W3: Near Visual acuity Flashcards
Importance of Near Vision
o Detection and monitoring of ocular disease
o Presbyopia- physiological changes in near vision with age
o Assessment of functional & quality of life
Reduced Snellen Chart
o For use at specified distance only (usually 40cm or 16” (in US)
o Typically record as Snellen Fraction e.g. 6/6 at 40 cm
o Must record the distance tested
o Can correlate distance and near acuities
Jagear notation
o Near acuity recorded with a J followed by
the ‘size’ – smaller numbers indicate smaller
print sizes (J1 – J20)
o Not standardized–different sizes between
different manufacturers even with the same
‘J’ number
Snellen fraction
Distance/ smallest M unit can be read
Adv and Dis of Bailey Lovie, Snellen, EDTRS, continuous text. pg21
Point spread function
Consider if tiny light point
If visual system had perfect optics the image on the retina would be identical to the original point of light.
o The relative intensity of this point of light were plotted as a function of distance, on the retina, such a plot would look like the dashed red line.
If visual system had perfect optics the image on the retina would be identical to the original point of light.
o The relative intensity of this point of light were plotted as a function of distance, on the retina, such a plot would look like the dashed red line
o To be able to resolve two stars, images on the retina
have to be a certain distance apar
Rayleighs criteria
Two points or lines are just resolved if the peak of one point spread function lies on the first trough of the other point spread function
radians = 1.22 λ/d