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3F: Low Vision > Telescopes Nd Telemicroscopes > Flashcards

Telescopes Nd Telemicroscopes Flashcards

1
Q

What is the height of a 20/20 letter when using 20 foot test distance?

A

20 feet (6096mm) Tan (5/60)=x/6096mm 0.001454442069=x/6096mm 8.87mm=x 5=5m of arc for 20/20 letter

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2
Q

These priced distance magnification

A

Telescope

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3
Q

What do telescopes consist of

A

2 lenses, an objective (closest to object) and an ocular (also known as eyepiece)

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4
Q

In low vision telescopes, the ____ is always a plus lense

A

Objective

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5
Q

Galilean telescope

A

-aka terrestrial telescope -Minus ocular, plus objective

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6
Q

Keplarian telescope

A

-AKA astronomical telescope -plus ocular, plus objective

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7
Q

Lens separation (d) in a Galilean telescope

A

(d)=1/F1+1/F2 Then the lenses are separated such that the secondary focal point of the objective coincides with the primary focal point of the ocular, they relate an afocal system (has not dioptric power)

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8
Q

Afocal telescopes

A

Lenses are separated such that the seoncday focal point of the objective coincides with the primary focal point of the ocular They are vergence amplifiers

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9
Q

What is the separation between the lenses in a keplarian telescope that has a +5D objective and a +20D ocular?

A

D=1/F1+1/F2 F1=+5D F2=+20D 1/5+1/20 D=0.25m=25cm

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10
Q

What is the separation between the lenses in this Galilean telescope? (F1=+5D; F2=-20D)

A

1/5+(-1/20) D=0.15m=15cm

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11
Q

Angular magnification

A

M=(-F[ocular])/F[objective])

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12
Q

What is the angular magnification of a galialean telescope that has a +5 objective and -20D ocualr?

A

M=-(-20)/5=4x

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13
Q

What is the magnification of a keplarian telescope that has a +5 objective and a +20 ocular

A

M=-20/5=-4X (image is inverted)

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14
Q

Uncorrected refractive error and Galilean focusable telescopes: -5D myope, -20D ocular, +5D objective

A

-5D of ocular power needed to correct refractive error D=1/5 + 1/-15=0.133 M=-(-15)/5=3x

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15
Q

Uncorrected refractive error and Galilean focusable telescopes. +5D hyperope, -20D ocular, +5D objective

A

I uncorrected hyperopia adds 5D of power to ocular D=1/5 + (1/-25)=0.16 M=-(-25)/5=5X

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16
Q

Viewing near objects through fixed focus telescopes

A

-telescopes are vergence amplifiers -much more acommodation is needed to view near objects with a telescope compared to without Approximate formula: L’=M^2L

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17
Q

An object viewed from 50cm away through a 5x telescope

A

L=1/.5=2 L’=(5^2)2=50D

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18
Q

Exit pupil

A

-image of the objective as formed by the ocular -the closer the exit pupils is to the entrance pupil of the eye, the larger the field of view

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19
Q

Exit pupil in Galilean

A

Inside scope

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20
Q

Exit pupil in keplarian

A

Located outside scope

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21
Q

Length of Galilean

A

Shorter

22
Q

Length of keplarian

A

Longer

23
Q

FOV in Galilean

A

Smaller

24
Q

FOV in keplarian

A

Larger

25
Q

Weight of Galilean

A

Lighter

26
Q

Weight of keplarian

A

Heavier

27
Q

Cost of Galilean

A

Less

28
Q

Cost of keplarian

A

More

29
Q

What is the power of an afocal telescope with an objective of +10D and an ocular of +30D? What kind of telescope is this

A

M=(-F2)/F1 M=(-30)/10 M=-3X Keplarian

30
Q

Determining the telescope power

A

Divide the diameter of the entrance pupil by the diameter of the exit pupil The diameter of the entrance pupil is found by measuring the diameter of the visible opening at the objective lens. The diameter of the exit pupil is found by measureing the small circle of light seen from the ocular side when holding the telescope away from the eye

31
Q

Diagnosis test sequence for Rxing telescopes

A

-determine best distance correction -measure BCVA -determine goal acuity -calculate magnification needed -demonstrate appropriate telescope magnification -explain available options

32
Q

RXing telescopes

A

-use your equation for determining needed magnification -mag=reference size/goal size -what you’ve got over what you need

33
Q

If the patients BCVA is 20/100 and they want to see 20/20, what telescope do they need

A

M=100/20 M=5X

34
Q

If the patients BCVA is 20/100 and their goal is 20/50, what telescope do they need?

A

M=100/50 =2X

35
Q

When Rxing telescopes for pateitns and they dont have a specific goal, what do we choose for a goal size?

A

20/40

36
Q

Your patient read 10/60 on the Feinbloom chart. They want to see the board from their seat (20 feet from the board) at school. Assuming 20/40 acuity would be adequate to do so, what is the min mag you would predict to meet their needs?

A

60/40=1.5X

37
Q

Things to consider when decision telescope format

A

How important is -FOV -cost -cosmesis -weight Do they need hands free -spec mounted? -monocular or binocular?

38
Q

Practical limits of telescopes

A

2x to 8x -telescopes beyond this range have an extremely limited field of view and it is difficult to maintain a steady image -Rx the lowest power that will be adequate

39
Q

Demonstrations of telescopes to patients

A

-pre-focus the telescope to be sure lenses are clean and scratch free -begin with a Galilean scope to see how well they understand using a telescope, typically 2.5x -if they struggle with a Galilean, they will need training before proceeding. Using a pair of binoculars, using an empty paper towel roll -if successful, show low power keplarian scope before moving to predicted telescope

40
Q

Hand held telescopes considerations

A

-power -Galilean vs keplarian -optical quality Conversion to clip on (allows same telescope to be used as hand held and spectacle mounted device)

41
Q

Using a telescope

A

-dousing typically requires 2 hands -hand holding telescope can serve as an occluder

42
Q

Hand held telescopes

A

-useful for spot checking -portable, inexpensive -can be worn around neck for convenience

43
Q

Numbers on the telescope 4 x 12

A

4x mag 12=objective diameter

44
Q

Telescope focusing at near

A

Some will focus at near

45
Q

Eschenback max TV; what type of telescope is it

A

Galilean

46
Q

Implantable miniature telescope centraSight

A

-implanted at the time of cataract surgery -only approved for endstage AMD -at least 65 years old -2.2 or 2.7x -lose peripheral vision in implanted eye -must be evaluated with an external telescope first

47
Q

Telemicroscopes

A

-combines features of telescope and microscope telescopes that is focuses at near -by adding a plus lens in front of the objective so that parallel light rays enter the telescope -by changing the tube length

48
Q

How do telescopes amplify vergence

A
49
Q

Magnification by a telemicroscope

A

Product of the magnification of the cap and that of the telescope -M(TMS)=M(cap)xM(TM) -M(TMS)=rF(cap)xM(TS) M(cap)=rF(cap)

50
Q

What letter size should a patient be abl to read if her near acuity with a +2.00D add over best correction is 0.5M/4M and she uses a 2x telescope wit ha 5D cap?

A

??

51
Q

What type of telescope is this

A
52
Q

What type of telescope is this

A

Galilean

3F: Low Vision (16 decks)

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