Lensometry, prism, and hand neutralization

Question 1

Where is the far point for a myopic eye?

Answer 1

before the cornea

Question 2

Where is the far point for a hyperopic eye?

Answer 2

after the retina

Question 3

Primary focal point

Answer 3

object-space focus. Distance from the optical surface to the primary focal point

Question 4

Primary focal point for a plus lens

Answer 4

point from which light must originate to emerge parallel from the lens. Image is at infinity

Question 5

Primary focal point for a minus lens

Answer 5

Point towards which the incident light must be directed in order for the image rays to emerge parallel

Question 6

collimated

Answer 6

rays are parallel

Question 7

secondary focal point

Answer 7

image-space focus

Question 8

secondary focal point for a plus lens

Answer 8

parallel rays from a distant point object are rejoined to form an image at that point.

Question 9

secondary focal point for minus lens

Answer 9

diverging rays seem to come from, after a parallel bundle of rays are refracted by a negative lens

Question 10

Plus lenses

Answer 10

magnify

Question 11

minus lenses

Answer 11

minify

Question 12

lensometer

Answer 12

lensmeter/focimeter/vertometer

Question 13

What does a lensometer measure?

Answer 13

sphere, cylinder, axis, add power, prism

Question 14

Down stream vergence

Answer 14

P/(1-[t/n2)P

Question 15

lens power

Answer 15

(n’-n)/r

Question 16

Approximate power

Answer 16

• P1+P2.
• Matches closely with back vertex power of minus lens.
• matches closely with thin lens
• cannot be used for manufacturing because error in app can be significant
• convenient/simple

Question 17

Front vertex power

Answer 17

also called the neutralizing power.
=P2/(1-[t/n2]P2) + P1
Used in hand neutralization

Question 18

Back vertex Power

Answer 18

Badal
Standard spectacle lens correction
Used for specifying power of spectacle lenses
=P1/(1-[t/n2]P1) +P2

Question 19

equivalent power

Answer 19

P1 + P2 + (t/n2)P1P2

Different lenses with the same EP can provide differring corrective powers

Question 20

Effective power

Answer 20

P/(1-[t/n2]P)

• used for things at a distance, needs to be addressed when RXing CL
• the further away, the more power is needed

Question 21

Why is effectivity important?

Answer 21

accomodative demand
CL calculations
Refractive surgery calcs

Question 22

Practice problem in this slide show

Answer 22

do it

Question 23

astigmatism

Answer 23

common vision condition that causes blurred vision. It occurs when the cornea is irregularly shaped or sometimes because the curvature of the lens in the eye

Question 24

spherecylindrical lenses

Answer 24

axes cannot exceed 180

Question 25

How does the angle increase for cyl

Answer 25

from patients left to right

Question 26

lensometry

Answer 26

- measures back vertex power in diopters | - uses a standard lens of known power

Question 27

What is the effect of prism?

Answer 27

- deviation of light rays towards the base of prism | - the image will appear to move towards the apex of the prism

Question 28

where does the image appear to move towards in a prism?

Answer 28

the apex

Question 29

magnification

Answer 29

m=-i/o

Question 30

Abbe number

Answer 30

- constringence or V-number - transparent material's chromatic dispersion - shows the variation of refractive index versus the wavelength - High Abbe, Low dispersion

Question 31

what can affect refractive index?

Answer 31

wavelength and temperature

Question 32

what color has the lowest index of refraction?

Answer 32

red

Question 33

what color has the highest index of refraction

Answer 33

violet

Question 34

How do you measure prism?

Answer 34

1 prism diopter creates a deviation of 1cm at 1 m 5 prism diopters creates a deviation of 5cm at 1m

Question 35

describe orientation of image in BI

Answer 35

image goes out

Question 36

describe orientation of image in BO

Answer 36

image goes in

Question 37

describe orientation of image in BU

Answer 37

image goes down

Question 38

describe orientation of BD

Answer 38

image goes up

Question 39

amount of prism

Answer 39

prism=P x d (in cm)

Question 40

prentice's rule

Answer 40

prims=P x d (in cm)

Question 41

hand neutralization for minus lens

Answer 41

with motion upright minified

Question 42

hand neutralization for plus lens

Answer 42

against motion inverted magnified

Question 43

How do you find front vertex?

Answer 43

hand neutralization

Question 44

how do you find back vertex?

Answer 44

lensometer

Question 45

What is the back surface power of first lens equal and opposite to in thick lens?

Answer 45

the neutralizing power in the second lens

Question 46

On a thick lens, if the unknown lens is placed in back...

Answer 46

you are determining the neutralization power of the unknown lens using the back vertex power of the known lens

Question 47

in thick lens, if the known lens is placed in back, you are determining...

Answer 47

the back vertex power of the unknown lens based on the neutralization power of the front lens (which might be different than the known back vertex power

Question 48

spherocylindrical

Answer 48

turning this type of lens gives a scissors motion