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
5.
6.
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 refractometer2-4 mins.
subjective refractometer4-8 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.
16.
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 radiationback
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.
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.