AUTOREFRACTOMETRY OR OPTOMETRY

AUTOREFRACTOMETRY OR OPTOMETRY

    Refraction being the most commonly performed optical procedure has been widely developed.
    Though the conventional technique of retinoscopic refraction is an excellent method of objective refraction, it is a time-consuming procedure and not every practitioner manages to accomplish it accurately.
    The refractometry (optometry) is an alternative method of finding out error of refraction by use of an optical equipment called refractometer or optometer.
    Refraction being the most commonly performed optical procedure has been widely developed.
    Though the conventional technique of retinoscopic refraction is an excellent method of objective refraction, it is a time-consuming procedure and not every practitioner manages to accomplish it accurately.
    The refractometry (optometry) is an alternative method of finding out error of refraction by use of an optical equipment called refractometer or optometer.

OPTICAL PRINCIPLES:

    The present day autorefractors (AR) are based on principles used in earlier attempts for automation of refraction.

    Most of autorefractometers are essentially based on following two principles:

    1. The Scheiner’s Principle

    2. The Optometer Principle

1. THE SCHEINER’S PRINCIPLE:

    Scheiner in 1619 observed refractive error of eye can be determined by using double pinhole apertures before the pupils. Following are his observations:

a. The parallel rays of light entering the eye from a distant object, which are normally focused on a point on the retina in an emmetropic patient(Fig A), are limited to two small bundles when double pinhole apertures are placed infront of the pupil. (Fig B)

b. In a myopic eye, the two ray bundles cross each other before reaching the retina and

two small spot of light are seen. (Fig C)

c. In the hypermetropic eye, the ray bundles are intercepted by the retina before they meet and

thus again two small spots of light are seen. (Fig D)

d. These two points of light can be coalesced to a single point by moving the double pinhole to the

far point of eye.

e. Thus from far point of eye, the refractive error of eye can be determined.

2. THE OPTOMETER PRINCIPLE:

    Porterfield, in 1759, coined the term optometer to describe an instrument for measuring the limits of distinct vision. The optical principle on which this instrument was based is now known as optometer principle.

    This principle permits continuous variation of power in refracting instruments. As shown in Fig, the autorefractometers based on this principle use a single converging lens placed at its focal length from eye (or spectacle plane) instead of interchangeable trial lenses.

    Light from target on far side of the lens enters the eye with vergence of different amounts

(ie zero (Fig B), minus (Fig C) or plus (Fig D) depending on position of target.

    The vergence of light in focal plane of optometer lens is linearly related to displacement of target.

    A scale with equal spacing can thus be made which would show number of dioptres of correction. Fig E

DEVELOPMENT OF OPTOMETERS

    The scheiner’s principle and optometer principle and their modifications have been used time and again to automate the clinical refraction. Presently, automated refraction has become a well-established technique.

    Numerous automated refractors have been devised during the last century. The modern electronic and computerized autorefractometers have rendered the previous optometers obsolete.

    However, a brief review of overall developments will be worthwhile, because in many respects optical systems of old optometers have been developed into those of their electronic successors and also because a reference to them is still made in research literature.

    In general, the development of optometers can be grouped as follows:

1. Early refractometers and

2. Modern AR

1. EARLY REFRACTOMETERS

Early Subjective OptometersThe earliest optometers developed during 1895-1920 were all subjective. These optometers required the patient to adjust the instrument for best focus or best alignment of parts of the target. These subjective optometers were unsuccesful because of instrument accommodation. Examples of ESO are 

1. Badaloptometer and 2. Young’s Optometer.

Early Objective Optometers

    Objective refractometers were developed to offer an alternative means of evaluating the optical correction of eye. However, these objective optometers were subjected to many of the uncertainities of retinoscopy with regard to accuracy of measurement.

    These, the so-called objective optometers, rely on the examiner’s decision on when the image is clearest or in coincidence setting. Thus, they were objective only in sense that patient’s subjective choice had been replaced by choice of experienced examiner.

These instruments were all based on optometer principle, and most of them incorporated scheiner’s principle as well. Three of these instruments that had been widely used in Europe in preference to retinoscopy are mentioned here because of their historic importance.

LIMITATIONS OF EARLIER OPTOMETERS:

    Three basic factors responsible for limited acceptance of Optometers in  Clinical refraction are as follows:

1. Alignment Problem,

2. Irregular Astigmatism 

3.Accommmodation.

MODERN REFRACTOMETERS

    With rapid development in electronics and microcomputers, a number of innovative methods and instruments for automated clinical refraction have appeared since 1960. Efforts have been made to eliminate the limitations of old refractors.

    The modern refractors can be grouped as 

1. Objective Refractometers

2. Subjective Refractometers.

Both Objective and Subjective AR are available commercially. A detailed description and comparison of major instruments, which are currently on market, is beyond the scope.

However, a general comparison of objective and subjective instruments and a brief description of some of instruments presently in use are given.

GENERAL COMPARISON OF SUBJECTIVE AND OBJECTIVE INSTRUMENTS

1. Source of Light

2. Time required for refraction

3. Information provided

4. Patient cooperation factors

5. Ocular factors

6. Over-refraction capability

7. Expected Results

OBJECTIVE AUTOREFRACTOMETERS

    Over the years, automated objective refractometers, often called merely AR, have evolved as high-tech devices as a result of electronic, electro-optical, charge-coupled device (CCD) cameras and computer revolutions.

    Presently, combination of automated refractors and automated keratometers are also in vogue.

COMMON CHARACTERISTICS OF AUTOREFRACTORS

1. Fixation Target and Control of Accommodation

2. Primary and Secondary Sources of Electromagnetic Radiation

3. Nulling versus Open-Loop Measurement Principle

4. Allowance for Ocular Refraction between Visible Light versus NIR

5. Allowance for the Plane of Reflection

6. Vertex Distance Consideration

1. Fixation Target and Control of Accommodation

A visible fixation target is provided in each instrument to help control the patient’sfixation and accommodation. The phenomenon known as proximal accommodationconfuses the determination of appropriate refractive correction.

Designers of automated refractors have often dealt with this by using visual fixation targets composed of colour photographs of outdoor scenes, with prominent central features in the distance.

Accommodation is most relaxed when a prominent feature is of low spatial frequency, when the visual scene has a wide band of spatial frequencies of observation, and when the patient identifies the scene as one typically seen at distance.

  Natural scenes have these characteristics, as do some other targets, such as Siemens stars or windmills. The abilities of these targets to successfully relax and stabilize accommodation when looking into an instrument under monocular or binocular conditions are suspect, and they depend greatly on the individual patient.

2. Primary and Secondary Sources of Electromagnetic Radiation

Primary:

Present day objective AR use near-infrared radiation (NIR) at wavelength between 780 and

950 nm as primary source of electromagnetic radiation because of following two reasons:

        NIR is efficiently reflected back from the fundus and 

        NIR is essentially invisibile to the patient.

Secondary: 

Used by objective AR is back scatter from the fundus. Operation of objective AR, the

method on which the determination of sphere power, cylinder power and cylinder axis,

depends on characteristics of the secondary source that are used by detection systems of

instruments.

3. NULLING VERSUS OPEN-LOOP MEASUREMENT PRINCIPLE

    Automated Retractors find the refractive error of the eye using either a nulling or an open-loop measurement principle:

Nulling principle refractometers

    Change their optical system until the refractive error of the eye is neutralized. Until null point is reached. The nulling instruments can be designed to function with higher signal/noise ratios, as the condition can be optimized near null point.

Open-Loop Principle Refractometers:

    The non-nulling instruments, make measurements by analysing the characteristics of radiation exiting the eye. Open-loop instruments are generally able to more quickly arrive at the refractive states because they are not required to alter their optical systems to move to the null point.

4.ALLOWANCE FOR OCULAR REFRACTION BETWEEN VISIBLE LIGHT AND NIR

    Since the eye is not achromatic, an allowance has to be made for the difference in ocular

refraction between visible light and whatever wavelength of IR is used. This is usually about

800-900nm, for which eye is 0.75-1.00 D hypermetropic relative to 550 nm. Provided the lenses

of the optometer itself are achromatic, their refractive power should not differ too greatly between

visible and NIRs.

5. ALLOWANCE FOR THE PLANE OF REFLECTION

    The plane of reflection within eye of visible radiation and IR may differ, and in any case either

or both of these may differ from the plane of the percipient layers of the retina. Thus, about

0.50-0.75 D allowance is to be made in addition to the effects of chromatic aberration.

    This suggests that IR is either being reflected from capillary bed of retina, about 0.3 mm in front of receptors, or it is reflected from several layers, the mean effect being equivalent to reflection from single place infront of receptors.

6. VERTEX DISTANCE CONSIDERATION:

    AR are constructed such that the full refractive error is determined at the plane of cornea (corneal plane refraction). Most modern AR have the option to convert corneal plane refraction into desired spectacle plane refraction by selecting from a range of vertex distances.

COMMERCIALLY AVAILABLE OBJECTIVE ARs

1. The Scheiner’s Principle

2. The Optometric Principle (Retinoscopic Principles)

3. The best-focus Principle

4. The knife-edge Principle

5. The ray-deflection Principle

6. The image size Principle