Visual acuity Shafee. The definition SPATIAL RESOLVING CAPACITY OF THE VISUAL SYSTEM Angular size of...

Click here to load reader

  • date post

    23-Dec-2015
  • Category

    Documents

  • view

    215
  • download

    1

Embed Size (px)

Transcript of Visual acuity Shafee. The definition SPATIAL RESOLVING CAPACITY OF THE VISUAL SYSTEM Angular size of...

  • Slide 1
  • Visual acuity Shafee
  • Slide 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
  • Slide 3
  • Slide 4
  • OPTICAL LIMITATIONS Diffraction pattern AIRY DISC =2.44/p Where = diameter in radians =wavelength of light p=pupil diameter 1/p Rayleigh criterion for resolution min = 1.22 /p (or) 2.3/p Where min in min of arc & p is in mm
  • Slide 5
  • When p=4.6mm, min = 0.5min of arc =2.3mm, = 1.0min of arcetc Refractive errors/focusing errors Small pupils=diffraction Large pupils=aberrations Chromatic Optical P=2.5mm (max VA..,MAR =0.92)
  • Slide 6
  • Neural limitations Packing density of neural receptors Neural interactions in retina Visual pathway Unstimulated receptor(2m-4m) 0.82 min of arc resolution.. 16.67mm assumed nodal point dist from retina optical limit
  • Slide 7
  • Tests of visual resolution Measuring the limits of visual performanceto discern & to detect & to recognize MINIMUM DETECTABLE MINIMUM SEPERATION RECOGNITION RESOLUTION
  • Slide 8
  • Minimum detectable Threshold size for being recognized against a background Width of image???!!!??? Dependence?? Diffraction..!! Very small contrast Min. size?!?
  • Slide 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
  • Slide 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
  • Slide 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
  • Slide 12
  • Letter optotypes Grid patterns 55 54 56 SNELLEN-serifs-British standard(D,E,F,N,H,P,R,U,V,Z)-54 grid MODERN-sanserifs- Sloan(C,D,H,K,N,O,R,S,V,Z)-55 grid
  • Slide 13
  • 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
  • Slide 14
  • Tumbling E Illiterate E - 55 Different orientation at each acuity level Four alternative Eight alternative
  • Slide 15
  • Numerical & pictorial targets Animals,Catoons,Numbersetc 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
  • Slide 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.
  • Slide 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.
  • Slide 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.
  • Slide 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.
  • Slide 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)
  • Slide 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
  • Slide 22
  • Events in the HISTORY Lea Hyvrinen 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.
  • Slide 23
  • Designation of VA Snellen fraction Decimal notation MAR logMAR VAR & fAS VE
  • Slide 24
  • logMAR Logarithm of MAR Eg., 20/20 MAR = 1 LogMAR = log10(1.0) = 0.00 20/40 MAR = 2 logMAR = log10(2.0) = 0.30
  • Slide 25
  • VAR VAR = 100 50 logMAR 20/20 logMAR = 0.00 100 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
  • Slide 26
  • Functional acuity score FAS = (VARod+VARos+3VARou)/5 VAR guides to the evaluation of permanent impairment 5:4 change btw MAR & VAR
  • Slide 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%) = 2.0777 0.0777 (MAR)
  • Slide 28
  • 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,5 th ed.,
  • Slide 29
  • Slide 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)
  • Slide 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
  • Slide 32
  • Bailey Lovie design principles 1.A logarithmic size progression 2.Same no. of letters at each size level 3.Spacing b/w letters and rows proportional to letter size 4.Equal and average legibility for letters at each size level logMAR units for lines and letters
  • Slide 33
  • Design features Logarithmic size progression Green, Sloan, Westheimer Peripheral visual acuities @ 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.)
  • Slide 34
  • Number of optotypes at each size level More letters more reliability Finer size progression more reliability SD = kp/n SD= standard Deviation K= constant p= size progression in log units N= no. of letters at each line