1. Visual acuity
Shafee

2. The definition SPATIAL RESOLVING CAPACITY OF THE VISUAL SYSTEM
Angular size of the object that can just be resolved
Limitations
Optical factors
Neural factors
Combination of the above two

4. OPTICAL LIMITATIONS Diffraction pattern – AIRY DISC ὼ=2.44λ/p ὼ ∝ 1/p
Where ὼ = diameter in radians
λ=wavelength of light
p=pupil diameter
ὼ ∝ 1/p
Rayleigh criterion for resolution
βmin = 1.22 λ/p (or) 2.3/p
Where βmin in min of arc & p is in mm

5. When p=4.6mm, βmin = 0.5min of arc =2.3mm, = 1.0min of arc…etc
Refractive errors/focusing errors
Small pupils=diffraction
Large pupils=aberrations
Chromatic
Optical
P=2.5mm (max VA..,MAR =0.92)

6. Neural limitations Packing density of neural receptors
Neural interactions in retina
Visual pathway
Unstimulated receptor(2μm-4μm)
0.82 min of arc resolution..
16.67mm assumed nodal point dist from retina
≈optical limit

7. Tests of visual resolution
Measuring the limits of visual performance…to discern & to detect & to recognize
MINIMUM DETECTABLE
MINIMUM SEPERATION
RECOGNITION RESOLUTION

8. Minimum detectable
Threshold size for being recognized against a background
Width of image…???!!!???
Dependence??
Diffraction..!!
Very small – contrast
Min. size?!?

9. Minimum separable
Least separation b/w two adjacent entities two be seen as two…
Commonly used to test human visual efficiency
Grating line targets…
DUTY CYCLE(1.0)..three line target
cpd – units
Spurious resolution – checker board targets

10. Recognition resolution
Most widely used for clinical testing
Correct identification / recognition
OPTOTYPES
Test targets used for these tests
Eg.,
Landolt C rings
Letters
Lh symbols..etc

11. Landolt rings Landolt ring target or Landolt C
External diameter is 5 times the STROKE width
Thus the internal diameter is 3 times the stroke width
Gap positions
4 directions
8 directions
Well defined and unambiguous detail as target

12. Letter optotypes Grid patterns
5×5
5×4
5×6
SNELLEN-serifs-British standard(D,E,F,N,H,P,R,U,V,Z)-5×4 grid
MODERN-sanserifs- Sloan(C,D,H,K,N,O,R,S,V,Z)-5×5 grid

13. N & H 2003 British bi{(C,H,N,V,Z)(K,R,D)(E,F,P,U)}
Standardization of variability and legibility of optotypes universally
Average legibility at each acuity level
Clues and combinations
Eg.,
N & H

14. Tumbling E Illiterate E - 5×5
Different orientation at each acuity level
Four alternative
Eight alternative

15. Numerical & pictorial targets
Animals,Catoons,Numbers…etc
Used in case of Infants, Toddlers..
No specific grid pattern and not standardized
Recognition better than usual optotypes
Threshold size of identification is very small

16. Events in the HISTORY
1843 German ophthalmologist Heinrich Kuechler wrote a treatise advocating the need for standardized vision tests and developed a set of three charts.
1854 Eduard von Jaeger published a set of reading samples to document functional vision. He published samples in German, French, English and other languages.
He used fonts that were available in the State Printing House in Vienna in 1854 and labeled them with the numbers from that printing house catalogue.

17. Events in the HISTORY
1861 Franciscus Donders coined the term visual acuity to describe the “sharpness of vision” and defined it as the ratio between a subject's VA and a standard VA.
1862 Hermann Snellen published his famous letter chart.
His most significant decision was not to use existing typefaces but to design special targets, which he called optotypes.
This was crucial because it was a physical standard measure to reproduce the chart. Snellen defined “standard vision” as the ability to recognize one of his optotypes when it subtended 5 minutes of arc, thus the optotype can only be recognized if the person viewing it can discriminate a spatial pattern separated by a visual angle of 1 minute of arc.

18. Events in the HISTORY
1875 Snellen changed from using feet to meters (from 20/20 to 6/6 respectively)
Today, the 20-foot distance prevails in the United States and 6 meters prevails in Britain.
1875 Monoye proposed to replace the fractional Snellen notation with its decimal equivalent (e.g., 20/40 = 0.5, 6/12 = 0.5,5/10 = 0.5)
Decimal notation makes it simple to compare visual acuity values, regardless of the original measurement distance.

19. Events in the HISTORY
1888 Edmund Landolt proposed the Landolt C, a symbol that has only one element of detail and varies only in its orientation.
The broken ring symbol is made with a "C" like figure in a 5 x 5 grid that, in the 20/20 optotype, subtends 5 minutes of arc and has an opening (oriented in the top, bottom, right or left) measuring 1 minute of arc.
This proposal was based in the fact that not all of Snellen's optotypes were equally recognizable.
This chart is actually the preferred visual acuity measurement symbol for laboratory experiments but gained only limited acceptance in clinical use.

20. Events in the HISTORY
1923 Soviet ophthalmologists Sergei Golovin and D. A. Sivtsev developed the table for testing visual acuity.
Later this table became known as Golovin-Sivtsev Table.
1959 Louise Sloan designed a new optotype set of 10 letters, all to be shown in each and every line tested, in order to avoid the problem that not all letters are equally recognizable.
The larger letter sizes thus required more than one physical line.
Louise Sloan also proposed a new letter size notation using the SI system stating that standard acuity (1.0, 20/20) represents the ability to recognize a standard letter size (1 M-b unit) at a standard distance (1 meter)

21. Events in the HISTORY
1976 Ian Bailey and Jan Lovie published a new chart featuring a new layout with five letters on each row and spacing between letters and rows equal to the letter size
This layout was created to standardize the crowding effect and the number of errors that could be made on each line, so letter size became the only variable between the acuity levels measured.
These charts have the shape of an inverted triangle and are much wider at the top than traditional charts.
Like Sloan's chart, they followed a geometric progression of letter sizes

22. Events in the HISTORY
Lea Hyvärinen created a chart, the Lea chart, using outlines of figures (an apple, a house, a circle and a square) to measure visual acuity in preschool children.
Hugh Taylor used these design principles for a "Tumbling E Chart" for illiterates, later used to study the visual acuity of Australian Aborigines.

23. Designation of VA Snellen fraction Decimal notation MAR logMAR
VAR & fAS VE

24. logMAR Logarithm of MAR Eg., 20/20 20/40 MAR = 1

25. VAR VAR = 100 – 50 logMAR 20/20 – logMAR = 0.00 20/200 – logMAR = 1.00
100 – 50(0.00) = 100
20/200 – logMAR = 1.00
100-50(1.00) = 50
20/2000 – logMAR = 2.00
100-50(2.00) = 0
20/16 – logMAR = 105

26. Functional acuity score
FAS = (VARod+VARos+3VARou)/5
VAR guides to the evaluation of permanent impairment
5:4 change btw MAR & VAR

27. Visual Efficiancy 1925 – Snell & Sterling
Use for legal and compensating purposes
Diffusing filters – Equal decrease in vision
20/20 – VE = 1.0/ 100%
20/200 – VE =0.2/20%
VE = 0.2(mar-1/9)
Log(VE%) = – (MAR)

28. Extended to quantification of VF & OM
Adopted by AMA
Extended to quantification of VF & OM
Overall efficiency – a product of acuity, fields & motility efficiency scores
MONOCULAR efficiency of both eyes with THRICE the weightage to the better eye
Guides to the evaluation of permanent impairement,5th ed.,

30. Chart Designs SNELLEN Bailey lovie TAYLOR – tumbling E chart
Ferris & colleagues – ETDRS – Sloan letters
Strong & Woo – columnar size progression with masking bars
Others( Johnston, Hyvarinen, Landolt)

31. SNELLEN CHART 7 Different lines One letter – largest at the top
Classical chart size sequence is 200,100,70,50,40,30&20
Modified Snellen charts
STANDARD Snellen charts

32. Bailey – Lovie design principles
A logarithmic size progression
Same no. of letters at each size level
Spacing b/w letters and rows proportional to letter size
Equal and average legibility for letters at each size level
logMAR units for lines and letters

33. Design features Logarithmic size progression
Green , Sloan , Westheimer
Peripheral visual dif retinal eccentricities on a logarithmic scale is constant
0.1 log units progression commonly used
1.2589:1 ratio(5:4 approx.)
1.0,1.25,1.6,2.0,2.5,3.2,4.0,5.0,6.3,8.0,10,12.5,16…
(6.0,7.5,9.5,12,15,19,24,30,38,48,60,75,95….)

34. Number of optotypes at each size level
More letters – more reliability
Finer size progression – more reliability
SD = k√p/n
SD= standard Deviation
K= constant
p= size progression in log units
N= no. of letters at each line

35. Letter Legibility Concilium Ophthalmological Universale – C
ETDRS – golden standard
LETTERS preferred to C&E
Guessing ratio
Relativity b/w gesture & vocal decisions
ETDRS – SLOAN letters
Bailey-lovie – 1968 British standard

36. Spacing b/w letters and words
Flom & colleagues – Contour Interaction
Crowding effect
BAILEY & RAASCH – expt.(0.5 – 3.0)
Two fold spacing improved VA score by 0.03,0.04&0.07 respectively for british, sloan& landolt rings
Wider Spacing – better VA

37. Chart formats Printed Panel Charts Projector Charts
Self illuminated
Back illuminated
Charts On Display Screens
150cd/m2
300cd/m2

38. Chart luminance Moderate photopic luminance+subdued room lightning
Recommendations –
85-300cd/m2(general)
Sheedy & colleagues – twice luminance(5%MAR)
160m2(stnd)
120m2 min(British stnd)
80-320cd/m2(clinical tolerance)&low contrast targets

39. Refractive correction
Unaided VA
Uncorrected VA
Habitual VA
Corrected VA/BCVA
Optimal VA
Pinhole VA& PAM
VA under Special Illumination

40. Testing distance Visual infinity…??!!??
Spatial constraints of examination room
10-30 foot range
Distance appropriate testing charts
Low vision
Pre-presbyopes – proximal accomodation

41. Testing procedures Monocular VA Binocular VA Occluder Hand-palm
Worst eye first if known
Binocular VA
Both eyes open
Better than/equal to VA of better eye usually

42. Can be started from a little better than expected VA Guess work?!?
Snellen
>50%
Landolt
20%
All above & None below

43. MISSING letters
JUMPING letters
SHUFFLING letters
ECCENTRIC viewing
FLIP CHARTS
Rosser – abbreviated charts
Camparini and colleagues – first letter in each row

44. Scores Row by Row scoring Letter by Letter scoring Widely practiced
Too coarse
Results not reliable
Partial credit - ± recording
Letter by Letter scoring
± recording
Equal letters – equal points for qualification

45. In Research More sampling Standardized procedures and instructions
Memorizing
Change in distance
Paired charts
Auto generated charts
S – charts

46. Pediatric tests Limited ability to respond to stnd. Test stimuli
Capability of patient to respond
Grating acuity tests
VEP tests
Preferential looking tests
Optokinetic nystagmus
Flash card tests – pictures, letters
Picture or Symbol charts

47. Grating Acuity Tests Striped/Checkered grating targets
Finest pattern that can elicit the response
Response can be elicited subj/obj
Cpd – MAR
MAR = 30/cpd
30 cpd grating equal to 20/20
3 cpd grating equal to 20/200

48. Visual Evoked Potential
For least responsive patients
Measuring electrical potentials due to vision
Flickering striped / checkered pattern
EP magnitude declines with finer detail
Smallest spatial frequency(in CPD) with a measurable response is taken as VA

49. Preferential looking tests
Dobson & Teller – Mc Donald & co-workers
Teller acuity cards – finest grating
Cardiff cards
Flanking lines on either side

51. Optokinetic nystagmus
Rotating drum/video with spatial pattern
“Railroad nystagmus” eye movements
Finest grating showing nystagmus movements

52. Flash card tests A little ability to respond to instructions
Pointing, naming, matching….
Bailey hall cereal test
LH symbols(Lea symbols)
Lighthouse flash cards(umbrella, apple, house)
Broken wheel test
Allen picture cards – simple lined drawings

53. Letter flash cards Naming & matching H, O, T, V – Mirror reversible
E & C

54. NEAR VISUAL ACUITY Arm’s length 40 cm stnd. Distance
Should be comparable with DVA if the illumination is same
But accommodation should be at rest
Pupillary constriction (in PSCC)
Typeset materials – sentences, paragraphs, unrelated words

55. Designation of NVA M units Sloan and Habel Lower case letters
5 mins of arc
1.0 M units subtends 5 min of arc at 1 meter distance
1.45 mm height
Snellen fraction – 0.40/1.0M
Jose & Atcherson – M = 0.7*smallest letter size(mm)

56. Points Printing industry 1 point = 1/72th of an inch
Small letters(a, c, e, m, n, o, r, s, u, v, w, x, z)- 4 p
Descenders (g, j, p, q, y) – 8 p
Ascenders( b, d, f, I, j, k, l, t) – 8 p
4/72 – 1.41mm – 8p = 1.0M
Caps – 8 p = 1.5 M

57. N Notation UK standard Times New Roman font Size in points
Recorded by smallest print read by the subject
Distance is specified
40 cm
Print size divided by 8 gives M units

58. Equivalent Snellen notation
Reduced snellen
40 cm usually assumed
cm = 0.4/1.0 = 20/50
Varies with distance
Inappropriate to use angular distance for height
Suggests 20 foot is suggested but irrelavent

59. Jaeger Notation J followed by number Used widely by ophthalmologists
No standardization
Smaller no., larger size
Should not be used

60. IMP things to remember Reading & letter chart acuity
Near visual acuity verses near vision adequacy
Resolution limit
Reading efficiency
Logarithmic scaling

61. Purposes Refraction and prescribing decisions Monitoring ocular health
Insurances
Treatment
Contrast
Disability glare
PAM

62. Thank you...!!!