autorefractors+phoropters

Question 1

which is the best starting point for subjective refraction?

Answer 1

• retinoscopy

Question 2

what is an auto-refractor?

Answer 2

• an instrument designed to automatically measure the refractive error

Question 3

what is an auto-refractor also known as?

Answer 3

• an automated refractor

- objective optometer

Question 4

what is an otopmeter?

Answer 4

• a device to measure refractive error that obviates or restricts the use of trial lenses

Question 5

what are the uses of auto-refractor?

Answer 5

• screening ( especially pre-screening in optometric practice)
• where speed and ease of use are required
  . widely used in the developing world
  . vision aid overseas
• measuring accommodation in research studies
• does the job of a retinoscope

Question 6

what information does RET provide that we can’t get through an auto-refractor?

Answer 6

• cataract

- quality of lens

Question 7

what are the early concepts of auto-refractors?

Answer 7

• placing an an optometer lens (e.g. -10) in front of the eye
• we would bring a target from unseen to seen ( moment of far point)
• use the RAF rule to measure distance of when object is first seen using the vergence equation
• L’=L+F

Question 8

what are the disadvantages of simple optometers ?

Answer 8

• large depth of focus
• target can stimulate accommodation
• scale is non-linear
• apparent size of target varies
• proximal accommodation
• we can’t measure astigmatism
• relies on subjective response ( px tells us when object is seen)

Question 9

what is the Scheiner principle?

Answer 9

• we use scheiner disc ( double pin hole effect)
• when px is emmetropic and looks at distance target image is coincident - so the two pinholes will form single image on eye
• in myopic eye - images are crossed and px will see two images
• in hypermetropic eye - images uncrossed and px will see two images

Question 10

what are the advantages of scheiner principle?

Answer 10

• better assessment of focus

Question 11

what are the disadvantages of scheiner principle?

Answer 11

• target can stimulate accommodation
• scale is non-linear
• when are images coincident
• proximal accommodation

Question 12

what do modern auto-refractors use?

Answer 12

• comprise of an infrared source, a fixation target and a badal optometer
• e.g. Badal optometer Topcon, shin-Nippon
• Scheiner (coincidence ) principle Nidek auto-refractor
• knife-edge test Humphrey

Question 13

how do auto-refractors use infra red radiation?

Answer 13

• we are looking at the reflex of infra red light
• auto-refractors use IR radiation between 800-900nm
• retina reflects IR better than visible light
• IR invisible to human eye - so no effect on patient comfort , pupil diameter

Question 14

why do we need to calibrate for use of infra red radiation in auto-refractors?

Answer 14

• different wavelength focus at different positions
• blue wave focus in front of the retina
• red waves focus behind the retina

Question 15

how do we calibrate for the use of infra red radiation in auto-refractors ?

Answer 15

• 1.6D of LCA between 480nm and 700nm
• an allowance must be made for the difference in ocular radiation between visible light and IR wavelength used
• in auto-refractors use infra red light source around 800 to 900nm- the eye is about 0.75 to 1.00D hyperopic compared with 550nm
• requires -0.50D to -1.00D adjustment
• also IR is reflected from deeper in fundus than visible radiation as the IR is reflected from sclera
• therefore a correction from 800nm to 550nm = approximately 0.50DS

Question 16

how to measure sphero-cylindrical error ?

Answer 16

• take measurements across different meridians
• variation of ‘ refraction’ in different meridians is sinusoidal
• in theory the auto-refractors only needs to measure the spherical component of the refractive error in 3 meridians to calculate the sphere/cyl axis refractive error
• in practice 6 meridians are measured

Question 17

why are 6 meridians measured in practice ?

Answer 17

• makes the estimate of refractive error significantly more accurate
• measurement is taken quickly

Question 18

what are the typical specification of auto-refractors ?

Answer 18

• up to +/-25DS
• up t0 +/-10DC
• minimal pupil diameter 2.0mm
• autofogging to relax accommodation
• some have subjective refraction facility ( internal targets for duochrome, x cyl, fan and block, subjective refraction)
• many also carry out keratometry

Question 19

what is the point of suitable target ?

Answer 19

• to control proximal accommodation as the px is looking at target close to them

Question 20

how to control fixation and accommodation?

Answer 20

• suitable targets ( looking into distance)
• proximal accommodation is a major source of error in early autos
• all modern instruments include some form of automatic, progressive fogging
• some try to control vergence

Question 21

how is progressive fogging carried out for +1.50 ?

Answer 21

1- first auto-refractor measures +1.00 then a +1.75 fogging lens added to relax accommodation

2. remove fogging lens
3. auto-refractor measures +1.25 then a +1.75 to relax accommodation
4. remove fogging lens
5. auto refractor measures +1.50 and then add +1.75 to relax accommodation
6. remove fogging lens
7. auto-refractor measures +1.50 again

Question 22

what are the different auto- refractors ?

Answer 22

. Nidek hand held

. Topcon TRK-2P which includes refractometer, keratometer, non-contact tonometer and pachymeter

Question 23

what are the drawback of IR optometers ?

Answer 23

• pupil diameter of 3mm or less produce may not give reading
• cost
• requires good fixation
• retinoscopy provides information on cataract assessment - retro-illumination
• proximal accommodation remains an issue ( tends to overminus- particularly with children)

Question 24

what are refractors heads/phoropters ?

Answer 24

• a contained unit allowing rapid lens presentation before each and both eyes by mechanical or electronic means

Question 25

what are the features included in a phoropter?

Answer 25

• sphere/cyl lens tracks
• cross cyl twirl
• occluder
• pinhole
• polaroid filters
• maddox rod
• rotary prisms
• vertex distance gauge
• near target
• convergence for near

Question 26

what are some extra features of electronic phoropter?

Answer 26

• input from auto-refractor
• input from focimeter
• immediate correction comparison
• automatic PD adjustment
• link with projection chart
• data storage
• data print out

Question 27

what is split cylinder method?

Answer 27

• patient are simultaneously presented with two targets

Question 28

what are the advantages of phoropters?

Answer 28

• speed: which helps stabilise accommodation and keep attention
• quick and easy to compare VA with two different lens powers
• accuracy of cyl axis- automatical alignment of JCC
• risley-rotary prism for fusional reserves
• patient comfort ( nose, ears)
• lens cleanliness
• social distance

Question 29

what are the additional advantages of electronic refractors ?

Answer 29

• comparison check:
  show old rx for each eye separately with change in cyl
• data transfer and storage
• practitioner comfort ( remote and less physically demanding)
• looks high tech- wow factor for patient

Question 30

what are the disadvantages of phoropters?

Answer 30

• the glass protecting lenses can steam up
• proximal accommodation
• variable vertex distance
• not always ideal for ( young children, presbyopic contact lenses, patients with visual impairment )
• barrier to communication
• can create an artificial visual environment for near vision testing