autorefractors+phoropters Flashcards
which is the best starting point for subjective refraction?
- retinoscopy
what is an auto-refractor?
- an instrument designed to automatically measure the refractive error
what is an auto-refractor also known as?
- an automated refractor
- objective optometer
what is an otopmeter?
- a device to measure refractive error that obviates or restricts the use of trial lenses
what are the uses of auto-refractor?
- 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
what information does RET provide that we can’t get through an auto-refractor?
- cataract
- quality of lens
what are the early concepts of auto-refractors?
- 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
what are the disadvantages of simple optometers ?
- 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)
what is the Scheiner principle?
- 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
what are the advantages of scheiner principle?
- better assessment of focus
what are the disadvantages of scheiner principle?
- target can stimulate accommodation
- scale is non-linear
- when are images coincident
- proximal accommodation
what do modern auto-refractors use?
- 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
how do auto-refractors use infra red radiation?
- 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
why do we need to calibrate for use of infra red radiation in auto-refractors?
- different wavelength focus at different positions
- blue wave focus in front of the retina
- red waves focus behind the retina
how do we calibrate for the use of infra red radiation in auto-refractors ?
- 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
how to measure sphero-cylindrical error ?
- 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
why are 6 meridians measured in practice ?
- makes the estimate of refractive error significantly more accurate
- measurement is taken quickly
what are the typical specification of auto-refractors ?
- 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
what is the point of suitable target ?
- to control proximal accommodation as the px is looking at target close to them
how to control fixation and accommodation?
- 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
how is progressive fogging carried out for +1.50 ?
1- first auto-refractor measures +1.00 then a +1.75 fogging lens added to relax accommodation
- remove fogging lens
- auto-refractor measures +1.25 then a +1.75 to relax accommodation
- remove fogging lens
- auto refractor measures +1.50 and then add +1.75 to relax accommodation
- remove fogging lens
- auto-refractor measures +1.50 again
what are the different auto- refractors ?
. Nidek hand held
. Topcon TRK-2P which includes refractometer, keratometer, non-contact tonometer and pachymeter
what are the drawback of IR optometers ?
- 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)
what are refractors heads/phoropters ?
- a contained unit allowing rapid lens presentation before each and both eyes by mechanical or electronic means
what are the features included in a phoropter?
- sphere/cyl lens tracks
- cross cyl twirl
- occluder
- pinhole
- polaroid filters
- maddox rod
- rotary prisms
- vertex distance gauge
- near target
- convergence for near
what are some extra features of electronic phoropter?
- input from auto-refractor
- input from focimeter
- immediate correction comparison
- automatic PD adjustment
- link with projection chart
- data storage
- data print out
what is split cylinder method?
- patient are simultaneously presented with two targets
what are the advantages of phoropters?
- 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
what are the additional advantages of electronic refractors ?
- 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
what are the disadvantages of phoropters?
- 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
