Visualacuity
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The document discusses various aspects of visual acuity, including: 1. It defines the resolving power of the eye and different levels of visual acuity including minimum detectable, minimum separable, minimum cognizable, and minimum discriminable. 2. It describes how the angle subtended at the nodal point of the eye determines whether two objects are seen as separate. 3. It discusses different tests used to measure visual acuity in adults and children, including Snellen charts, Landolt C charts, and preferential looking tests for infants.
Visualacuity
- 2. Resolving power of the eye or the ability to see two separate objects as separate.
- 3. The angle subtended at the nodal point of the eye by the physical dimensions of an object in the visual field.
- 4. Receptor theory: If the images fell on two cones separated by an unilluminated cone, then the points of light would be perceived as two distinct sources. Width of foveal cones-2.5microns Means that two distinct points only can be recognized as separate when they subtend an angle of 1’ of arc.
- 5. When light from a point source passes through a small aperture ,it doe not produce a bright dot as an image,rather it forms a difffuse circular disc known as AIRY DISC due to diffraction property of light. Airy dics is surrounded by much fainter concentric rings Two “point” (unresolved) sources are resolved from each other when separated by at least the radius of the airy disk. According to reyleigh,2 closely spaced airy discs are distinct if they are farther apart than the distance at which principal maximum of 1 coincides with first minimum of 2nd airy disc
- 7. Minimum Detectable ( visible) Minimum Separable (resolution) Minimum cognizable (recognition) Minimum Discriminable ( hyperacuity)
- 8. MINIMUM DETECTABLE: Ability to determine whether or not an object is present in a visual field. The limit for this kind of acuity is ~ 1 arc second.
- 9. Discrimination of 2 spatially separated targets. Assessment of function of fovea centralis Distance between 2 targets is specified by angle subtended at nodal Point Of eye. Normal angular threshold of discrimination for resolution 30-60 seconds of an arc. (minimum angle of resolution).
- 10. : not only discrimination of the spatial characteristics of the test pattern but also the pattern with which PERSON has previous experience (Identification of faces, letters, symbols, pictures etc.)
- 11. Ability to determine whether or not two parallel and straight lines are aligned in the frontal plane. The threshold value of verneir acuity is only 2-10 seconds of an arc.
- 12. Criteria: Normal testing distance (distance VA): 6m For near : 33cm/ 40cm. Illuminated visual acuity chart. Proper room illumination. Steps: Unaided VA Aided VA Pinhole VA
- 13. 6m distance
- 14. Overall Illumination of test object. Contrast of letters. Pupil size Cognitive ability Ocular health Refractive Status
- 15. Stimulus related Luminance of test object Geometrical configuration Contrast difference of stimulus from background Influence of wavelength Exposure duration of stimulus Interaction effects of 2 targets Observer related Retinal locus of stimulation Pupil size Accommodation Effect of eye movement Optical elements of eye
- 16. DOT VISUAL ACUITY TEST CATFORD DRUM TEST STYCAR GRADED BALLS TEST RECOGNITION ACUITY TEST SNELLENS CHART LIPMANS HOTV CHART SNELLEN E CHART LANDOLTS C CHART ALLEN PICTURE CARD TEST LIGHT HOUSE TEST CARDIFF ACUITY BAILEY HALL CEREAL TEST
- 17. OPTOKINETIC NYSTAGMUS TEST PREFERENTIAL LOOKING TEST VISUALLY EVOKED RESPONSE
- 19. Snellen Fraction is the most common notation of acuity. The distant acuity is usually tested by snellen chart. Consist of a series of black capital letters on a white board,arranged in lines,each progressively diminishing in size. Each letters fits in a square,, sides of which are 5 times the breadth of constituent lines Assume 5x5 grid for letters with detail separation of 1/5 of letter size.
- 21. Log of the MAR used to notate the acuity Used in research and low vision ◦ Their chart and its derivatives follow the principles: Geometric progression of size & spacing by 0.1 Log unit or 4/5 ◦ Found that the relative spacing between letters and the number of letters per row can cause substantial variation in acuity scores
- 22. It is flat and typical “V” shape. Testing distance 4m. Every line has 5 letters . Point one (0.1) logarithmic progression. (5:4) Between two letter space is the width of a single letter. Between two line space is the height of the lower line letter. Acuity becomes worse as the log MAR value increases Decrease brightness line by line
- 23. Letter to letter Acuity assessment Every letter that is correctly read deducts 0.02 from that line Example: If the patient reads all the letters of the logMAR 0.40 line (20/50) and two letters from the 0.30 line (20/40), it is scored as 0.36 Snellen Equivalent to this would be 20/50+2
- 24. With toddlers and slightly older 'verbal' children, simple pictures constructed on Snellen's principles may be used. A very effective test is the 'E-test' in which the examiner holds cards on which the letter E is printed, in various positions and in various sizes. If the test is treated as a game, the child standing 6 m away will readily respond on request by indicating the direction of the letter with his hand or by holding a similar card in the same position so long as he sees it. A similar test is the Landolt C chart. This is the chart containing a series of broken rings, with each gap subtending an angle of 1 minute at nodal point at a given distance SHERIDAN GARDINER HOTV TEST-Child is handed charts with H,O,T,V and are asked to match with letters in distant chart
- 26. Dot visual acuity test- child is shown illuminated box with black dots of different size.smallest dot identified denotes visual acuity of child. Miniature toy test- Child is shown a miniature toy from distance 10 ft and asked to pick the pair from assortment. CHILDREN 1-2 YRS MARBLE GAME TEST-child asked to put marbles into a box with each eye seperately with other occluded, vision of the eye is then noted as USEFUL OR LESS USEFUL SHERIDAN BALL TEST-Rolling ball on white background asking child to pick it up. Smallest sized ball to which child gives good response is rough way to estimate VA.
- 27. OPTOKINETC NYSTAGMUS Nystagmus is elicited by passing succession of black and white stripes through subjects field of action.the visual angle subtended by a smallest strip width that still elicits an eye movement is measure of VA.OKN reaches level of 6/30 at 6 months and 6/6 by 20-30months PREFERENTIAL LOOKING TEST Infant prefers to look at stripped pattern more than homogenous fields,in this examiner is hidden behind a screen in which 1 projects a homogenous surface on 1 side and black & white stripes on other .the 2 stimuli are alternated randomly The location of striped pattern is varied randomly from left to right and fineness of stripes is gradually reduced until there is no longer any correlation between judged direction of infants gaze and location of stripe. VA determined by this method ranges from approximately 6/240 in newborns to 6/60 at 3 months and 6/6 at 36 months of age.
- 30. EEG recording made from occipital lobe in response to visual stimuli. FLASH VER-integrity if macula and visual pathway PATTERN REVERSAL VER-Rough estimate of VA. VER studys show that VA in infants 6/120 at 1 month,6/60 2 months,6/6-6/12 at 6months to 1 year. CATFORD DRUM TEST DETECTION ACUITY TEST Child observes an oscillating drum with black dots of varying size.the smallest dot that evokes pendular eye movement denotes level of visual acuity.
- 32. Based on preferential fixation of cards which have a picture optotype and a blank located vertically.the picture optotypes are of same sizes but have been specially drawn with 2 dark lines with a white space in between such that picture is visible only at a particular distance or closer, these are called vanishing optotype as they vanish at farther distance The child can identify the picture by verbalizing,pointing or fixation preference.
- 34. DENSE CATARACT The LOTMAR RODENSTOCK laser interferometer forms a diffraction pattern of parallel lines on the retina even through a moderate cataract. Utilizes coherent white light/helium neon lazer generated interface fringes that are projected into retina through ocular media,width of fringes correspond to VA.The patient is asked to identify the orientation of progressively finer lines, to establish the visual acuity likely to be regained after surgery. REF ERR need not be corrected as projn of fringes occur over 3 dimensions and is not affected by ammetropia Potential acuity meter ( GUYTON MINKOUSKI) This is attached to a slit lamp & projects a tiny Snellen chart through pinhole clear area onto the retina around a lens opacity and the patient is required to read the alphabets.ref errors are compensated by apparatus
- 35. Visual acuity for near Near vision is tested by asking the patient to read the near vision chart, kept at a distance of 35 cm in good illumination, with each eye separately. In near vision charts, a series of different sizes of printer type are arranged in increasing order and marked accordingly Commonly used near vision charts are as follows: 1. Jaeger’s chart. In this chart, prints are marked from 1 to 7 and accordingly patient’s acuity is labelled as J1 to J7 depending upon the print he can read. 2. Roman test types. According to this chart, the near vision is recorded as N6, N8, N10, N12 and N18 (Printer’s point system). 3. Snellen’s near vision-graded thickness of different lines is about 1/17th of distant vision .in this event the letter equivalent to 6/6 will subtend an angle of 5 minutes at an average reading distance.
- 37. Snellen near vision chart
- 38. DUKE ELDERS PRACTICE OF REFRACTION KHURANA OPTICS AND REFRACTION PRADEEP SHARMA STRABISMUS SIMPLIFIED VARIOUS INTERNET SOURCES