Physiological optics 3 rd lecture

1. Dr. Mohammad Shehadeh Physiological optics3rd lecture

2. Interference • When two waves of light travel along the same path, the effect produced depends upon whether or not the waves are in phase with one another. • If they are in phase, the resultant wave will be a summation of the two, and this is called constructive interference • If the two waves of equal amplitude are out of phase by half a cycle , they will cancel each other out: destructive interference. • The final effect in each case is as if the waves were superimposed and added (in the algebraic sense) to each other. • Phase differences of less than half a cycle thus result in a wave of intermediate amplitude

4. Destructive interference occurs within the stroma of the cornea. The collagen bundles of the stroma are so spaced that any light deviated by them is eliminated by destructive interference. • Interference phenomena are also utilised in optical instruments. One example is low reflection coatings which are applied to lens surfaces. The coating consists of a thin layer of transparent material of appropriate thickness. • Light reflected from the superficial surface of the layer and light reflected from the deep surface of the layer eliminate each other by destructive interference

6. Diffraction : definition • When a wave front encounters a narrow opening or the edge of an obstruction, the wave motion spreads out on the far side of the obstruction. • It is as if the edge of the obstruction acts as a new centre from which secondary wave fronts are produced which are out of phase with the primary waves.

8. The intensity of the light falling on zone AB is reduced to some extent by interference between the primary and secondary waves • The light falling on zone BC is derived from secondary waves alone and is of much lower intensity.

9. When light passes through a circular aperture, a circular diffraction pattern is produced. • This consists of a bright central disc surrounded by alternate dark and light rings. • The central bright zone is known as the Airy disc.

12. Diffraction effects are most marked with small apertures, and occur in all optical systems including lenses, optical instruments and the eye. • In the case of lenses and instruments, the diffraction effect at the apertures used is negligible compared with the other errors or aberrations of the system. • In the case of the eye, diffraction is the main source of image imperfection when the pupil is small. • However, the advantage of a large pupil in reducing diffraction is outweighed by the increased effect of the aberrations of the refractive elements of the eye

13. Limit of Resolution; Resolving Power • the smallest angle of separation (w) between two points which allows the formation of two discernible images by an optical system • The limit of resolution is reached when two Airy discs are separated so that the centre of one falls on the first dark ring of the other.

15. Tests of Visual Acuity –Resolving Power of the Eye • Babies are best examined when alert and not hungry. • Fixation with either eye should be central, steady and maintained (CSM). • The best target is a face (especially that of the mother), a toy, or a television cartoon. • A strong preference for one eye, indicated by an aversive response to occlusion of that eye, squint, nystagmus, roving gaze, and eye poking, all suggest poor visual acuity.

16. Visually directed reaching develops between two and five months of age. When the vision is poor, the movements are exploratory in nature • Much can be deduced from watching babies playing. • The ability of a child aged 15 months or older to pick up a tiny coloured 'hundreds and thousands' sweet suggests near visual acuity equivalent to 6/24 or better and the absence of a serious visual defect. • However, good near visual acuity may develop in the presence of high myopia.

17. 'hundreds and thousands

18. Catford drum • t has been found that targets of decreasing size may be presented until they become too small to be fixed and nystagmus is no longer elicited

19. Catford drum • comprises a white cylinder marked with black dots of increasing size corresponding to visual acuities ranging from 6/6 to 6/60 when viewed from 60 cm. • The drum is masked by a screen except for a rectangular aperture which exposes a single spot. • This spot is made to oscillate horizontally and stimulates corresponding eye movement if seen by the child. This test overestimates the acuity both because the target is moving and because the test is conducted at a short working distance

20. The STYCAR (Screening Test for Young Children And Retards) • rolling balls are ten white polystyrene spheres ranging in size from 3.5 mm to 6 cm in diameter. They are rolled across a well illuminated contrasting floor 3 m from the child. Pursuit eye movements indicate that they are seen.

21. preferential looking • Preferential looking can be used to assess the visual acuity of infants based upon their turning their head or eyes towards a patterned rather than a uniform target. • A black and white square wave grating (alternating black and white stripes) is presented simultaneously with a plain grey target of equal size and average luminance. • Children with better vision are able to see a finer grating and turn towards it. • Examples : teller and cardiff acuity cards

23. Occlusion test

24. Visual evoked potentials

25. Visual evoked potentials • Visual evoked potentials are the electrical responses generated in the occipital cortex by visual stimulation of the eye. The stimulus used is either a black and white square wave grating or a chequerboard pattern in which the pattern reverses at a set frequency

26. An optotype assisted VA tests • It is a symbol, the identification of which corresponds to a certain level of visual acuity. • All tests employ black letters or pictures on an opaque or retro-illuminated white background in order to maximize contrast

27. Testing of young children requires them to match the optotype letter or symbol on a card shown by the examiner who is 6 m or 3 m away by pointing to one of a group of matching letters on a key card.

28. Examples: 1- at age 2 years: kay pictures which is a picture naming test

29. 2- at age 3 years : matching of single leter optotype as in Sheridan-Gardiner test But it overestimates VA in amblyopic eye because it eliminates the crowding phenominon Keeler LogMAR crowded test is more accurate in amblyopia

30. Sheridan-Gardiner test

32. 3- at age 4 years: most children will be able to perform a linear snellen acuity test: • It is an optotype testing of the literate involves the naming of letters. • The test is based on the theory that the smallest object which can be resolved by the eye subtends the same visual angle at the nodal point of the eye as a cone photoreceptor, i.e. one minute of arc

33. The test employs a chart with rows of letters of diminishing size. Each row is accorded a number indicating the distance in metres at which a person with normal visual acuity should correctly identify the letters • The bars and spaces of each letter subtend an angle of one minute of a degree

34. The test chart is normally read from 6 m (20 feet). Thus, a subject who identifies the letters on the '12' line from 6 m has 6/12 vision (20/40) • 'Normal' visual acuity is 6/6 (20/20) although young adults often achieve 6/4 acuity.

36. LogMAR (LOGarithm of the Minimal Angle of Resolution) visual acuity charts • (e.g. the Bailey-Lovie test) • are more precise than the Snellen test because they have a regular progression in the size and spacing of the letters from one line to the next and the same number of letters on every line

37. Fig. 1.12 LogMAR chart for testing visual acuity LogMAR chart for testing visual acuity

38. Typical snellen chart

39. An offset of 3-5 seconds of arc is normally discernible. This is less than the limit of Snellen visual acuity and is therefore also called hyperacuity. An offset of 3-5 seconds of arc is normally discernible. This is less than the limit of Snellen visual acuity and is therefore also called hyperacuity. An offset of 3-5 seconds of arc is normally discernible. This is less than the limit of Snellen visual acuity and is therefore also called hyperacuity. Vernier acuity is the smallest offset of a line which can be detected • An offset of 3-5 seconds of arc is normally discernible. • This is less than the limit of Snellen visual acuity and is therefore also called hyperacuity.