Chapter 4: Instruments B

Question 1

What are the 3 stages of retinoscopy?

Answer 1

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

Question 2

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

Answer 2

real image of the light source formed between patient and observer

Question 3

What type of mirror is used for retinoscopy?

Answer 3

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

Question 4

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

Answer 4

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

Question 5

What happens in the reflex stage of retinoscopy?

Answer 5

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

Question 6

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

Answer 6

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

Question 7

What happens at the projection stage of retinoscopy?

Answer 7

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

Question 8

In which direction does the retinoscopy reflex move in hypermetropia?

Answer 8

with the illuminating light

Question 9

In which direction does the retinoscopy reflex move in myopia?

Answer 9

against the direction of the light

Question 10

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

Answer 10

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

Question 11

What happens as the neutral point is approached in retinoscopy?

Answer 11

movement of luminous reflex appears to move more rapidly

Question 12

What correction must be made after retinoscopy?

Answer 12

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

Question 13

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

Answer 13

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

Question 14

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

Answer 14

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

Question 15

What does a keratometer (or ophthalmometer) measure?

Answer 15

radius of curvature of the central area of the cornea

Question 16

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

Answer 16

4mm in diameter

Question 17

What is the radius of curvature of the emmetropic eye?

Answer 17

7.8mm

Question 18

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

Answer 18

45 D

Question 19

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

Answer 19

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)

Question 20

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

Answer 20

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

Question 21

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

Answer 21

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

Question 22

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

Answer 22

doubling the image seen by the examiner

Question 23

How is image doubling achieved in the von Helmotz keratometer?

Answer 23

two rotating glass plates

Question 24

How does the keratometer of von Helmotz work?

Answer 24

• 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

Question 25

Can the von Helmotz measure astigmatism and how?

Answer 25

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

Question 26

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

Answer 26

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

Question 27

How does quartz act as a prism?

Answer 27

* **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

Question 28

How does a Javal Schiotz keratometer work?

Answer 28

* 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

Question 29

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

Answer 29

* 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

Question 30

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

Answer 30

curvature of contact lenses

Question 31

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

Answer 31

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

Question 32

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

Answer 32

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

Question 33

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

Answer 33

a smaller base curve will cause it to fit more tightly

Question 34

What is the most common computerised analysis of corneal topography?

Answer 34

CVK (computerised videokeratography)

Question 35

How does CVK work?

Answer 35

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

Question 36

What is Raster photogrammetry?

Answer 36

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

Question 37

What is a compound microscope used for?

Answer 37

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

Question 38

What are 6 instruments which use a compound microscope?

Answer 38

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

Question 39

What is a specular microscope?

Answer 39

modified microscope allowing examination/ photography of corneal endothelium

Question 40

How does the compound microscope work?

Answer 40

* 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

Question 41

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

Answer 41

just outside anterior focal point of the objective lens

Question 42

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

Answer 42

real, inverted, magnified image

Question 43

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

Answer 43

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

Question 44

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

Answer 44

vertically and horzontally inverted

Question 45

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

Answer 45

Porro prism

Question 46

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

Answer 46

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

Question 47

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

Answer 47

multiple lens elements used

Question 48

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

Answer 48

* 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)

Question 49

What is a slit lamp?

Answer 49

* 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)

Question 50

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

Answer 50

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

Question 51

How are slit lamp microscopes shortented?

Answer 51

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

Question 52

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

Answer 52

bank of Galilean telescopes of different powers incorporated

Question 53

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

Answer 53

bank of Galilean telescopes of different powers incorporated

Question 54

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

Answer 54

Porro prism

Question 55

Can the von Helmotz measure astigmatism and how?

Answer 55

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

Question 56

What correction must be made after retinoscopy?

Answer 56

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