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Lensometry Flashcards

1
Q

the purpose of the lensometer is to….

A

to measure the vertex power of the lens

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2
Q

we measure the vertex power of a lens to… (3 things)

A
  1. measure pts habitual correction
  2. verfiy the rx of new glasses receiced from the lab (power, axis, bifocal add, PD, prism)
  3. verfiy dioptric power of CL
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3
Q

the lensometer measures the vertex power of the len’s ______

A

surface

NOT FOCAL LENGTH

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4
Q

Reciprocal of the distance btwn a surface of the lens and its secondary focal point

A

vertex power

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5
Q

the common term for measure of power of a lens using a lensometer is…

A

neurtralize

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6
Q

vertex power is not true….

A

focal lenght

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7
Q

the back vertex power is…..

A

measured from the back surface (faces eye)

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8
Q

the front vertex power is….

A

meausred from the surface that faces light

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9
Q

which is most commonly used vertex power in opthalmic optics

A

back vertex power

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10
Q

where the optical axis of a lens intersects the front and back surfaces

A

optical center of a lens

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11
Q

distance btwn the visual axes of the pts eye

A

PD

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12
Q

distance btwn optical centers of the 2 ophthalmic lenses

A

spectacle PD

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13
Q

what are the measurments of PD, spectacle PD, and optical center of a lens given in

A

mm

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14
Q

for spectacle pd, you typically want to match the _______ of the pts lenses to the _______ of the 2 eyes

A

optical center

visual axes

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15
Q

a compact holder (transparent plexiglass) used for inspection and neutralization of soft CL

A

wet cell

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16
Q

the eyecup is a rubber ring that protects the outer-most lens and your eyeglasses lens from damage. it also helps block peripheral light. the focus ring allows focusing the instrument to your indiv. eye

A

eyepiece

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17
Q

the metal ring rotates the ______

A

reticle

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18
Q

rotates target and indicates direction of principle meridians of the lens

A

axis wheel

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19
Q

indicate power in diopters of the lensometer

A

power wheel/drum

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20
Q

platform where the glasses are placed w/ a brace to hold the glasses in place

A

lens stage

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21
Q

the handle of this allows to lock the lenses in place

A

lens holder

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22
Q

what the back surface of the lens rest in

A

lens-stop

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23
Q

how do you change vergence of rays in the lensometer (2)

A
  1. moving the target (mires)

2. moving the light source

24
Q

brings the target (light) to infinity when the instrument is set to zero (plano) and there is no unknown lens placed to be measured

A

standard or badal lens (or collimator lens)

25
these are the cross-hatched series of lines (2 names)
target | mires
26
moved to effect a chnage in divergence and thus measure the power of the unknown
light source | ALWAYS KEEP A SPARE!
27
some lensometers (topocon) have a prism measuring device called _____ mounted at the end of the objective
Risley prism | make sure its set to 0 and 90 b4 starting
28
the telescope contains these 3 things in order of the lensometer
eyepiece reticle objective
29
the badal system is the
standard lens target light source
30
with zero setting and NO glasses, target's image (mire) is seen at ______
infinity
31
when an unknown lens is introduced, the image is blurry bc....
the telescope is no longer collimated (parallel rayss) and the image of the target (mire) is blurry
32
what do you need to do to bring the image to focus
move the target (mires) by an amount equal to distance it takes to make the rays parallel again ( in focuse)
33
what happens to the light and the mires when a + lens is introduced
light is converged and mires are out of focus
34
how do you clear the image of the + lens
target is moved closer, generating more divergent rays
35
what happens to the light when a - lens is introduced
light is diverged
36
how do you clear the image of a - lens
target needs to be moved further away to generate more converging rays
37
every ____mm of movement equals 1D of power
2.5
38
what are the steps to measuring a SV lens 1. 2. focus the _____ 3. determine _______ 4. calibrate ______ 5. position _______ 6. center the ______ 7. check the ______ 8. chck the ______ and ______ 9. mark the ______ 10. repeat ______ for the _____ lens 11. turn _____ 12. _____results
1. Turns lensometer on 2. focus the black reticle 3. determine which is sphere and cyl lines (thinner lines is sphere) 4. calibrate the mires (focus w/ drum)w 5. the glasses ( right lens first) on the stage 6. lens 7. sphere power 8. cyl power and axis 9. optical center 10. repeat 5-9 for OS 11. turn off 12. record
39
when you focus the reticle, first turn the eyepice all the way ______ and then slowly _____ until the reticle is shart and clear
counter-clockwise | clockwise
40
it your pts optical axis (center of pupil) is not aligned w/ the opitcal center of the lens (pt PD and spec PD dont match) there will be an ...
induced prismatic effect
41
the prismatic effect depends on the....
power of the lens | and how far away the pupil is from the lens' opticla center
42
how do you measure induced prism
calculate (prentice rule) | measred w/ lensometer
43
what is prentice's rule
prism power = F x d / 10 F=power of the lens d=decentration in mm
44
what does the direction of the prism base depend on
1. the location of the pt's line of sight | 2. whether the lens is minus or plus
45
2 prisms are placed base to base
plus lens
46
2 prisms are placed apex to apex
minus lens
47
to meausre prism power.... 1. dot each lens using pts _____ 2. place glasses in ______ 3. determine location of the ____ o fmires on the ring 4. use concentric rings of ____ to calculate amount of prism 5. determin base _____
1. PD 2. lensomter 3. center 4. reticle 5. direction of prism
48
w/ a risley prism, center ___ on reticle then ___prism amnt from the scale
mires | record
49
what do you do when the prism is too large that it is outside your view
use hand held prism to bring the image in
50
in a line bifocal, the carrier lens is the _______ | the segment is the.....
distance vision rx | near add
51
which vertex is used to determine the bifocal add
front vertex power
52
in trifocals: the in btwn distance and near (power usually 50% of near add power)
intermediate add
53
the greatest plus power, near (reading) portion
near add
54
btwn 4D and 8D there is a ____ difference in the add
0.25D
55
markings indicate ad power in ...
progressive lenses
56
the lens stage and holder contains 2 things:
``` marking device (3 ink dots) and lens stop ```
57
what is the distance portion of the lens called
carrier- holds near and intermiediate power lenses

PPO (14 decks)

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