VA & CS

1. Age

   • young- dev of fovea + neural connections= better VA

   • old- pupil miosis, reduced ret lum= reduced VA

2. Retinal eccentricity- density of cones and gang cells reduces with increasing RE- VA=6/30 at edge of macula

3. Neural limits- spots of light must fall on seperate photoreceptors seperated by 1 unstimulated photoreceptor

4. Pupil size- optimum = 2-3mm

   • small pupil= reduce ret lum= bad BUT reduce abber= good

   • large pupil= increase ret lum= good BUT increase abber=bad

5. Luminance- room = 30% of test chart luminance. Va plateus at high lum

6. Contour Interaction- crowding effect of optotypes

   • if number of letters on each line varies= crowding varies

   • crowding occur due to adjacent letters/ lines around each letter

7. Rx

1. Detection / discrimination- presence of an object against a plain background.

2. Localisation- spatial location of one target to next

3. Resolution- seperation of elements

4. Recognition- seperation of elements + identifying wha pattern/shape is.

** Resolution and Recognition are dependent on photoreceptor spacing + rx Bigger Rx= bigger blur circle = more overlap at retina

MAR

• smallest angle between 2 points that can be resolved by the eye

• 1arc minute - in healthy eye

AS

• size of the object given as an angle

• depends on distance and linear size of object

• measured in cycles per degree (cpd)

• 6/6= 1’= 30cpd

• higher SF= sharp edges of letter= smaller detail= harder to resolve

• bigger Rx affects high SF before low= edges disappear before filling

1. high contrast optotypes

2. well illuminated

3. optotypes recognisable by px

4. opto= equal legibility- some letters easily recognisable than others

5. opto= consistent spacing btwn lines

6. consistent number of opto on each line

7. cann be scored exactly

8. measures range of VA: no ceiling limit ( lack of large letters)

   no truncation( lack of small letters)

• no easy way to score VA

• unequal number of letters on each line

• irregular spacing btwn each letter = greater contour interaction

• irregular spacing between each line of letters

• has optotypes which are of unequal legibility

• British letters on a 5x4 grid

• AKA present day LogMAR chart

• scored logMAR

• each letter changes by 0.02logMAR

• each line is 1.26x smaller than above

• lines smaller than 6/6= -ve logMAR

  *to change from Snellen to LogMAR, divide lower number by 6 then log ans. eg. 6/24= log4= 0.60 logMAR

• 5x5 Sloan letters

• Uses LogMAR scoring

• Comes in various formats :

  • diff distances

  • diff letters

  • symbols

  • low contrast

• charts can be used at different distances:

  • 1/2 dist= add 0.03logMAR

  • 2x dist= subtract 0.03logMAR

    • lower logMAR score = better VA

• if whole line read= record logMAR score

• if line partly read= take score for line and add 0.02 for every letter missed

• for below example, work 1 line at a time:

  • for 0.2 line, add 0.02 for any letters missed, 2x0.02=0.04 = 0.24

  • for 0.1line, subtract 0.02 for any letters seen, 1x0.02=0.02

  • final score : 0.24- 0.02= +0.22

• CT= minimum difference in contrast that is detected

• CS= 1/CT, expressed as a log

• 0.5% contrast= 2.301 logCS

• 99% contrast= 0.004logCS

  • 0 is the biggest change in contrast i.e black on white

  • higher CS= easier to see

older charts= vistech CST, tested at 3m. had to identify orientation of grating but eye is less sensitive to oblique gratings

• Bailey Lovie + ETDRS availble in low contrast

• Pelli Robsonn chart

  • each triplet chnges contrast by 0.15logCS

  • used at 1m

  • letters are large so Rx doesnt matter

  • each letter is o.o5logCS

• Mars letter chart similar to Pelli Robson but hand held at 50cm

• CS reduced inocular disease:

  • Keratoconus

  • corneal dystrophies

  • CL wear

  • Cataract

  • Glaucoma

  • AMD

  • optic neuritis

  • Amblyopia

• indicates daily tasks that VI px stuggle with

• measuring px contrast reserve

graph of CS against SF

peaks at 3-5cpd= 6/60 to 636

at high SF we need high CS to see

cataract affects high and low SF

• if px has any ocular dieases mentioned

• older rivers

• px with VI (visual impairments) to slect LVAs (low vision aids)

• px who complain vision isnt clear with normal VA