Autorefractometry: principle and procedure.

1. BY: Dr. Samarth mishra

2.  The refractometry( optometry) is an alternative method of finding
out the error of refraction by use of an optical equipment called
REFRACTOMETER/ OPTOMETER

3. OPTICAL PRINCIPLES
 SCHEINER PRINCIPLE:
 scheiner{1619} observed that refractive error of eye can be
determined by using double pinhole apertures.
 Parallel rays of light entering the eye from a distant object,are
limited to two small bundles when double pinhole apertures are
placed in front of the pupil.
 Myopic bundles cross each other before reaching the retina
two small spots of lights are seen.
 Hypermetropic rays are intercepted by the retina before they
meet again two small spots are seen.

4.  These two points of light can be coalesced to a single point by
moving the double pinhole to the far point of eye.
 So from far point of eye,refractive error of eye can be found.
 Myopia
 Hyper-
metropia

7.  THE OPTOMETER PRINCIPLE:
 Porterfield{1759} coined term optometer.
 use a single converging lens placed at its focal length from the
eye instead of interchangeable trial lenses.
 Light from the target on far side of lens enters the eye with
vergence of zero/minus/plus, depending on the position of target.
 Vergence of light in the focal plane of optometer lens is linearly
related to the displacement of target.

8.  A scale with equal spacing can thus be made which would show the
no. of dioptres of correction.
( optometer principle)

9. EARLY REFRACTOMETERS
 Early subjective optometers:
 During 1895-1920 were all subjective.
 Needed the pt. to adjust the instrument for best focus.
 Were unsuccessful because of instrument accomodation.
 E.g : badal optometer,
young optometer.
 Early objective optometers:
 Depends on examiner’s decision on when the image is clearest.
 Mainly based on optometer principle.

10.  LIMITATIONS OF THE EARLIER OPTOMETERS:
a) ALIGNMENT PROBLEM
b) IRREGULAR ASTIGMATISM
c) ACCOMODATION
 ALIGNMENT PROBLEM:
-As per scheiner principle both pinhole apertures must fit within
patient’s pupil.
-If the pt. fixation wanders or he/she move the head,the reading
is invalid.
-Thus,considerable cooperation is required!!

11.  IRREGULAR ASTIGMATISM:
-in irregular astigmatism, the best refraction over the
whole pupil maybe different in contrast to the two small
pinhole areas of the pupil.
 ACCOMODATION:
-on looking into the instrument,pt tends to
accommodate instrument myopia.
-alters the actual refractive status of the pt.

12. GENERAL COMPARISON OF SUBJECTIVE &
OBJECTIVE INSTRUMENTS:
o Source of light:
objective refractometers use invisible infrared
light.
subjective refractometer  use visible light.
o Time required for refraction:
objective refractometer2-4 mins.
subjective refractometer4-8 mins.

13. o Information provided:
objective refractometer provide less info.
subjective refractometer supply more info.
o Pt cooperation factors:
objective refractometer req less pt cooperation.
subjective refractometer req more cooperation .
In general ,it has been observed that children >5yrs of
age can be refracted with objective refractometer,while
for subjective refractometer use the child should be
>8yrs of age.

14. o ocular factors:
-ocular diseases may limit the performance of
refractometers.
-in hazy media causing a drop in VA<6/18 ,the objective
refractometers donot function properly.
-in macular diseases objective better than subjective
refractometers.
o Over refraction capability:
- over-refraction in pts using spectacles,contact lenes,iol
is difficult with objective refractors d/t reflection.

15.  Some common charectristics of ARs:
fixation target & control of accomodation:
-fixation target is provided to help control the patient’s
fixation & accommodation.
-mostly coloured photographs of outdoor scenes with
prominent central features in the distance are used.
-[accommodation is most relaxed when]:
a prominent feature is of low spatial frequency natural scenes
the visual scene has a wide band of spatial have
frequencies for observation. these
when the pt identifies the scene as one seen charecteristics.
at distance.

17.  Primary source of electromagnetic radiation in AR
-near infrared radiation (NIR)
- 780-950 nm.
-NIR is efficiently reflected back from fundus.
-is invisible to pt.
 Secondary source of electromagnetic radiationback
scatter from fundus.

18.  Nulling principle refractometers:
-change their optical system until the refractive error of
eye is neutralized. i.e until end point is reached.
-can function in higher signal/noise ratio.
• Open loop principle refractometers: (non-nulling)
-make measurements by analyzing the characteristics of
the radiation exiting the eye.
-more quick as they don’t require to change their optical
systems.

19.  objective refractometers are based on one or
more of following working principles:
 The scheiner principle
 The optometric principle
 The best focus principle
 The knife edge principle
 The ray-deflection principle
 The image size principle.

20.  ARs BASED ON SCHEINER PRINCIPLE:
-dates back to christopher scheiner,1619.
-light from a primary point source at near was collimated
through a condensing lens passed through the “scheiner’s
disc” directed to the eye.
-modern version of scheiner principle is accomplished with
infrared light emitting diodes(IR-LEDs) that are optically
presented in substitution for the apertures in a scheiner disc.

21.  AR based on scheiner principle:
acuity systems 6600
grand seiko(RH burton’s BAR 7 in the USA;BAR 8 with
autok)
nidek (marco’ s AR-800 & 820 in the USA;ARK 900 with
autoK)
takagi
topcon

22.  Basic working features of scheiner’s principle
autorefractors:
nulling refractors: these are nulling refractors that
optically substitute IR-LEOs for the apertures of a
traditional scheiner’s disc.
concept of badal optometer: is used to accomplish
projection of NIR into the eye,collection of fundal reflex
& determination of refractive status.

23. corneal reflex: is removed & the vitreoretinal reflex is
attenuated by a corneal reflex block introduced into the
path of radiation returning from the fundus.
specialized photodetection device: to analyse the
position of fundus images created by the optical train.
meridional refractive errors: are neutralized & the two
primary meridia of the eye are found by a second nulling
process.
refractive power endpoints: these ARs can reach
refractive power endpoints at speeds of 100 D/SEC.

24.  ARs BASED ON RETINOSCOPIC PRINCIPLE:
 Called autoretinoscopes. Based on following two chr.
 Direction of motion of the observed fundus reflex: with the
direction of motion of incident radiation.
 E.g bausch & lomb ophthalmetron.( no longer available in
market)
 Speed of motion of observed fundus reflex: with respect to the
motion of incident radiation.
E.g nikon NR-5500
nikon retinomax
tomey TR-1000
carl zeiss meditec “acuitus”
nidek OPD-scan

25.  Basic working features of retinoscopic principle ARs:
source optical train of an autoretinoscope imitates the
function of a streak retinoscope.
motion of incident rectangular beams is usually created
by a slotted drum rotating about a source of NIR.
refractors based on the analysis of the direction of
motion of the retinoscopic fundus reflex are nulling
refractors.
- neutralization by use of badal optometer.
autoretinoscopes based on the analysis of the speed of
motion of fundus reflex are open loop(non-nulling)

26. corneal reflex: is masked from photodetection as it falls
on the spaces between photocells.
vitreoretinal & corneal reflexes can be filtered by the
polarization of incoming NIR to the eye & the removal of
polarized NIR returning from the eye in the fundus
image.
autoretinoscopes are meridional refractors.
photodetection devices are usually composed of 2-4
photocells.

27. ARs BASED ON BEST FOCUS PRINCIPLE:
Utilizes the automatic detection of a change of image
contrast at the fundus.
i.e by capturing the vergence of incident radiation
necessary to bring about max contrast.
dioptron
canon autorefractor ( based on best focus
principle)
hoya autorefractor

28.  Basic working feature of dioptron:
( rest no longer available)
are both nulling & meridional refractors.
neutralization is achieved with the use of badal optometer.
refractive endpoint is obtained when the referred image of a
secondary fundus source attains highest contrast at the plane
of a photo detection device.
vitreoretinal reflex
corneal reflex, can be filtered by the polarization
& removal of polarized NIR
returning from the eye
coaxial reflex

29.  ARs BASED ON KNIFE-EDGE PRINCIPLE:
knife edge principle is related to retinoscopy,& it is the basis
of photorefraction.
it is a retroreflective method using the same entrance & exit
pupil of the device under test.
Basic working principle:
use the concept of reciprocity such that radiation from the
fundus reflex is returned to the primary source.
are nulling refractometers.
Cylinder power & axis are neutralized with the use of two
stokes lenses.
coaxial reflexes from the extensive common optical path can
be reduced by tipping of common-path elements.

30. ARs BASED ON RAY-DEFLECTION PRINCIPLE:
E.g : canon R 30
hoya
welch-allyn sure-sight(hartmann -shack
handheld)
VISX waveScan(hartmann -shack wavefront
refractor)
bausch & lomb zywave (hartmann-shack wavefront
refractor)
alcon ladarwave(hartmann-shack wavefront
refractor)
topcon KR-9000PW

31. BASIC WORKING PRINCIPLE:
 Are open loop (non-nulling) meridional refractors.
 Design is similar to that of autoretinoscope & to a
scheiner principle refractor in that discrete,fixed
pupillary areas are used.
 Measures the linear deflection of fundus image.
 Primary source & photodetectors are fixed.
 Corneal reflex removed by placing a central aperture in
a plane conjugate to the pupil in the detection path.
 Coaxial reflexes from the few common path elements
can be filtered by the polarization & removal of
polarized NIR returning from the eye.

32.  ARs BASED ON IMAGE-SIZE PRINCIPLE:
E.g: grand seiko( RH burton’s handheld BAR 600 in USA)
grand seiko WR 1500K( a “see-through” instrument)
topcon RM-A7000
basic working principle:
 Size of optical image on the retina is a function of the
refractive error.
 The refractive status therefore be determined by
measuring the size of an annular secondary fundus
source.

33.  Detection system consists a fundus camera: a
CCD(charge coupled device) camera is used as detector.
 Design is same as scheiner principle refractors.
 Instrument measures the size of fundus image in three
or more different meridia.
 Video imaging of fundus reflex accomplished by fundus
camera.
 Image analysis of video image performed by computer
programme.
 Refractive power found by open-loop(non-nulling)
process.

34.  SUBJECTIVE AUTOREFRACTORS:
 VISION ANALYSER:
introduced by humphrey in 1975.
was combined with humphrey lens analyser to form
over-refraction system.
 SR-IV PROGRAMMED SUBJECTIVE REFRACTOR:
uses optometer principle.
an axially moving cylindrical lens to achieve smoothly
variable spherocylindrical powerover a wide range.

35.  SUBJECTIVE AUTOREFRACTOR-7:
instrument has spherical optics only.
since no astigmatic correction is possible, and VA
cannot be measured with spheres, this is only a
screening instrument.

36. THANK
YOU