7. Optical prescriptions and spectacle lenses Flashcards
Spherical lenses
DS
Cylindrical lenses
DC with axis
Simple transposition of spheres
add the values together
Simple transposition of cylinder
Sum, sign, axis
o Sum: add the sphere and cylinder together
o Sign: change the sign of the cylinder (keep the value the same)
o Axis: add 90 (if at or less than 90) or subtract (if greater than 90)
Toric lens transposition
Transpose, numerator, BC axis, other DC
o Transpose so that DC has same sign as BC
o Numerator: DS – base curve
o BC axis: axis from (i) add/subtract 90
o Other DC: add DC from (i) to BC
Detection of lens type
view a cross through the lens and move it
Spherical lenses
cause no distortion to the cross, moved from up and down
Convex spherical lens cross test
opposite direction –> AGAINST movement
Concave spherical lens cross test
same direction –> WITH movement
astigmatic lenses cross test
distort the cross (unless axis coincides with lines), with rotation causing a scissor movement
minus astigmatic lens cross test
with cross
plus astigmatic lens cross test
against cross
neutralisation of power
o Once lens nature known via above – strength can be determined
o Using a lens of the opposite type until when superimposed there is no movement of the cross neutralised
o The unknown lens power is therefore the opposite sign
neutralising astigmatic lenses
– each meridian must be done separately
back vs front vertex pwoer
o Ideally need to be done via back vertex power but this can be in-practical
o Can use front vertex power, but more inaccurate
Inaccurate for curved lenses more than +2.0D
Can lead to an error of 0.50D for more power lenses
Geneva lens measure
o Can determine the surface power of the lens by measuring the surface curvature
o Total power of thin lens equals sum of its surface power
calibriation of geneva lens measure
o Calibrated for lenses made of crown glass (refractive index 1.523)
focimeter measurements
vertex power of lens, axis of astigmatic lenses, major powers of an astigmatic lens and power of prism
formed by
focusing system and observation system
focusing system of the focimeter
moveable illuminated target + fixed collimating lens (renders light parallel when located at F1)
observation system of the focimeter
focused at infinity) and graticule and protractor
measuring with the focimeter
Target is moved until image is focused
The distance moved is directly equal to the power in dioptres
Spherical lens = target is dots
Astigmatic lens = target is lines
In prisms the image with be displaced in proportion per 1D
Label A-C
A-collimating lens
B-viewing telescope
C-test lens
Automated focimeter
the degree to which a beam of light is deflected as it passes though a lens depends on the focal and prismatic power of the lens and the distance from its optical centre
