snellen chart design 1.3 Flashcards
what is British standard?
. develops standard for manufactured goods
. the aim is to support industry in producing high qaulity products
what do the letters by each line on snellen chart mean?
. the number by each line is letter size in meters
. it is the distance at which letter height subtends 5 min of arc or the gap/width subtends at 1 min of arc
what is limb width?
the size of gap in the letters , limb width subtends at 1 min of arc at the eye
at what angle does the limbs and height subtend in a 12m letter?
. A 12 m letter will have limbs subtending at 1 min of arc at 12m
. height subtends 5 min of arc at 12 m
what letter styles do British standards recommend in a snellen chart?
sans-serif letters on a 5 x 4 grid
why are 5 x 4 sans-serif letters used ?
they are more difficult to identify in the presence of a focusing error
why are sloan letters not used?
Sloan letters are easier to read when there is a focusing error
Why is a restricted range of letters recommended for sight test charts?
It improves the reliability of the chart by removing easy and difficult letters and using mid-range legibility letters
what are some common confusion pairs?
. U and V
. F and P
. H and N
details in the letters must be resolved to distinguish them
what is the progression of letter sizes on successive lines of a British Standard Snellen Chart?
3 , 4 , 5 , 6 , 9 , 12, 18, 24, 36 ,60
What is the common ratio for the progression of letter sizes in a Snellen chart?
there is no geometric progression of letter size
Between which successive lines on a Snellen Chart is there large jump in letter size?
6 to 9
24 to 36
36 to 60
what is the letter width in snellen chart ?
4 units
what is the letter spacing from 60 m to 18 m ?
3-5 units
what is the letter spacing from 12 m to 3m ?
4-6 units
