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Visual Optics > 1.3 - Snellen Chart Design > Flashcards

1.3 - Snellen Chart Design Flashcards

1
Q

What are the features of Snellen chart design

A

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

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2
Q

What does British Standard Institute do

A

Develops standards for manufactured products

Some standards become international standards (ISO) on manufactured products and packaging

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3
Q

What is aim of British Standard Institute

A

Support industry in producing high quality products - become more widely marketable throughout the world

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4
Q

How is British Standard Institute relevant to snellen chart

A

State the minimum requirements for devices here test chart - snellen chart to meet BS that determines what a snellen chart should conform to

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5
Q

What does the number by each line on a snellen chart represent

A
  • 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
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6
Q

How is the number on side of snellen chart line written

A

This is the number in the denominator of the Snellen fraction
This is written to record vision

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7
Q

Does size of letter on snellen chart relate to its actual size

A
  • 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!!
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8
Q

How do you work out height of letter

A

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

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9
Q

What angle does letter E and limb width/ gap size subtend

A

5 mins of arc for whole letter

1 min of arc for gap size and limb width

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10
Q

Working out limb width/gap size of a ‘6m’ Snellen E

A
  1. Tan x = O/A: O = Tanx x A
  2. 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
  3. O = Tan ( 0.016 ) x 6
    = 1.75mm
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11
Q

What height does 12m letter subtend

A

5’ arc

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12
Q

What limb does 12m letter subtend

A

1’ arc

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13
Q

What height does 60m letter subtend

A

5’ arc

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14
Q

What limb does 60m letter subtend

A

1’ arc

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15
Q

How to find letter size according to scale - height letter subtends

A

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

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16
Q

What would height of letter on 24m letter subtend

A

20’ arc because….

  • At 6m: 5’ arc
  • 6m to 24m = x4 ( 4x larger )
  • 5’ arc x 4 = 20’ when displayed at 6m
17
Q

What are the 3 types of letter styles

A
  • 5 x 5 Serif snellen ( original )
  • 5 X 5 Non - serif Sloan
  • 5 x 4 San - serif British
18
Q

What letter style is used in BS chart

A

Sans-serif letters on a 5 x 4 grid

Sans is ‘ non ‘

19
Q

What is serif

A

Little additions to letters

20
Q

Which letter style is easier to recognise

A

Non - serif sloan

Non serif is shorter and fatter than usual letters

21
Q

Which letters are more difficult to identify in presence of focussing error

A

5 x 4 Sans - serif

22
Q

Difference between Serif and Non - serif fonts

A
  • Sans serif = without little additions at end of limbs

- Serifs = helps readability in large sections of text

23
Q

How many letters are used in test charts

A

10

24
Q

What is specific about letters chosen in snellen chart

A

Not all letters are equally recognisable

25
Q

What letter is easiest to recognise and why

A

L

  • Few crowded limbs
  • Open letter
  • No other letter with similar form
26
Q

What letter is hardest to recognise

A

S

27
Q

What legibility of letters are used

A
  • Mid range legibility

- = remove very easy and very difficult letters = not all letters are used

28
Q

Why is the letter A hard

A

Less obvious as it looks like crowded letter, small gap at apex of A but no other letter with similar triangular shape

29
Q

How to make snellen chart/test reliable

A
  • Include one or more “difficult” letters on each line

- Include letter pairs commonly confused on each line (“common confusion pairs”)

30
Q

What are the 3 common confusion pairs

A
  • UV
  • FP
  • HN
31
Q

Why is it hard to recognise letters of confusion pairs

A

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

32
Q

Progression of Letter Size in snellen chart

A

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)

33
Q

What letter sizes does BS recommend

A

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

34
Q

What is the common ratios of the BS chart

A

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

35
Q

What is letter width on British charts

A

4 units

36
Q

What is letter spacing on British charts

A

For 60m to 18m lines the letter spacing is 3-5 units

For 12m – 3m line the letter spacing is 4-6units

37
Q

Why does spacing need to be specified

A

Because of crowding

Certain eye conditions where people suffer from crowding

38
Q

What is crowding

A

Surrounding detail affects ability to recognise letter.gap width
Crowded space - how close things are together can affect results

39
Q

What is the Line spacing on BS chart

A

20mm between successive lines on a full chart

large letters top of chart look more closely spaced than bottom

Visual Optics (18 decks)

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