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Duke Elder: Optics (Alice) > Chapter 4: Instruments B > Flashcards

Chapter 4: Instruments B Flashcards

1
Q

What are the 3 stages of retinoscopy?

A
  1. Illumination stage - light shone to illuminate pt retina
  2. Reflex stage - image of retina formed at pt far point
  3. Projection stage - illumination moved across fundus to locate image at far point
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2
Q

What acts as a light source for illuminatin gthe patient’s retina in retinoscopy?

A

real image of the light source formed between patient and observer

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3
Q

What type of mirror is used for retinoscopy?

A

in between a plane mirror and concave mirror - essentially a concave mirror of focal length slightly exceeding observer-patient distance

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4
Q

What are 2 advantages of the type of mirror used in retinosopy and how is this achieved on the instrument?

A
  1. light behaves in same way as for plane mirror
  2. light is brighter due to concave mirror properties
    -> condensing lens at intermediate position on shaft of retinoscope
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5
Q

What happens in the reflex stage of retinoscopy?

A

image A1B1 of illuminated retina (AB) formed at patient’s far point

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6
Q

What are the 3 rays used to draw diagram of reflex stage of retinoscopy?

A
  1. ray from A - travels along principal axis
  2. ray from B which travels parallel to prinicipal axis until principal plane, where it is deviated through patient’s anterior principal focus Fa
  3. ray from B which travels undeviated through patient’s nodal point

In emmetropia the latter 2 rays are parallel

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7
Q

What happens at the projection stage of retinoscopy?

A

osberver views image A1B1 of illuminated retina from convenient distance - does not see actual image but rays from it as illuminated area in patient’s pupil

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8
Q

In which direction does the retinoscopy reflex move in hypermetropia?

A

with the illuminating light

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9
Q

In which direction does the retinoscopy reflex move in myopia?

A

against the direction of the light

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10
Q

What happens 1) visibly and 2) in terms of scientific optics when the neutral point or point of reversal is reached in retinoscopy?

A
  1. visibly: no movement of reflex - diffuse bright reflex seen
  2. optically: patient’s far point coincides with observer’s neutral point
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11
Q

What happens as the neutral point is approached in retinoscopy?

A

movement of luminous reflex appears to move more rapidly

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12
Q

What correction must be made after retinoscopy?

A

add -1.5 for working distance of 66cm or -1.0 for 1m to patient’s refractive error

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13
Q

What are 3 types of instruments which can be used to assess corneal curvature?

A
  1. Placido’s disc
  2. von Helmotz keratometer
  3. Javal-Schiotz keratometer
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14
Q

what is a Placido disc and what type of lens does it use?

A

convex lens mounted in an aperture in centre of a disc bearing concentric black and white rings. eaminer looks through central aperture and observes image of disc reflected from pt cornea

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15
Q

What does a keratometer (or ophthalmometer) measure?

A

radius of curvature of the central area of the cornea

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16
Q

How large is the central/ axial area of the cornea?

A

4mm in diameter

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17
Q

What is the radius of curvature of the emmetropic eye?

A

7.8mm

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18
Q

What dioptric power does a corneal radius of curvature of 7.5mm correspond to?

A

45 D

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19
Q

What is the equation used to calculate dioptric power of the cornea from its curvature?

A

D = n2 - n1 / r
where D is power in dioptres, r is radius of anterior corneal curvature in metres. n2 would be refractive index of tears (1.3375) and n1 is refractive index of air (1)

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20
Q

What equation is used to calculate the radius of corneal curvature from the keratometer?

A

r = 2u x 1/O
where r = radius, u =the focal distance of the viewing telescope from the cornea (a constant), O is object size

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21
Q

What is the key difference between the von Helmotz keratometer and the Javal Schiotz keratometer?

A

von Helmotz has fixed object size (image size adjusted to measure curvature) while in Javal Schiotz the oject size is varied to achieve standard image size

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22
Q

How can the natural movements of the patient’s eye be overcome for keratometry?

A

doubling the image seen by the examiner

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23
Q

How is image doubling achieved in the von Helmotz keratometer?

A

two rotating glass plates

24
Q

How does the keratometer of von Helmotz work?

A
  • beam of light passes through graticule onto cornea, image of graticule reflected
  • passes back into instrument through the 2 glass plates, inclined to each other
  • plates displace light laterally producing 2 virtual images
  • **angle of inclination **of plates varied by observer until edges of 2 images touch
  • distance between their centres equals dimaeter of graticule image from which corneal curvature can be calculated
25
Q

Can the von Helmotz measure astigmatism and how?

A

yes - can be rotated to allow measurement of this similar to Javal-Schiotz keratometer

26
Q

How is the image doubled in the Javal-Schiotz keratometer?

A

using Wollaston prism which is incorporated into the viewing telescope (2 rectangular quartz prisms cemented together)

27
Q

How does quartz act as a prism?

A
  • double refracting substance - splits single beam of incident light to form 2 polarised emergent beams
  • 2 prisms are cemented with grain at right angles, 2 beams are at fixed emergent angle
  • **dispersion neutralised **by second prism forming sharp image
28
Q

How does a Javal Schiotz keratometer work?

A
  • pair of mires, one rectangle and one stepped shape, mounted on curved arms (lanterns + coloured window)
  • distance between mires adjusted until images just touch
  • each steps of mire A equals one dioptre of corneal power - if aliged correctly in one meridian but overlap in another indicates degree of corneal astigmatism
29
Q

How can the axis and also magnitude of astigmatism be measured in with Javal Schiotz keratometry?

A
  • if inner images aligned correctly in one corneal meridian but overlap in another, degree of corneal astigmatism can be calculated
  • the two images are displaced vertically in all but the two principal meridians of cornea
30
Q

In addition to measuring curvature of the cornea what else can keratometers be used for?

A

curvature of contact lenses

31
Q

How does the information from computerised analusis of corneal topography vs keratometry differ?

A

keratometry measures central zone only, computerised obtains info on larger area

32
Q

What are 4 situations when analysis of corneal topography is useful?

A
  1. corneal astigmatism
  2. contact lens fitting
  3. refractive surgery
  4. early diagnosis and monitoring of keratoconus
33
Q

How does changing the base curve influence how tightly a contact lens will fit to the corneal surface?

A

a smaller base curve will cause it to fit more tightly

34
Q

What is the most common computerised analysis of corneal topography?

A

CVK (computerised videokeratography)

35
Q

How does CVK work?

A

Placido’s disc projected onto 5-6mm diameter of cornea, reflection converted into digital image (red=steeper)

36
Q

What is Raster photogrammetry?

A

2D pattern projected on precorneal tear film after installation of fluorescein dye (like net over mount) - reflection gives info on height of surface

37
Q

What is a compound microscope used for?

A

provides magnified view of a near objects (vs telescope which magnifies distant objects)

38
Q

What are 6 instruments which use a compound microscope?

A
  1. slit lamp (2x at 13-14 degrees)
  2. operating microscope (2x at 13-14 degrees)
  3. keratometer
  4. pachometer
  5. applanation tonometer
39
Q

What is a specular microscope?

A

modified microscope allowing examination/ photography of corneal endothelium

40
Q

How does the compound microscope work?

A
  • 2 convex lenses (objective and eyepiece)
  • object O placed just outside anterior focal point of object lens
  • real, inerted magnified image formed behind objective lens
  • eyepiece lens placed so image formed by objective lens falls at or close to its principal focal plane, Fe
  • eyepieces acts as loupe and further magnifies image seen by observer
41
Q

Where is the object placed to be viewed by the compound microscope?

A

just outside anterior focal point of the objective lens

42
Q

What is the nature of the image formed by the objective lens of the compound microscope?

A

real, inverted, magnified image

43
Q

Where does the image formed by the objective lens of the compound microscope lie?

A

some distance behind object lens, falling at or close to principal focal plane of eyepiece lens

44
Q

What is the nature of the final image formed by the compound microscope?

A

vertically and horzontally inverted

45
Q

What is used to obtained an erect, non-inverted final image from the compound micrscope?

A

Porro prism

46
Q

How are the 2 compound microscopes mounted in operating microscopes and the slit lamp?

A

at 13-14 degrees to each other to give binocular, stereoscopic view

47
Q

How are spherical, coma and chromatic abberation reduced and smooth zoom achieved for optical instruments with compound microscopes?

A

multiple lens elements used

48
Q

What factor must be done to smoothly change image magnification from a compond microscope e.g. in a slit lamp?

A
  • simplest zoom effect obtained by placing single movable concave lens between the microscope lenses
  • several mobile lens elements must be used to achieve constant object-image distance (compensated zoom system)
49
Q

What is a slit lamp?

A
  • low-powered binocular compound microscope linked to adjustable bright light source
  • narrow vertical slit of light projected onto eye
  • light and microscope correspond to same point (have common focal plane + common axis of rotation in this plane)
50
Q

Why is there considerable distance between the microscope of a slit lamp and patient’s eye?

A

have long working distance - allows observer to carry out manoeuvres e.g. removing foreign body from cornea + room to interpose devices e.g. +90D lens / 3-mirror contact lens to inspect vitreous + retina

51
Q

How are slit lamp microscopes shortented?

A

by the incorporation of prisms (also invert image to appears erect/right way round)

52
Q

How can the magnification of the observation system be varied in slit lamps?

A

bank of Galilean telescopes of different powers incorporated

53
Q

How can the magnification of the observation system be varied in slit lamps?

A

bank of Galilean telescopes of different powers incorporated

54
Q

What is used to obtained an erect, non-inverted final image from the compound micrscope?

A

Porro prism

55
Q

Can the von Helmotz measure astigmatism and how?

A

yes - can be rotated to allow measurement of this similar to Javal-Schiotz keratometer

56
Q

What correction must be made after retinoscopy?

A

add -1.5 for working distance of 66cm or -1.0 for 1m to patient’s refractive error

Duke Elder: Optics (Alice) (19 decks)

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