observer

Answer 1

The standard observer has a visual acuity that just allows him/her to identify a letter with a stroke width subtending 1’ arc

Question 2

Why do we need a standard observer

Answer 2

We need a baseline measure for comparison
Develop a standard for comparison
To know how well we should do

Question 3

What is form vision

Answer 3

The ability to recognise an object from its “form”/shape

e.g. A - easy to recognise due to its form - triangular shape - unusual

Question 4

What is resolution

Answer 4

The ability to detect fine detail e.g. gaps

Question 5

What does recognition of letters require

Answer 5

Recognition of letters as they get progressively smaller requires both form vision and resolution

Question 6

What is visual acuity

Answer 6

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

Question 7

What is vision

Answer 7

Smallest line of letters read with the unaided eye ( no refraction correction I.e. no glasses/CL’s)

Question 8

When is VA and vision same for a px and when is it different

Answer 8

• If you don’t wear glasses/CL’s i.e. no refractive correction = unaided vision and va are. same
  BUT
• If you have refractive error i.e. wear glasses/CL’s = VA different to unaided vision where they don’t wear refractive correction

Question 9

How can vision be measured

Answer 9

Both eyes = binocular

One eye = monocular

Question 10

Which one is better - monocular or binocular

Answer 10

Binocular - gives advantage when using both eyes

Question 11

How can vision/va be recorded

Answer 11

Snellen Fraction V ( most common )
Minimum Angle of Resolution (MAR)
Decimal V

Question 12

What is the snellen fraction

Answer 12

see equations in notebook

Question 13

What does V = 6/18 mean

Answer 13

6: Testing distance – 6m is standard in the UK
(20 feet in the USA)

18: Distance at which the limbs of the last line of letters correctly identified subtend1’ arc. Equivalently the angular height of the last line of letters correctly identified is 5’ arc

= all letters on 18m line correctly identified

Question 14

What is minimum angle of resolution

Answer 14

• Angular separation of two points or lines at the resolution acuity limit
• Eye’s ability to see fine detail
• Angle subtended by limb of letter at observer for last line of letters correctly identified
• Angle A

Question 15

What is MAR for standard observer

Answer 15

1’ arc

Question 16

How to convert snellen to decimal

Answer 16

Decimal V = 1/MAR

MAR is in minute of arc
e.g. 6/5 = 1.2

Question 17

Recording Snellen V/VA

Answer 17

• If all letters are correctly identified on the final line read, record as the Snellen fraction e.g. 6/5 if the 5m line was completely read
• If only 2 letters are correctly identified on the 6/6 line, record as 6/9 +2. This tells us all letters on the ‘9m’ line were correctly identified + 2 letters on the next (6/6) line
• If, for example, 3 letters are incorrectly identified on the 6/5 line then record 6/5 -3

The +/- number should be as small as possible so 6/5 - 3 rather than 6/6 + 5

Question 18

What to do if VA below 6/60

Answer 18

• Reduce distance to chart until Px can read top letter. If at 3m then record 3/60 ( numerator = testing distance )
• Counting Fingers. Record CF @25cm if Px could count fingers at 25cm ( 25cm = distance from subject ). Counting fingers should be at less than 1m.
• Hand movements. Record HM if Px can detect your hand moving at 1/2m
• Perception of Light. Record PL if Px can detect your pen torch. (Can also record if they can accurately detect which quadrant the light is projected into) - can px accurately project ie. they know where the light source is - quadrant perception = projecting light into different parts of eye
• No Perception of LightRecord NPL if Px cannot detect light

Question 19

Practical Matters

Answer 19

• Do not ask someone with vision/VA below 6/60 to jump out of the chair. They are sight impaired and may trip in a dark consulting room!
• Increase the letter size above ‘60m’ on computer charts
• Use a single ‘60m’ letter E, which you hold at different distances. Rotate it to test if it is resolved.
• Counting fingers should be at less than 1m. If 1m or more then use increased letter sizes or the ‘60m’ E described above