13. Optics of Low Vision Aids Flashcards
how do most optical system act in LVA
increasing the angle subtended by the object at the eye thus producing an enlarged retinal image
magnifying power can be defined as
retinal image size with use of instrument / retinal image size without use of instrument
what lens can be used in glasses
high power simplex convex lens
ie enhanced presbypoic correction
what does this show
as the object moves nearer to the first principal focus F1 the virtual image becomes larger and is situated further from the eye
field of vision with a convex lens is dependant on
size or aperture of the lens
the greater the eye-lens distance
the smaller the field of vision
convex cyclindircal lenses
bar shaped lens
no refractive powers or only a low converging power in its long axis
and a high concerging power in cross section
procduces vertical magnficaiton
A: +2.50DS
B and E: Principle plane
C and F: Retina
D: +5.00DS
what makes up a galilean systemt
Convex objective lens
Concave eye piece lens
Which are both separated by difference of their focal lengths
a galilean system produces an image with what features
Erect, magnified
Not distorted by curvature of field, or astigmatism
benefits of a galilean systemt
Compact and light
Suitable as magnifying aid mounted on spectacle frame
can adapted for near and distance
how does a galilean system work
magnifies by increasing the anlge subtended by the onekct at the eye
angular magnification of a galilean systemt is
angle of emergence / angle of incidnce
mathematically the angular magnification of a galilean system is
M = Fe/ Fo
where fe is power of the eye piehce lens in diptres
and Fo is power of objective lens in dioptres
