Visual acuity

Visual acuity

1. Visual Acuity OPTOM FASLU MUHAMMED

2. Vision

3. Vision Vision or visual perception is a complex integration of light sense, form sense, contrast sense & colour sense. Visual acuity is considered a measure of Form Sense

4. Visual Angle In term of visual angle , VA is defined as the reciprocal of the minimum resolvable VA measured in minutes of arc for a standard test pattern Angle subtended at the nodal point of the eye by the physical dimensions of an object in the visual field

5. Contd……. Depends up on Size of the object Distance from the eye

6. Components of visual acuity Minimum Visible Resolution Recognition Minimum Discriminable

7. Minimum Visible The ability to determine whether or not object is present.

8. Resolution Discrimination of two spatially separated targets is termed resolution The normal angular threshold of discrimination for resolution measures 30 – 60 sec of an arc . It is usually called Minimum Angle of Resolution ( MAR ) E

9. Recognition Not only discriminates the spatial characteristics of the test pattern but also identifies the pattern with which one has had an experience Familiar with the set of test figures The test targets are called optotypes G L N

10. Minimum Discriminable Minimum Discriminable refers to spatial distinction by an observer when the threshold is much lower than the ordinary acuity Vernier acuity

11. Factors affecting VA Stimulus related factors Luminance of test objects Geometrical configuration of the stimulus Contrast of the stimulus from the surround Influence of wavelength Exposure duration Interaction effects of the two targets

12. Contd…….. 2. Observer related factors Retinal locus of stimulation Pupil size Accommodation Effect of the eye movement Meridonial variation in acuity Optical element of the eye Developmental aspects

13. Principle measures of VA Unaided VA Habitual VA Optimal VA

14. Standard testing conditions Letter Size VA involves the ability to detect a gap subtending an angle of 1 minute of arc

15. Testing Distance 6 m distance ,Parallel rays of light.

16. Contd………. If testing distance must be less than 20 feet ,the use of a mirror system should be considered.

17. ILLUMINATION Illumination for VA testing can be consider in terms of either illuminance or luminance. ILLUMINANCE 12 to 20 foot candles LUMINANCE 10 to 16 foot lamberts.

18. Contd…….

19. Visual acuity Notations

20. Decimal Acuity Dividing the numerator of the Snellen fraction by its denominator. D.A decrease with increase in gap (or letter size.) D.A =8.726mm/height of letter. e.g :- 6/6 =1 6/12 =0.5

21. PERCENTAGE ACUITY D.A is multiplied by 100. If D.A is 1 P.A acuity is 100% D.A 0.1 P.A 10%

22. SNELLEN ACUITY Snellen Fraction =Testing Distance/Designation of smallest line read or Snellen Fraction =Testing Distance/Distance at which the smallest letter read subtends an angle of 5’ of arc

23. Contd…….. Snellen fraction stated either metric or english units at 6m distance or 20 feet The numerator of the snellen fraction is almost always 6 or 20 depending on whether the metric or english system is used To convert decimal acuity into the snellen fraction ; it is necessary only to multiply the numerator of the reciprocal of the decimal acuity

24. Contd…….. Eg: 0.5 1/ 0.5 = 2 2(6)= 12 or 2 (20) = 40 6/ 12 or 20/40

25. Contd………. Convert English Acuity to Metric Acuity Eng D.A Metric Acuity 20/20 1 6/6 20/200 0.1 6/60 20/10 2.0 6/3

26. Contd……… Communicating with patients Snellen fraction = testing distance / distance at which a normal eye can see the smallest letter read

27. Thank u ……….