1.3 - Snellen Chart Design Flashcards
What are the features of Snellen chart design
Letter size ( progression of it )
Test type font
Number of letters per line – varies as you go down, increases as you go down
Range of letters used - not every letter of alphabet
Progression of letter sizes - not random
Letter spacing
Line spacing
What does British Standard Institute do
Develops standards for manufactured products
Some standards become international standards (ISO) on manufactured products and packaging
What is aim of British Standard Institute
Support industry in producing high quality products - become more widely marketable throughout the world
How is British Standard Institute relevant to snellen chart
State the minimum requirements for devices here test chart - snellen chart to meet BS that determines what a snellen chart should conform to
What does the number by each line on a snellen chart represent
- Letter size in metres - represent size of letters on that specific line
- It is the distance at which a letter height subtends 5’arc at eye or the gap/limb width subtends 1’ arc at eye
- They relate to the physical size of letters through angle and distance
- e.g. 6m letter = at 6m, the height of a letter on the 6m line will subtend 5’ arc
- e.g. 18m letter = at 18m, the height of a letter on the 18m line will subtend 5’ arc
How is the number on side of snellen chart line written
This is the number in the denominator of the Snellen fraction
This is written to record vision
Does size of letter on snellen chart relate to its actual size
- NO - it is not directly related to the physical size of letters in metres, only through angle and distance = doesnt represent actual size of letter
- A ‘12m’ letter does NOT mean it is 12m high!!
How do you work out height of letter
Tan x = O / A SO
O = Tanx x A
Tan x is the angle in minutes of arc converted to degrees ( 1 degree = 60 mins ) e.g. at 6m its 5’ arc which is 0.083 degrees
A = distance letter is from eye
What angle does letter E and limb width/ gap size subtend
5 mins of arc for whole letter
1 min of arc for gap size and limb width
Working out limb width/gap size of a ‘6m’ Snellen E
- Tan x = O/A: O = Tanx x A
- Work out what 1 min of arc is in degrees ( because working out gap size/ limb width which is 1 min of arc )…
60 mins of arc = 1 degree
1 min of arc = 0.016 degree - O = Tan ( 0.016 ) x 6
= 1.75mm
What height does 12m letter subtend
5’ arc
What limb does 12m letter subtend
1’ arc
What height does 60m letter subtend
5’ arc
What limb does 60m letter subtend
1’ arc
How to find letter size according to scale - height letter subtends
A ‘60m’ letter is 10x larger than a ‘6m’ letter
A ’18m’ letter is 3x larger than a ‘6m’ letter
You multiply the minutes of arc at 6m i.e 5’ to the distance
What would height of letter on 24m letter subtend
20’ arc because….
- At 6m: 5’ arc
- 6m to 24m = x4 ( 4x larger )
- 5’ arc x 4 = 20’ when displayed at 6m
What are the 3 types of letter styles
- 5 x 5 Serif snellen ( original )
- 5 X 5 Non - serif Sloan
- 5 x 4 San - serif British
What letter style is used in BS chart
Sans-serif letters on a 5 x 4 grid
Sans is ‘ non ‘
What is serif
Little additions to letters
Which letter style is easier to recognise
Non - serif sloan
Non serif is shorter and fatter than usual letters
Which letters are more difficult to identify in presence of focussing error
5 x 4 Sans - serif
Difference between Serif and Non - serif fonts
- Sans serif = without little additions at end of limbs
- Serifs = helps readability in large sections of text
How many letters are used in test charts
10
What is specific about letters chosen in snellen chart
Not all letters are equally recognisable
What letter is easiest to recognise and why
L
- Few crowded limbs
- Open letter
- No other letter with similar form
What letter is hardest to recognise
S
What legibility of letters are used
- Mid range legibility
- = remove very easy and very difficult letters = not all letters are used
Why is the letter A hard
Less obvious as it looks like crowded letter, small gap at apex of A but no other letter with similar triangular shape
How to make snellen chart/test reliable
- Include one or more “difficult” letters on each line
- Include letter pairs commonly confused on each line (“common confusion pairs”)
What are the 3 common confusion pairs
- UV
- FP
- HN
Why is it hard to recognise letters of confusion pairs
Similar shapes - details in letters must be resolved to distinguish them
FP - to distinguish these as different, need to distinguish gap between 2 horizontal limbs
Progression of Letter Size in snellen chart
There is no common ratio and hence no geometric progression of letter sizes.
Some jumps in size are large: 6 to 9, 24 to 36
and 36 to 60 (ratio ≥ 1.5)
What letter sizes does BS recommend
BS 4274 recommends letter sizes (m): (3, 4), 5, 6, 9, 12, 18, 24, 36, 60
3,4 = additional lines that might appear on some charts
What is the common ratios of the BS chart
The common ratios are:
1.33, 1.25, 1.20, 1.50, 1.33, 1.50, 1.33, 1.50, 1.67
= ratios vary slightly between 1.20 and 1.67
What is letter width on British charts
4 units
What is letter spacing on British charts
For 60m to 18m lines the letter spacing is 3-5 units
For 12m – 3m line the letter spacing is 4-6units
Why does spacing need to be specified
Because of crowding
Certain eye conditions where people suffer from crowding
What is crowding
Surrounding detail affects ability to recognise letter.gap width
Crowded space - how close things are together can affect results
What is the Line spacing on BS chart
20mm between successive lines on a full chart
large letters top of chart look more closely spaced than bottom
