Visual acuity, visual acuity types and factors

Types of visual acuity and factors

1. BY. Farah Naheed B.Sc (Hons.)Optometry and Orthoptics Pakistan Institute Of Ophthalmology , Al- Shifa Trust Eye Hospital Rawalpindi. Distance V isual Acuity

2. Visual Acuity Visual Angle Basics of Acuity charts Acuity charts Types of Visual Acuity Factors Affecting V/A Clinical Measurements Procedure of testing V/A Contents

3. I know your situation is like this right now …BUT……

4. Vision ……?

5. Resolving power of the eye or the ability to see two separate objects as separate. Visual Acuity…?

6. The angle subtended at the nodal point of the eye by the physical dimensions of an object in the visual field. Nodal Point…? Visual Angle…

7. Basics of Acuity charts… The distant acuity is usually tested by snellen chart. Consist of a series of black capital letters on a white board. Each letters fits in a square. Assume 5x5 grid for letters with detail separation of 1/5 of letter size.

8. Snellen chart E-chart C-chart( Landolt ring or Broken ring chart) Visual Acuity Charts:

9. Minimum Detectable ( visible) Minimum Separable (resolution) Minimum Recognizable (recognition) Minimum Discriminable ( hyperacuity) Types of Visual Acuity :

10. It is also called Visible . Ability to determine whether an object is present or absent in a visual field. The min. angle of detection is 1 seconds of arc. Minimum Detectable :

11. B) A White Dot against Black BG A) A Black Dot against White BG

12. It is also called Resolution . Ability to determine a group of points or lines as separate objects. The min. angle of resolution is 30-60 seconds of an arc. Minimum Separable

13. It is also called Recognition . It involves discrimination of the spatial characteristics of the test pattern with which he has previous experience. Forexample: Identification of faces etc. Minimum Recognisable:

14. Minimum Discriminable It is also called Hyperacuity. Ability to determine whether or not two parallel and straight lines are aligned. Forexample: Vernier Acuity.

15. A few of you right now …

16. Overall Illumination of test object Contrast of letters Pupil size Cognitive ability Refractive error Factors affecting visual acuity

17. Criteria: Normal testing distance (distance VA): 6m Illuminated visual acuity chart. Proper room illumination. Steps: Unaided VA Aided VA Pinhole VA Clinical Measurements of Visual Acuity:

18. For testing distant visual acuity, the patient is seated at a distance of 6m from the Snellen’s chart. The patient is asked to read the chart with each eye separately. When the patient is able to read up to 6 m line, the visual acuity is recorded as 6/6, which is normal. Procedure of testing:

19. 4) Depending upon the smallest line which the patient can read from the distance of 6 m, his vision is recorded as 6/9, 6/12, 6/18, 6/24, 6/36 and 6/60, respectively. 5) If he cannot see the top line from 6 m, he is asked to slowly walk towards the chart till he can read the top line. Depending upon the distance at which he can read the top line, his vision is recorded as 5/60, 4/60, 3/60, 2/60 and 1/60, respectively. 6) If the patient is unable to read the top line even from 1 m, he is asked to count fingers (CF) of the examiner. His vision is recorded as CF-3’,CF-2’, CF-1’

20. When the patient fails to count fingers, the examiner moves his hand close to the patient’s face. If he can appreciate the hand movements (HM), visual acuity is recorded as HM positive. When the patient cannot distinguish the hand movements, he is taken into the dark room and a light is focused on his eye and he is asked to say when the light is on and when it is off. If he succeeds in doing this,If vision is recorded as PL + ve and if not it is recorded as PL – ve .