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Cornea > Ametropia/Ophthalmic Optics > Flashcards

Ametropia/Ophthalmic Optics Flashcards

1
Q

What is the definition of the “far point of accommodation (punctum remotum)”?

A

The point of conjungate focus of the retina when accommodation is at rest. Far point of an eye that is emmetropic is at infinity.

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2
Q

What is the definition of the “near point of accommodation (punctum proximum)”?

A

The point of conjugate focus of the retina when accommodation is at its maximum. The near point of an eye that is emmetropic (with or without corrective lenses) is the reciprocal of its maximum accommodation in diopters.

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3
Q

What is dynamic retinoscopy?

A

An objective clinical procedure to determine the point of conjugate focus of the retina when the viewing distance is at near.

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4
Q

Define Visual Acuity

A
  1. Ability to discriminate details of an object within the field of vision. The reciprocal of the threshold angle that is formed by the extent of the object or distance between objects subtended at the nodal point of the eye.
  2. Acuteness or clearness of vision which is dependent on the sharpness of the retinal focus, sensitivity of the nervous elements and the interpretative faculty of the brain.
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5
Q

Define “ductions”

A

Under MONOCULAR conditions, the movement of the eye by the EOMs.

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6
Q

What is sensory fusion?

A

The process by which stimuli seen separately by the two eyes are combined, synthesized or integrated into a unitary percept.

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7
Q

What is the Interval of Sturm?

A

The interval between the two foci created by the refractive power of the two principal meridians.

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8
Q

The most in-focused image within the Interval of Sturm is referred to as?

A

Circle of Least Confusion

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9
Q

During retinoscopy, if the image of the reflex seen at the plane of the pupil is broad and moving quickly then are u further from the patients far point or closer?

A

The far point of the patients eye is CLOSER to the plane of retinoscope and that meridian is almost neutralized.

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10
Q

Where is the far point of the eye located (generally) if during retinoscopy the observer sees “against motion”?

A

The eye’s far point is located between the retinoscope and the pupil.

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11
Q

During manual keratometry, the diameter of the reflected image is a direct function of what?

A

Corneal Curvature - The steeper the corneal curvature, the smaller the diameter of the reflected image.
The mires from a spherical cornea will have equal diameters, whereas those from a toric cornea with have unequal diameters.

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12
Q

The keratometer actually measures the average curvature over a ?mm area of the cornea

A

The central 3mm of the cornea

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13
Q

Javal’s Rule is used to predict what?

A

The amount of refractive astigmatism from measurements of corneal toricity. The formula states that RA = [1.25 x Change in K] + or - 0.50D. Factor of 1.25 corrects or compensates for the cylinder lens power difference between the corneal plane and spectacle plane. The value of 0.50D represents the amount of physiological toricity (produced by the crystalline lens). If the corneal toricity is against the rule, the value of 0.50 D in the formula is added (since the magnitude of the toricity produced by the lens is approximately 0.50D of against the rule). 0.50D is subtracted if the corneal toricity is with-the-rule. 0.50D is dropped if the toricity is oblique.

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14
Q

The Keratometer has a range of measurements from 36.00 D to 52.00 D. How can the ranges be extended?

A

Placing a +1.25D lens (increases mire size) in front of the aperture can extended the range to 61.00 D. By placing a -1.00 (decreases mire size) will reduce the range to a 30.00 D.
General rule if a conversion table is unavailable:
If using a +1.25: add 9D to the drum reading
If using a -1.00: subtract 6D from the drum reading.

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15
Q

Classification of VA:

  1. Detection?
  2. Vernier Acuity?
  3. Resolution?
  4. Recognition?
A
  1. Detection: determination of the presence or absence of an object. Minimum human threshold angle is one second of arc.
  2. Vernier: discrimination of lateral misalignment of two separate objects. Minimum human threshold is 3 seconds of arc. Basis for stereoscopic acuity
  3. Resolution: discrimination of two or more spatial separated objects or elements of a single object. Basis for Snellen Acuity. Minimum human threshold is ~30seconds of arc
  4. Recognition: capacity to recognize an object that must be RESOLVED first.
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16
Q

Define Snellen Fraction

A

d/D = test distance (usually 20ft or 6m) divided by the distance at which the smallest optotype recognized subtends 5 minutes of arc at the nodal point of the eye.
Ex. 20/20 means that at a test distance of 20ft the smallest optotype recognized would subtend a 5 minute angle when viewed at 20ft.

17
Q

What is the size of a 20/20 optotype for clinical purposes?

A

8.73mm = 6m x tan(5’) = 8.73mm
Ex. a 20/40 optotype is twice as large (17.4mm high) and will subtend 5’ of arc when viewed at 40 ft.
Easy rule to determine optotype size: Divide snellen denominator by 20 and then multiply by 8.7.

18
Q

VA measured at any test distance can be converted to the equivalent at any other distance. What is the formula?

A

Divide 20 by the new test distance and multiply that factor to lowest optotype read at that distance.
Ex. 20/40 at distance of 10ft.
(20/10) = 2 x 40 = 80. So the conversion would yield a VA of 20/80 at 20 ft.

19
Q

What is LogMAR acuity?

A

It is the log of the minimum angle of resolution and is based on the logarithm of the inverse of decimal acuity.
Ex. 20/40 converted to logMAR
1. First convert to decimal = 20/40 = 0.5
2. Reciprocal is 1/0.5 = 2
3. Take Log of 2 = 0.3
The higher the LogMAR from 0.00 (20/20) the worse the VA

20
Q

Describe the 3 degrees of Fusion

A
  1. First-Degree: defined as simultaneous, binocular perception of dissimilar objects that are projected in the same visual direction. Ex. one eye sees a square and the other a circle - patient would see a circle and square superimposed.
  2. Second-Degree: occurs when similar stimuli seen separately by the eyes are combined or integrated into a single percept. Ex. Red Lens test - one red light and one white light are seen as orange.
  3. Third-Degree: highest degree and is exhibited by a perception of depth resulting from fusion of disparate retinal images.
21
Q

What is Panum’s Fusional Area?

A

Theoretical area about the fovea of each eye within which disparity can occur and result in stereopsis.

22
Q

Name 2 ways to clinically assess 2nd-degree fusion (tested at far and near)

A
  1. Red Lens Test
  2. Worth-4-Dot
    Grades of Fusion:
    Grade A: normal response where 2 images are fused into 1
    Grade B: an initial perception of diplopia with an almost immediate fusion
    Grade C: Constant diplopia (Exo or Eso)
    Grade D: Constant Suppression
23
Q

If an observer does not note any eye movement when performing the Alternating Cover Test but the patient says the image moves in the same direction as the paddle - does the patient have an exo or eso deviation?

A

Exo - fixation target jumps in the SAME direction as the occluder.

24
Q

Increasing the Abbe value of a lens does WHAT to the chromatic aberations?

A

Abbe is inversely proportional to Index of Refraction (n) so increasing Abbe decreases the amount of chromatic aberrations (increasing n = increasing the amount of abberations).

25
Q

What is the Index of Refraction of the following materials?

  1. Glass
  2. CR-39
  3. Polycarb
  4. Trivex
A
  1. Glass = 1.52
  2. CR-39 = 1.50
  3. Polycarb = 1.59
  4. Trivex = 1.53
26
Q

What is specific gravity?

A

Specific Gravity refers to the weight of a lens. Trivex is the lightest material

27
Q

When performing hand neutralization, what direction would you expect to see if you have a (-) spherical lens?

A

With motion. If you are neutralizing a (+) lens then you would initially see against motion. Remember light bends toward the apex of a prism.

28
Q

Optical Prisms refract white light toward the base or apex?

A

The light is refracted toward the base of the lens. Shorter, higher energy wavelengths (blue) are refracted MORE.

29
Q

Do strong plus lenses create a pincushion or barrel effect?

A

Strong (+) lenses increase magnification at the lens edge creating a PINCUSHION. Strong (-) lenses create a BARREL effect. Decreasing the frame size can help decrease these effects.

30
Q

Which surface is the base curve on?

A

F1 -always for single vision.

  • If (+) cylinder form then the BC is the weaker or flatter of the 2 meridians (remember that (+) cylinder is on the front of the lens)
  • If (-) cylinder then the front spherical curve is the BC (because the cylinder is ground in into the back of the lens and BC is always on the front).
31
Q

If a lens is ground in (+) cylinder form - which surface is the bifocal segment?

A

The segment and the cylinder surfaces are always opposite each other. So if a lens is ground in (+) cyl then the F1 surface contains the cylinder so the segment is on the F2 surface (if the segment were on the F1 surface then it would be ground away during fabrication).

32
Q

When using a lens clock to measure the F1 surface which scale is read off the clock?

A

The (+) scale is read on the lens clock because the F1 surface is always (+).

33
Q

Would you expect to find a difference in the overall power reading (in the lensometer) between a lens thats warped and one that is not warped?

A

No. The power reading is the same but you would get a variation in the curvature when using a lens clock.
Ex. Minus cylinder lens - you would expect the front surface to be spherical but a warped lens may give you a different result.

34
Q

The lens clock is calibrated for an index of refraction of 1.53. If determining the power of a lens with a lower n value - would you expect the overall power calculated to be higher or lower than what you would expect?

A

If n

35
Q

Which surface are modern bifocal segments added to?

A

F1

36
Q

1 prism diopter equals how many degrees or light deviation?

A

0.57 degrees

OR 1 degree = 1.75 prism diopters

37
Q

What are the ANSI standards when evaluating unwanted horizontal and vertical prism?

A

HORIZONTAL:
*For lenses of +/- 2.75 or below in the HORIZONTAL meridan: prism amount shall not exceed 2/3 or 0.66 prism diopters

*For lenses of +/- 2.75 D or greater in the HORIZONTAL meridan: difference between ordered PD and actual DBOC not to exceed 2.5mm

VERTICAL:
*For lenses of +/- 3.75 D or below in the VERTICAL meridian: shall not exceed 1/3 or 0.33 prism diopters

*For lenses of +/- 3.75 D or greater in the VERTICAL meridian: vertical difference between PRPs not to exceed 1mm

Cornea (7 decks)

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