Autorefractors and Phoropters Flashcards
uses of autorefractors
- screening - esp pre screening in optometric practice
- where speed and ease of use are required - widely used in developing world
- measuring accom in research studies
- does the job of a ret (although an unintelligent one as we can judge speed and quality of reflex
definition of an autorefractor
= an instrument designed to automatically measure the refractive error
also known as an automated refractor or an OBJECTIVE optometer
- optometer = a device to measure refractive error that obviates or restricts the use of trial lenses
disadvantages of a simple optometer
- large depth of focus - may not get a definitive end point as image becomes and stays clear over a range
- target can stimulate accom
- scale is non linear
- apparent size of target varies
- proximal accom
- cant measure astigmatism very well
- relies on subj response
scheiners principle
emmetropic eye - images coincident
myopic eye - images crossed
hypermetropic eye - images uncrossed
advantage of scheiner principle
better assessment of focus
disadvantages of scheiner principle
- target can stimulate accom
- scale is non linear
- when are images coincident
- proximal accom
principles of autorefractors
- autorefractors use IR radiation bet 800-900nm
- retina reflects IR better than visible light
- IR invisible to human eye - so no effect on patient comfort, pupil diameter
calibrations used in autorefractors
FOR LCA
- need to calibrate for use of IR in autos - 1.6D of LCA bet 480 and 700nm
- an allowance must be made for the diff in ocular refraction from visible light and IR wavelength used
- autos use IR light source around 800 to 900nm - the eye is about 0.75 to 1.00 hyperopic compared with 550nm
- requires -0.50 to -1.00 adjustment
Also IR is reflected from deeper in the fundus than visible radiation
- visible light reflected from ILM
- IR reflected from sclera mostly
- correction from 800 to 550nm = approx 0.50
measuring sphero cylindrical error
- variation of refraction (min blur) in diff meridians is sinusoidal
- in theory autos only needs to measure spherical component of refractive error in 3 meridians to calculate the sph/cyl error
- in practise 6 meridians are measured - makes est of error more accurate
typical specs of autos
- up to +/-25DS
- up to +/-10DC
- min pupil diamter 2.0mm - small pupil makes it harder
- autofogging to relac accom
- some have subj refraction facility
- many also carry out keratometry
how to control fixation and accom
suitable targets - hot air balloon at end of road
proximal accom
- major source of error in early autos
- all modern instruments include some form of automatic progressive fogging
- some try to control convergence
drawbacks of IR optometers
- pupil diameters of 3mm or less may not give a reading
- cost
- requires good fixation
- ret provides information on cataract assessment - retro illumination
- proximal accom remains an issue - tends to over minus, esp with kids
what is an aberrometer
- instruments designed to objectively measure monochromatic aberrations of the eye using wavefront sensors
- most freq used to measure higher order aberrations but can also measure second order refractive errors similar to standard autos
what is a refractor head / phoropter
- a contained unit allowing rapid lens presentation before each and both eyes by mechanical or electronic means
mechanical feautures of phoropters
- sph/cyl lens tracks
- cross cyl twirl
- occluder
- pinhole
- polaroid filters
- maddox rod
- rotary prisms
- vertex distance gauge
- near target
- convergence for near
