Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

KMK - Physiological Optics > 10 - Physiological2 > Flashcards

10 - Physiological2 Flashcards

1
Q

Reduced eye model

  • interior eye
  • cornea to anterior focal point
  • cornea to posterior focal point
  • total eye power
A

n = 1.33

  1. 67mm (axial length)
  2. 22mm

60D

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Far point

A

aka punctum remotum (PR)

Where the eye is looking without accommodation to have light perfectly focused on the retina

i. e. the point conjugate to the axial retinal point
i. e. a point object object at the far point will result in the formation of a point image and the retina, and vice versa

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Far point sphere

A

When the eye rotates, the far point is no longer just a point, but traces a spherical surface - known as fps

Center of curvature = center of rotation of the eye (~27mm behind typical spectacle plane)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

How to find far point of ametropic eye

A

With correcting prescription (F):

Far point = abs(1 ÷ F)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Near point

A

aka punctum proximum (PP)

Same as far point, but with MAX ACCOMM

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

Reduced eye model

-emmetropia

A

Incoming plane waves converge to a point on the retina
-retina located at Ƒ’ (secondary fp of lens), which must account for the IR used for the interior eye

For reduced eye model: power +60D, internal n=1.3
-retina located at 1.3/60m, or about 22mm from the lens

The far point for an emmetrope is optical infinity (same for corrected my/hyperopes)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Reduced eye model

-myopia

A

Eye is too strong (>60D) -> light from incoming plane waves converge in front of the retina

The far point for a myope is between the eye and infinity (i.e. will sit in front of the retina)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Describe night myopia

A

Pt becomes more myopic under low light levels

Due to combo of:

  • incr spherical aberrations
  • light levels that are too low to fully relax accomm when viewing distance
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Myopia trends

A

At birth ~25-50%
By 1yo, few children are myopic
By 6yo, -0.50 or more in only 2%
-incr in prevalence b/w ages 6 and 20, reaching 20% by 20yo
Incr somewhat in later years due to nuclear sclerotic lens changes

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

Reduced eye model

-hyperopia

A

Eye is too weak (<60) -> light from plane waves converge to a point behind the retina

Far point located behind the retina, considered virtual

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Optically speaking, a correcting lens should be place so that…

A

Its secondary focal point coincides with the far point of the ametropic eye

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Hyperopia

  • latent
  • manifest
  • absolute
  • facultative
A

Accommodated/covered up with accomm

Found in subjective refraction

Amount that cannot be corrected in a hyperope whose rx is too large to be neutralized by accomm

Amount that can be neutralized by accomm

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Hyperopia trends

A

Prevalence ~6% in children 6-15yo
-unlike myopia, prevalence doesn’t change with age for this range

A hyperopic child approx 5yo, will likely be:

  • hyperopic at age 14 if orig >1.50 D
  • emmetropic at 14 if orig 0.50 to 1.25 D
  • myopic at age 14 if orig <0.50 D

Ages 20-40:

  • expected to be relatively constant
  • decr in accomm ability may functionally highlight otherwise non-burdensome hyperopia

Over age 45, hyperopes + emmetropes tend to show an incr with age

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Measuring interpupillary distance

-PD ruler

A

1) dr sit at 40cm and closes OD
2) pt fixates on dr OS, PD measured (OS edge -> OD edge) = NEAR
3) with ruler in place dr closes OS/opens OD, pt fixates on dr OD
4) measure again = DISTANCE

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Measuring interpupillary distance

-pupilometer

A

1) dr sets ometer at 40cm/infinity
2) place ometer against bridge of nose/at spec plane
3) ometer light produces corneal reflex visible to dr
4) dr aligns vertical hairline with location of corneal reflexes
5) both binoc and monoc PD can be read directly from instrument

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Measuring interpupillary distance

-using a pupilometer reduces…

A

Errors resulting from parallax

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

Measuring visual acuity

-resolution vs recognition acuity

A

Resolution: pt distinguishes a pattern from a uniform patch of equal luminance (e.g. Teller cards)
-typical young adult has cutoff of 40-60 cycles/degree (0.75 MAR)

Recognition: gives info about ability to resolve high frequencies
-not useful for pts who show probs at other frequencies (e.g. cataracts)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

Measuring visual acuity

-minimum detectable acuity

A

Essentially asks what is the thinnest possible wire that’s visible
-e.g. vernier lines

Avg person’s MDA (~1 arcsec) is much better than resolution/recognition

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

Measuring visual acuity

-hyperacuity

A

Person’s ability to sense directional relationships

  • e.g. whether 2 lines are parallel (ability to so is very good, threshold on order of several arcsecs)
  • believed to be the result of higher cortical processing
20
Q

Measuring visual acuity

-MAR and snellen

A

MAR = minimum angle of resolution
-typically measured in arcminutes (1/60 of a degree)

Snellen fraction = 1/MAR
-fraction is typically multiplied by 20/20 to obtain standard form

20/20 = 1 MAR = 0 logMAR

21
Q

Measuring visual acuity

-snellen letter size

A

Each leter on 20/20 subtends 5 arcmin at 20 ft

Each distinct bar = 1 arcmin (think of letter E)

20/20 = distance/letter size

22
Q

Measuring children’s visual acuity

  • examples of recognition acuity
  • examples of recognition acuity matching
A

Birthday cake, tumbling E, landolt C

STYCAR, HOTV, lea symbols

23
Q

Measuring children’s visual acuity

-allen vision test

A

Ages 2+

Child names series of images on cards at close range -> pt closes/occludes one eye, dr determines longest dist at which child can resolve images

Acuity expressed as x/30
-where x is distance (ft) at which child can read

24
Q

Measuring

  • corneal curvature
  • corneal thickness
A

Curv: keratometer (only central 3mm) or topographer (whole k)

Thick: pachymeter

  • old measured dist bw purkinje images from ant + post k
  • modern methods use ultrasound
25
Q

Optical principles of static retinoscopy

A

Streak of light reflected from pts cornea, refracted as it passes thru the cornea and focus, and focused to the FAR POINT of eye
-image = retinal reflex

Examiner observes both ret reflex + light reflected from exterior of pt’s eye (streak)

Myope = against motion (eye is too strong, neutralize with minus)
Hyperope = with motion (eye is too weak, neutralize with plus)
26
Q

Retinoscopy prescription equation

A

RX = F - WD

WD assumed 1.5 D unless otherwise stated

27
Q

Retinoscopy

  • astigmatism
  • e.g. +3D vertical streak, +1D horizontal streak
A

Horizontal streaks = vertical meridian
Vertical streaks = horizontal meridian

+3D vertical streak, +1D horizontal streak = +0.50 -2.00 x 180

28
Q

Dynamic retinoscopy

-purpose

A

Measure accommodative accuracy/response

29
Q

Dynamic retinoscopy

-types and descriptions (3)

A

MEM: target at reading dist/Harmon, lenses quickly placed in front of pt

Nott’s: retinoscope moves in/out (no lenses)

Mohindra: aka “near retinoscopy”, used for determining refractive state of children/infants

  • performed in dark @ 50cm
  • monocular
  • adjustment factor 1.25D must be subtracted from sphere component of lens powers
30
Q

Optics of autorefractor

A

Lens in machine placed at a distance = focal length from spectacle plane

  • target on other side of lens moves back-forth, continuously changing vergence of light at spec plane
  • effectively same as keeping the target stationary and using a trial lens whose power can change continuously
31
Q 
Stenopaic slit example:
-pt sees most clearly when the slit is along 180 axis
-with it at 090, refraction is -1.25D
-with it at 180, refraction is -0.25D
What is the pt’s refractive error
A

-0.25 -1.00 x 180

Position at which stimulus is most clear = perpendicular to the most plus meridian of the eye

32
Q

Describe a JCC lens

A

Principle meridians have equal power but opposite in sign

  • red is the positive meridian /negative axis
  • white is the negative meridian/positive axis
33
Q

Technique for astigmatic dial

A

1) fog the eye to ~20/50
2) find lines that appear SHARPEST AND DARKEST
3) add minus cyl until all lines app equal (power in direction of darkest line)
4) add minus sphere until best acuity obtained

34
Q

Astigmatic dial

  • why we don’t need to adjust sphere power to maintain spherical equivalent as minus cyl is added
  • rule of 30
A

Added cyl power is collapsing the interval of sturm

For the clearest line, the axis in minus cyl is the lowest number given times 30
-converts pt’s view to dr’s view

35
Q

Duochrome test

-optics

A

Utilizes chromatic aberration

  • green focuses before the red - the difference is ~0.50 D for the typical filters used
  • goal is to ensure retina falls about half-way between the 2 foci (0.25 D from each, green slightly ahead, red slightly behind retina)
36
Q

Duochrome test

  • mnemonic
  • color vision-impaired pts
A

RAM GAP: red add minus, green add plus

Chromatic abberations underly = independent of color vision, so works with color-impaired
-however, must corrected to at least 20/30 to be used

37
Q

Equalization for pre-presbyopic pts

A

May accomm different amounts in each eye

Use fogging, prism techniques, or prism-dissociated bio-ocular balance

38
Q

Accommodation optics

A

An object located closer to the eye than the far point -> rays striking the eye that are too divergent -> lens gainst power

39
Q

Define near point

A

Point conjugate to the retina when the eye is achieving its maximum accommodation
-exactly like far point, but for fully accomm

40
Q

Describe range of clear vision

A

All point between near and far points

41
Q

Accommodation and age:

  • 10 yo
  • 20 yo
  • 50 yo
A

14D

11D

<2D

Recall Hofstetters: 18.5-(.3)(age)

42
Q

List and describe techniques to find presbyopic add other than age-based (3)

A

Half-amp: as general rule, pt should have at least half their amps in reserve to maintain clear, comfortable vision
Theoretical add = WD - (.5*Amps)

Balancing NRA/PRA: with a tentative add in place, perform NRA/PRA and balance so prescribed add is in the middle
-e.g. with +1.50 add, get +1.75/-0.25, halfway = +0.75, so add that to the tentative and = +2.25 D

FCC: with subjective rx add plus until vertical lines initially appear darker/clearer, gradually decr plus until report even, the total plus power over subjective is the add
-if don’t report even, use one click of plus above horizontal (so vert just better)

43
Q

Age-expected add power (40-60 yo’s)

A

40-42 add +0.75 D
Every 2-year step add 0.25D (43-45 +1.00, 46-47 +1.25, etc.)
…
58-60 add +2.50 D

44
Q

Aphakia

-possible problems

A

Aniseikonia, diplopia (monocular correction)

Reduced VF, pincushion distortion (binocular correction)

Correction assoc with ring scotoma -> jumping effect (“jack-in-the-box”)

Additional convergence demands

45
Q

Astigmatism

  • simple
  • compound
  • mixed
  • equally mixed
A

S: one line on retina, one front/behind

C: both lines in front/behind retina

M: one line on each side

EM: one on each side such that the CoLC falls on the retina

46
Q

Astigmatism ranges

  • WTR
  • ATR
  • oblique
A

WTR: 90 ± 30 degrees

ATR: 0 ± 30 degrees

Oblique: whatever’s between/not covered by those

KMK - Physiological Optics (10 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions