9 - LV/Magnification Flashcards Preview

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Flashcards in 9 - LV/Magnification Deck (23)
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1

Describe angular magnification

Result of an incr in retinal image size of an object by introducing an optical system (i.e. LENSES USED) in bw the object and the eye
-hand-held/collimating
-stand/non-collimating

2

Difference bw lateral and angular magnification

L: compares object size to image size

A: compares original retinal image size to retinal image size when viewed thru an optical system

3

Hand-held magnifiers
-describe

High plus-powered lens

Puts object of interest at the primary focal point of the magnifier
-this creates an image at infinity by collimating outgoing light
-no need for accommodation when using

4

Hand-held magnifiers
-an object at the primary focal point

Once there, the HHM-object system can be moved anywhere in front of the eye and total magnification will remain the same
-as lens-eye distance changes, the spectacle mag of the lens changes, however there’s an equal but opposite change in RDM = 0

5

Hand-held magnifiers
-what changes/doesn’t change when HHM is moved closer/farther from eye

Changes: linear FOV (incr when brought closer to eye)

Does not change: angular/total mag, accommodation (not needed still)

6

Stand magnifiers
-describe

High plus-powered lens mounted such that object to lens disparity is FIXED
-for most SMs, the object is located INSIDE the primary focal point
-upright, magnified, virtual image
-need for accommodation (light is still diverging as it leaves)

7

Telescopes
-describe

For magnifying distance objects

Both light in and out is parallel/plane = no need for accommodation

8

Telescopes
-entrance and exit pupils

EnP = objective lens

ExP = image of objective lens thru ocular lens

9

Telescopes
-meaning of label A x B

A = magnification
B = diameter entrance pupil (dent) in mm

E.g. 4 x 45 = 4x mag, 45mm dent

10

Telescopes
-field of view dependent on (2)

Diameter of EnP - incr dent will incr exit pupil = incr FOV

Diameter of ExP - will continue to incr FOV by incr dex until dex becomes larger than the pt’s pupil, at which point the pupil is the limiting factor

11

Telescopes
-Keplarian

Both plus lenses

Inverted, real, magnified image
-need another lens to upright -> longer tube, heavier

AS and EnP are the objective
ExP is behind ocular (outside/in free space) = LARGER FOV bc can align with pupil
-typically very small = DIM image

12

Telescopes
-Galilean

Positive objective, negative ocular lens

Upright, magnified image
-objective lens forms real image that becomes a virtual object for ocular lens

AS and EnP are the objective
ExP is inside telescope = SMALLER FOV
-typically larger = BRIGHT image

Limited to 4x mag

13

Telemicroscopes
-describe

Telescope + reading cap

Used for viewing near object
-reading cap acts as a HHM - object located at primary focal point -> plane waves enter TS

14

Spectacle-mounted telescopes
-center fit
-bioptic fit

C: in LOS = continuous viewing, requires minimal training, shouldn’t walk with

B: in upper portion of spectacle lens, used for spotting, significant training required
-uses include classroom work, driving, traveling, grocery stores

15

Reverse telescopes
-use
-describe
-alternative

Expand FOV - pts with RP, advanced glaucoma, etc.

Pt looks thru objective lens = minifies objects
-must have good central acuity
-most often hand-held, 2.5-4x Galilean

Minus lenses - esp successful with -5 to -10D

16

VFD
-most effective method
-prism orientation

Scanning techniques

Base toward defect

17

Classifying vision
-normal
-near-normal
-moderate LV
-severe LV
-profound LV
-near-blind

Snellen:
12 to 25
30 to 60
70 to 160
200 to 400
500 to 1000
Worse than 1000

18

M notation

1M
-subtends 5 arcmin at 1m
-equals 20/20
-linearly = 1.45mm (or 1/16th of an inch)

19

Just noticeable difference

Smallest lens power change the pt can detect

JND = snellen ÷ 100

E.g. 20/400 pt’s JND is 4D
-when doing trial frame use +/- 2.00 D flippers

20

For a pt that uses eccentric viewing, the point located adjacent to the scotoma that is used for viewing is called

Preferred retinal locus (PRL)

21

Examples of contrast sensivity charts (3)

Pelli-Robson: large letters, uniform size

Vistech system: sine-wave gratings

Bailey-Lovie: 3 charts, each at diff level contrast

22

Filters
-neutral density filters
-blue blockers/amber tints

NDF: reduce glare, good for pts with photophobia, all wavelengths are transmitted equally so no effect on contrast

BB/AT: reduce transmission of short-wavelengths -> reduced glare and enhanced contrast

23

VF testing
-central scotomas are detected most effectively with

Scanning laser ophthalmoscope