Autorefractors Flashcards
what is the definition of an auto refractor
any instrument that measures refractive error - it usually excludes those that use trial case lenses
what are 2 other names for auto refractor
- an optometer
- an automated refractor
what is the usual type of target of an auto refractor
an image which looks like its at a distance e.g. house at the end of a road, in order to relax the patient’s accommodation
what are the 4 uses of auto refractors
- screening e.g. pre-screening in practice
- where speed and ease of use is required - widely used in developing world
- measuring accommodation in research studies
- basically does the job of a retinoscope - but unintelligent retinoscope
what are the 3 components of a simple optometer
- target
- optometer lens
- scale
where is the target placed and how does this differ for a hype rope compare to a myope, in a simple optometer
placed on first focal point of optometer lens
- for myope: need to move target in
- for hyperope: need to move target out
for target to be clear
what type of lens is used in a simple optometer and why
a +ve lens put in front of the target
it changes most hyperopes into myopes, so that it can measure the patient’s far point (which will now be somewhere in front of px’s eye) & estimate their refractive error
what is the usual type of target of an auto refractor
an image which looks like its at a distance i.e. infinity e.g. house at the end of a road, in order to relax the patient’s (proximal) accommodation
what type of lens is used in a simple optometer and why
a +ve lens put in front of the target
it changes most hyperopes into myopes, so that it can measure the patient’s far point (which will now be somewhere in front of px’s eye) and estimate their refractive error
how do the dioptre steps on the scale of a simple optometer differ for a myope compared to a hyperope
dioptre steps get closer as you get more myopic and further apart as you get more hyperopic
list and explain the 5 main disadvantages of a simple optometer
- large depth of focus: if you move the target over a large range, the patient will still see a clear target
- target can stimulate accommodation: e.g. from the print of the target
- scale is non-linear: bigger gaps in dioptres for hyperopes and smaller gaps in myopic range
- apparent size of target varies: as you bring the target closer, its retinal image size gets bigger, so the target gets easier to see as you move closer to the eye
- proximal accommodation: px knows the target is not at infinity, so they apply proximal accommodation
what is a scheiner disc
a double pinhole disc, one pin hole at top of pupil and one pinhole at bottom pf pupil, a ray goes through the top pinhole and another ray goes through the bottom pinhole
how does the images of the target appear with the scheiner principle for an emmetropic eye
the images appear coincident
how do the images of the target appear with the scheiner principle for an emmetropic eye
the images appear coincident
how do the images of the target appear with the scheiner principle for a myopic eye
the images are crossed - before they hit the retina
the top ray of the pinhole reaches the bottom axis of the eye and the bottom ray of the pinhole reaches the top axis of the eye
how do the images of the target appear with the scheiner principle for a hyperopic eye
the images are uncrossed - before they hit the retina
the top ray of the pinhole reaches the top axis of the eye and the bottom ray of the pinhole reaches the bottom axis of the eye
what can you do to check if a patient is myopic with the scheiner principle
if you cover the top pinhole, the bottom image will disappear
indicating that the images were crossed and that you need to move the target in
list and explain the 5 main disadvantages of a simple optometer
- large depth of focus: if you move the target over a large range, the patient will still see a clear target
- target can stimulate accommodation: e.g. from the print of the target
- scale is non-linear: bigger gaps in dioptres for hyperopes and smaller gaps in myopic range
- apparent size of target varies: as you bring the target closer, its retinal image size gets bigger, so the target gets easier to see as you move closer to the eye
- proximal accommodation: px knows the target is not at infinity, so they apply proximal accommodation
what is an advantage of the scheiner principle
better assessment of focus - the target is clear over a range of distances, giving a more precise end point for assessing the refractive error
list 4 disadvantages of the scheiner principle
- target can stimulate accommodation
- scale is non-linear: bigger gaps in dioptres for hyperopes and smaller gaps in myopic range
- apparent size of target varies: as you bring the target closer, its retinal image size gets bigger, so the target gets easier to see as you move closer to the eye
- proximal accommodation: px knows the target is not at infinity, so they apply proximal accommodation
what causes longitudinal chromatic aberration
light of different wavelengths are refracted by different amounts
which wavelengths correspond to myopic eyes
shorter (blue) wavelengths
which wavelengths correspond to hyperopic eyes
longer (red) wavelengths
which wavelengths correspond to emmetropic eyes
medium (green) wavelengths
how much D of longitudinal chromatic aberration is between 480 - 700nm (visible spectrum)
~1.6D
how much D of longitudinal chromatic aberration is between 480 - 700nm (visible spectrum)
~1.6D
what needs to be done with the difference in ocular refraction between visible light and IR wavelength used with auto refractors
an allowance needs to be made for the difference in ocular refraction
in auto refractors usually 800-900nm is used
how much D is 800-900nm worth, which is used in auto refractors and what needs to be done to make it correspond to the visible wavelength of 550-555nm
0.75 - 1.00D hyperopic compared with 550nm
so need to take off about 0.75D e.g. if reading in auto refractor gives +3.00D, it will actually be +2.25D (0.75 off)
how much the power changes depends on what wavelength is used in the optometer. but you must take some D off the result in the auto refractor, so that the power corresponds to 550-555nm
how much D is 800-900nm worth, which is used in auto refractors and what needs to be done to make it correspond to the visible wavelength of 550-555nm
0.75 - 1.00D hyperopic compared with 550nm
so need to take off about 0.75D e.g. if reading in auto refractor gives +3.00D, it will actually be +2.25D (0.75 off)
how much the power changes depends on what wavelength is used in the optometer. but you must take some D off the result in the auto refractor, so that the power corresponds to 550-555nm
LCA has low inter subject variability, so its the same for everyone
list 4 main point about infrared optometers
- transmission of ocular media is good: % of light that gets through to the cornea, aqueous, lens, vitreous to the back of the eye is about 80-90% gets through to the fundus at about 500-800nm and at about 800nm transmission is still goof at about 80%
- removes stimulus to accommodation: since IR is invisible to the patient, there is no stimulus to accommodate to & can’t see target
- reflectance of fundus ~40% (820nm): 40% of light that goes into the eye is reflected back from the fundus which is good enough amount for detection at 820nm
- needs electronic detector: to decide if the images are crossed or uncrossed as the patient and practitioner can’t see the IR
list 4 disadvantages of the scheiner principle
- target can stimulate accommodation
- scale is non-linear: bigger gaps in dioptres for hyperopes and smaller gaps in myopic range
- apparent size of target varies: as you bring the target closer, its retinal image size gets bigger, so the target gets easier to see as you move closer to the eye
- proximal accommodation: px knows the target is not at infinity, so they apply proximal accommodation
list the 2 advantages of infrared optometers
- better assessment of focus
- target cannot stimulate accommodation (as IR is invisible)
list 3 disadvantages of infrared optometers
- scale is non-linear: bigger gaps in dioptres for hyperopes and smaller gaps in myopic range
- apparent size of target varies: as you bring the target closer, its retinal image size gets bigger, so the target gets easier to see as you move closer to the eye
- proximal accommodation: px knows the target is not at infinity, so they apply proximal accommodation
what 4 problems does the badal optometer solve
- better assessment of focus
- target can stimulate accommodation
- scale is non-linear
- apparent size of target varies
what is the one thing that the badal optometer cannot solve
proximal accommodation
where in the fundus is IR reflected compared to visible radiation
from deeper in the fundus
where in the fundus is IR reflected compared to visible radiation
from deeper in the fundus (from sclera) than visible radiation
which layer of the fundus is visible light reflected from, e.g. from ret and why is this a disadvantage
internal limited membrane = first layer of the fundus
not great for measuring refractive error as want to get reflectance from deeper structures
what happens as a result of IR radiation being reflected from the sclera and what needs to be done to correct it
eye appears to be longer i.e. more myopic than it really is
correction from 800nm to 550nm = approx -0.50DS, this cancels out with the 0.75D hyperopia from LCA
how does an auto refractor measure sphere-cylindrical errors/astigmatism and what is the advantage to this method
the variation of refraction in different meridians is sinusoidal i.e. power changes along different axis, so is not just a 2 principle meridian, and also refractive error doesn’t go in 0.25D steps but are actually continuous
- in practice, the auto refractor measures refractive error from 6 different meridians
- this makes the estimate of refractive error significantly more accurate
- it takes these 6 measurements very quick
how does an auto refractor measure sphere-cylindrical errors/astigmatism and what is the advantage to this method
the variation of refraction in different meridians is sinusoidal i.e. power changes along different axis, so is not just a 2 principle meridian, and also refractive error doesn’t go in 0.25D steps but are actually continuous.
in theory, an auto refractor only needs to measure the sphere-cylinder component of refractive error in 3 meridians to calculate the sphere/cyl axis refractive error.
- in practice, the auto refractor measures refractive error from 6 different meridians
- this makes the estimate of refractive error significantly more accurate
- it takes these 6 measurements very quick
how is small pupils a problem with auto refractors
auto refractors can’t get enough IR back out of the eye i.e. low light intensity (same problem with ret)
how much DS can a typical auto refractor get a reading up till
± 25DS
how much DC can a typical auto refractor get a reading up till
± 8DC
what element does a typical auto refractor have in order to relax accommodation
autofogging to prevent proximal accommodation
what subjective refraction facilities do some typical auto refractors have
internal targets:
- duochrome
- x cyl
- fan and block
- subjective refraction
as well as detecting refractor error, what other measurement can most auto refractors carry out
keratometry
how do targets in auto refractors cope with proximal accommodation
go in and out of focus
other than automatic or progressive fogging, what other method can auto refractors use to control proximal accommodation
some try to control convergence
list the steps of how an auto refractor uses progressive fogging to work out a persons refractive error which is +1.50D
- first time auto refractor measures +1.00D
- then fogs the eye with a +1.75D to relax accommodation
- it then removes the +1.75D fogging lens, the eye will accommodate, but not quick enough
- the auto refractor takes a measurement before the eyes can accommodate, if +1.25D
- so the eye is now a bit more +ve as the accommodation is a bit more relaxed
- it then again adds the +1.75D fogging lens to relax accommodation
- it removes the fogging lens
- and gets a reading of +1.50D (more +ve so can see more relaxing of accommodation)
- it adds the +1.75D fogging lens
- removes the fogging lens
- gets a reading of +1.50D - accommodation not relaxed anymore, so auto refractor gets as much +ve power as it can get from the eye
- end point = +1.50D
what must be done after taking auto refractor readings of a patient’s refractive error
must do subjective refraction before prescribing
name 3 hand held auto refractors
- nikon retinomax
- nidek palm arc
- welsh allyn suresight
what is the: weight measurement wavelength time for each measurement and cost of the nikon retinomax hand held auto refractor
- weight: approx 1KG
- measurement wavelength: 860nm (IR)
- time for each measurement: 0.01ms
- cost: £10,000
list 5 drawbacks of IR optometers
- pupil diameters of 3mm or less produce an undetectable output
- saturated output if patient links
- cost
- proximal accommodation remains an issue, even with all the tricks of automatic fogging etc
- they will always remain an unintelligent ret
