7. Optical prescriptions and spectacle lenses

Question 1

Spherical lenses

Answer 1

DS

Question 2

Cylindrical lenses

Answer 2

DC with axis

Question 3

Simple transposition of spheres

Answer 3

add the values together

Question 4

Simple transposition of cylinder

Answer 4

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)

Question 5

Toric lens transposition

Answer 5

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

Question 6

Detection of lens type

Answer 6

view a cross through the lens and move it

Question 7

Spherical lenses

Answer 7

cause no distortion to the cross, moved from up and down

Question 8

Convex spherical lens cross test

Answer 8

opposite direction –> AGAINST movement

Question 9

Concave spherical lens cross test

Answer 9

same direction –> WITH movement

Question 10

astigmatic lenses cross test

Answer 10

distort the cross (unless axis coincides with lines), with rotation causing a scissor movement

Question 11

minus astigmatic lens cross test

Answer 11

with cross

Question 12

plus astigmatic lens cross test

Answer 12

against cross

Question 13

neutralisation of power

Answer 13

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

Question 14

neutralising astigmatic lenses

Answer 14

– each meridian must be done separately

Question 15

back vs front vertex pwoer

Answer 15

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

Question 16

Geneva lens measure

Answer 16

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

Question 17

calibriation of geneva lens measure

Answer 17

o Calibrated for lenses made of crown glass (refractive index 1.523)

Question 18

focimeter measurements

Answer 18

vertex power of lens, axis of astigmatic lenses, major powers of an astigmatic lens and power of prism

Question 19

formed by

Answer 19

focusing system and observation system

Question 20

focusing system of the focimeter

Answer 20

moveable illuminated target + fixed collimating lens (renders light parallel when located at F1)

Question 21

observation system of the focimeter

Answer 21

focused at infinity) and graticule and protractor

Question 22

measuring with the focimeter

Answer 22

 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

Question 23

Label A-C

Answer 23

A-collimating lens
B-viewing telescope
C-test lens

Question 24

Automated focimeter

Answer 24

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

Question 25

tinted lenses absoptive

Answer 25

Cr, NiCr, MgF2, SiO

Question 26

tinted lenses reflectvie

Answer 26

Cr NiCr

Question 27

UV filters

Answer 27

CR39 lenses absorb UV light shorter than 370nm

Question 28

Photochromatic lenses

Answer 28

o Changes transmission based on intensity of incident radiation
o Become darker in brighter lights
o Darkening more rapid thatn lightening

Question 29

heat and photochromatic lenses

Answer 29

o Heat opposes the effect of light – darken more easily when cold

Question 30

glass photochromatic lenses

Answer 30

colourless silver-halide crystals suspended in borosilicate

Question 31

plastic photochromatic lenses

Answer 31

o Organic photochromic compounds can be used for plastic lenses

Question 32

anti-reflective coatings

Answer 32

o Reflection can be reduced by addition a material with the thickness of which is a quarter of the wavelength of incident light
o Causes destructive interference