1.4 - Using a snellen chart Flashcards
What is the standard observer
The standard observer has a visual acuity that just allows him/her to identify a letter with a stroke width subtending 1’ arc
Why do we need a standard observer
We need a baseline measure for comparison
Develop a standard for comparison
To know how well we should do
What is form vision
The ability to recognise an object from its “form”/shape
e.g. A - easy to recognise due to its form - triangular shape - unusual
What is resolution
The ability to detect fine detail e.g. gaps
What does recognition of letters require
Recognition of letters as they get progressively smaller requires both form vision and resolution
What is visual acuity
Smallest line of letters read with spectacles or contact lenses worn (provided they correct all defects of focus)
What is vision
Smallest line of letters read with the unaided eye ( no refraction correction I.e. no glasses/CL’s)
When is VA and vision same for a px and when is it different
- 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
How can vision be measured
Both eyes = binocular
One eye = monocular
Which one is better - monocular or binocular
Binocular - gives advantage when using both eyes
How can vision/va be recorded
Snellen Fraction V ( most common )
Minimum Angle of Resolution (MAR)
Decimal V
What is the snellen fraction
see equations in notebook
What does V = 6/18 mean
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
What is minimum angle of resolution
- 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
What is MAR for standard observer
1’ arc