AUTOREFRACTOMETRY: PRINCIPLES  & PROCEDURE

BY: Dr. Samarth mishra

The refractometry( optometry) is an alternative method of finding out the error of refraction by use of an optical equipment calledà REFRACTOMETER/ OPTOMETER

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.

Myopicà bundles cross each other before reaching the retinaà two small spots of lights are seen.

Hypermetropicà rays are intercepted by the retina before they meet àagain two small spots are seen.

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.

Myopiaà

Hyper-metropiaà

THE OPTOMETER PRINCIPLE:

Porterfield{1759}à 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.

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

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.

LIMITATIONS OF THE EARLIER OPTOMETERS:

1. a) ALIGNMENT PROBLEM

2. b) IRREGULAR ASTIGMATISM

3. 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!!

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 accommodateà instrument myopia.

-alters the actual refractive status of the pt.

GENERAL COMPARISON OF SUBJECTIVE & OBJECTIVE INSTRUMENTS:

Source of light:

objective refractometersà use  invisible infrared light.

subjective refractometer à use visible light.

Time required for refraction:

objective refractometerà2-4 mins.

subjective refractometerà4-8 mins.

Information provided:

objective refractometerà provide less info.

subjective refractometerà supply more info.

Pt cooperation factors:

objective refractometerà req less pt cooperation.

subjective refractometerà 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.

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.

Over refraction capability:

– over-refraction in pts using spectacles,contact lenes,iol is difficult with objective refractors d/t reflection.

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  àwhen the pt identifies the scene as one seen charecteristics.

at distance.

Primary source of electromagnetic radiation in ARà  

            -near infrared radiation (NIR)

            – 780-950 nm.

            -NIR is efficiently reflected back from fundus.

            -is invisible to pt.

Secondary source of electromagnetic radiationàback scatter from fundus.

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.

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.

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.

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

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.

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.

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 ànikon NR-5500

              ànikon retinomax

              àtomey TR-1000

              àcarl zeiss meditec “acuitus”

              ànidek OPD-scan

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)

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. 

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

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

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.

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

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.

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.

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.

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.

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.

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