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R. Sumalini and P. Satgunam
Fig. 1.10 The LEA numbers near visual acuity chart
particularly when handling charts used to assess
contrast sensitivity or low-contrast visual acuity,
as the discoloration of the charts can make the
low-contrast stimuli more challenging to resolve,
thereby underestimating the patient’s contrast
values.
1.5 Conclusion
High- and low-contrast visual acuity and contrast
sensitivity measures are useful screening parameters to help the clinician understand the patient’s
complaints. They are also important outcome
measures to quantify the effectiveness of any
intervention or to understand the disease progression. Inadequate awareness of the techniques
used to measure these visual function parameters
among clinicians can lead to errors in
documentation and intervention. Although these
are commonly noted as quick measures, appropriate thresholding techniques should be used
while recording to avoid over/underestimating
the same. Vulnerable groups such as children
(particularly those with special needs) and individuals with visual impairment are likely to need
extra time and care, as there could be a wider
test–retest difference in their visual function
parameters owing to the severity of the condition
and overall developmental delays.
Acknowledgments We thank Ms. Shakthi Pradheepa for
her help with the gures.
Funding Funding Hyderabad Eye Research Foundation,
Hyderabad, India.
Disclosure None.
1 Visual Acuity: High Contrast andLow Contrast
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Autorefraction: Objective
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Estimation ofRefractive Error
2
ShrikantR.Bharadwaj
2.1 Introduction
Uncorrected refractive error is the leading cause
of visual impairment and one of the most common causes of avoidable blindness globally [1,
2]. Inaccurately corrected refractive errors also
lead to undesirable consequences such as suboptimal visual resolution, asthenopia, binocular
vision anomalies, and amblyopia in children; all
of these adversely affect the individual’s quality
of life and daily activities [3–5]. Therefore,
screening for uncorrected refractive errors and
accurate correction is vital in eye care [6].
Identifying and correcting such errors in a typical
eye examination are a two-step process, commonly designated as “objective” and “subjective”
refraction. Objective refraction involves the eye
care practitioner estimating the refractive error of
the eye through one of several “objective” techniques, while subjective refraction involves rening the refractive error estimates to optimize the
patient’s vision by eliciting “subjective”
responses from the patient. The endpoint of
objective refraction is typically used as the start-
S. R. Bharadwaj (*)
Brien Holden Institute of Optometry and Vision
Sciences, Prof. Brien Holden Eye Research Centre,
L V Prasad Eye Institute, Hyderabad, India
e-mail: bharadwaj@lvpei.org
ing point of subjective refraction. This chapter
will focus on specic automated techniques commonly used in objective refraction. Retinoscopy,
the present-day gold standard for objective
refraction, against which all other autorefractors
are compared will be discussed along with subjective refraction techniques in other chapter(s)
of this book.
Several techniques have been developed over
the years to objectively estimate the eye’s spherocylindrical refractive error [7, 8]. Given the automated nature of refractive error estimation, sans
the intervention of a human observer, this technology has come to be broadly referred to as
“autorefraction,” and the instrumentation associated with autorefraction is referred to as “autorefractors.” This chapter will focus briey on the
measurement techniques used in these autorefractors, their accuracy, vis-à-vis, retinoscopy,
repeatability, and their scope of use in tertiary eye
care settings versus mass screening of uncorrected refractive error in public health settings.
Given the intended purpose of this book as a
ready reckoner for optometrists and ophthalmologists about the different technologies involved
in automated refraction, only broad overviews of
these technologies will be provided. For specic
technical details on the relative performance of
different autorefractors, the readers are referred
to the recent publications by Padhy etal. [7] and
Venkataraman etal. [8].
© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024
T. Das, P. Satgunam (eds.), Ophthalmic Diagnostics, https://doi.org/10.1007/978-981-97-0138-4_2
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2.2 Technology
those that measure the refractive status of each
eye separately (monocular techniques) or of both
2.2.1 Measurement Principle
ofAutorefractors
eyes simultaneously (binocular techniques), (2)
those that control for the accommodative status
of the eye by allowing presentation of real-world
Autorefraction technology may be categorized
into several types based on instrument design and
measurement principles (Table 2.1). Each technology has advantages and disadvantages that
determine its accuracy, precision, and utility in
tertiary-level care or mass screening in public
health endeavors. Depending on the instrument
design, autorefractors can be categorized as: (1)
Table 2.1 Detailed characteristics of currently available common refractive error measurement techniques
Characteristics Retinoscopy
Working
principle
Operating range No specic
Invasiveness of
measurement
Portability Portable Table-top Portable Portable
Measurement
time
Fixation
distance
Pupil size
dependence
Binocular
viewing/
measurements
Participant
cooperation
Examiner
training
Near-triad
measurements
Examples of
commercial
devices
Data on operating ranges and measurement time are obtained from the manufacturer-prescribed user manual of each
instrument. Table adapted from Padhy etal. [7]
Foucault knife-edge Scheiner pupil | ray
operating range
Non-invasive
Measurements
performed from a
distance
Measurement takes
time
Distant target Near target with simulated
Measurements
become challenging
with pupil miosis
Viewing can be
binocular, but the
measurement is
monocular
Needs limited
cooperation
Useful in
challenging cases
Examiner training is
time-consuming
Only measurement
of accommodation is
possible
Welch Allyn 18245
Elite HPX Streak
Retinoscope
Open/closed-eld
autorefractors
deection | best-focus
+22 D to −25 D for sphere
and±10.00 D for cylinder
Non-invasive
Participant stabilized on
forehead rest
~5 measurements per
second
distance viewing
Stable measurements for
pupil diameter up to 2mm
Only monocular viewing
and measurements
Participant cooperation is
essential for reliable
measurements
Examiners can be trained
within a short time
Only measurement of
accommodation is possible
(in closed-eld, only the
measure of refractive error
is possible)
Nidek TONOREF™ III |
HandyRef-K | ShinNippon NVision-K 5001
targets through an open-eld viewing design or
those that “fog” the stimulus using a closed-eld
viewing design, and (3) those that use the entire
pupil area for estimation of refractive error or
those that use only the central pupil for estimation of refractive error. These autorefractors use
one of several measurement principles, including
wavefront analysis, Scheiner disc principle, best
Wavefront-based
autorefractors Photorefractors
Wavefront
aberrometry
±10.00 D for sphere
and±6.00 D for
cylinder
Device touches the
face for
measurement
~10 measurements
per second
Distant target Distant target
Measurement
accuracy may vary
with pupil diameter
Only monocular
viewing and
measurements
Participant
cooperation is
critical for reliable
measurements
Examiners can be
trained within a
short time
Only measurement
of accommodation is
possible
E-see | Visionix Eye
Refract |
WaveAnalyzer 700
Eccentric infrared
photorefraction
±7.50 D for sphere
and±3.50 D for
cylinder
Non-invasive
Measurements are
performed from a
distance
<1s per
measurement
Measurement
accuracy decreases
with pupil miosis
Viewing and
measurement can
be binocular
Needs limited
cooperation
Useful in
challenging cases
Examiners can be
trained within a
short time
Near-triad can be
measured in sync
with each other
Welch Allyn Spot
Vision Screener |
PlusoptiX A12C
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focus principle, proprietary rotary prism technology (i.e., ray-deection), and photorefraction.
Autorefractors dependent on wavefront analysis typically shine a coherent light source in the
infrared wavelength into the eye and capture the
light reected from the retina onto a Shack–
Hartmann-based wavefront sensor. The sensed
wavefront, at the level of the eye’s pupil, is then
broken down into component Zernike polynomials from which the sphero-cylindrical refractive
error is calculated [9]. The lower-order aberration
of defocus (Z
) corresponds to the spherical
2,0
refractive error, while the cylindrical error of the
eye is computed from a combination of the defocus and the two astigmatism terms [vertical astigmatism (Z
) and oblique astigmatism (Z
2,2
2,-2
)]
terms of the Zernike polynomial series [9]. The
readers are referred to the excellent review on the
topic of clinical applications of wavefront-based
refraction by Bruce and Catania [10] for details
on the measurement of the eye’s wavefront aberrations and the Zernike polynomial-based representation of the eye’s wavefront aberrations (the
Z-notation stated above) and the various dependencies of the eye’s wavefront aberration prole
(e.g., pupil diameter, viewing distance) [10].
Autorefractors that use Scheiner’s disc principle essentially perform the same measurement as
the wavefront-based technique, with the exception that the reected light from the retina is converted into a diplopic image using Scheiner’s
pupil. In addition, the dioptric position of the sensor within the autorefractor where the diplopic
components converge to become a single image
is considered as the refractive error of the eye.
The orientation of Scheiner’s pupil is changed to
estimate the refractive error of the eye across different meridians to compute the astigmatic
refractive error of the eye. Autorefractors relying
on the ray deection technique, in principle,
work similarly to the previous technology, but
given the proprietary nature of this technology,
the exact details of its operation remain unknown.
Likewise, autorefractors, relying on the best
focus technique, attempt to determine the conjugate focus of an internal target in different meridians to determine the sphero-cylindrical refractive
error of the eye.
Photorefractors (also called photoscreeners
[11]) determine the spherocylindrical refractive
error by sending in infrared light from a series of
LEDs placed slightly eccentrically from the camera aperture and analyzing the pattern of luminance prole formed across the pupil by the light
reected from the retina (Fig.2.1a). An emmetropic eye has a uniform pattern of luminance
across the pupil, while those with refractive
errors tend to have a graded prole, the orientation of which is dependent on whether the light is
focused in front of the retina (myopia) or behind
the retina (hypermetropia) (see example of luminance prole with induced myopia in Fig.2.1b).
This luminance prole is quantied using standard linear regression analysis and converted into
units of diopters (D) using a defocus calibration
factor (Fig.2.1c). The luminance prole formed
across the pupil and, hence, the refractive error
estimated by the photorefractor depends on several factors, such as the overall light intensity
reected off the retina, the pupil diameter, and
the subject’s ethnicity [12, 13]. All these limit the
operating range of the photorefractor and make
the refractive error estimates prone to large interindividual and inter-ethnic variations (Fig.2.1c,
d) [12, 13].
All these autorefractor designs are available as
tabletop or handheld designs that determine their
portability. As an aside, some autorefractors also
come with the advantage of being able to estimate additional ocular parameters that are associated with the refractive error of the eye (e.g.,
pupillary diameter and vergence eye position as
part of the near-triad keratometry to estimate corneal curvature and axial length measurements to
determine the source of the uncorrected refractive error).
2.2.2 Relative Advantages/
Disadvantages ofDierent
Autorefractors
Each autorefractor design has certain advantages
over the others and may be useful in specic
settings or for particular populations (Table2.1).
For instance, photorefraction can rapidly obtain
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Fig. 2.1 An eccentric infrared photorefractor used for
estimating the eye’s refractive error (Panel a). The infrared light that is mounted eccentric to the camera aperture
is seen in this panel. The luminance prole across the
pupil in a representative emmetropic subject without (top
panel) and with (bottom panel) induced refractive error
(Panel b). The pupil was uniformly illuminated at the
baseline, while the luminance prole gradually changed
from bright to dark from the top to the bottom of the pupil,
with induced myopia in the right eye (bottom panel). The
slope of this luminance prole is calibrated into diop-
simultaneous estimates of refractive error from
both eyes and the near-triad from a remote distance; this is advantageous when scanning
infants, children, and special-needs populations,
where cooperation may be limited (Table 2.1,
Fig.2.1a). Variability in the defocus calibration
prole and the operating range of the photorefractor (Fig.2.1c, d) limit its utility as an accurate
ters—the defocus calibration slope. The intersubject variability in the defocus calibration slope may induce
variability in the refractive error measurements of photorefraction (Panel c). The refractive error (anisometropia) measured by the photorefractor was plotted as a
function of the induced refractive error (anisometropia)
for two representative subjects (Panel d). The vertical
arrows in this panel indicate the values where the refractive error measurements saturate, reecting the operating
range of the photorefractor
and precise device for estimating the objective
refractive error of the eye. This technology is,
therefore, conned to being mainly used as a
screening tool for refractive errors in a public
health setting [11]. However, photorefractors are
useful in research settings that investigate the
near-triadic properties of the visual system [14,
15], and have more recently also been used to
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identify ocular refractive pathologies like keratoconus [16] and spasm of near reex [17].
Much like photorefraction, autorefraction
technologies based on wavefront aberrometry
also use the entire pupil to determine the refractive error status of the eye [7, 8]. Due to the inher-
ent dependence of wavefront aberrometry on
pupil size, these autorefractors have built-in technologies that estimate pupil diameter and scale
the wavefront measurements to that pupil diameter. The refractive error estimates, therefore,
may vary slightly depending on the pupil size.
Given that this technology estimates several
orders of wavefront errors of the eye, they may be
more useful in estimating the refractive error in
people with highly aberrated optics (e.g., eyes
with keratoconus or post-corneal transplantation). However, this remains a theoretical possibility for now, and additional scientic evidence
must be gathered to support or refute this
possibility.
Autoreectors based on the Scheiner disc,
best focus, or ray-deection principles only utilize the central pupil to measure refractive error
[7, 8]. These become the choice of measurement
for patients with high refractive errors outside the
operating range of other techniques, with high
accuracy and repeatability (Table2.1). The ability of technology to control the accommodative
status of the eye to minimize instrument myopia
and reveal the maximum hyperopic refractive
error has been subject to intense scientic investigation. In general, technologies within this
domain that have an open-eld viewing design
have been shown to exert better control over the
accommodative state of the eye relative to their
counterparts with closed-eld designs that
attempt to minimize the impact by using blurred
xation stimuli with a linear perspective.
However, the refractive error output from these
autorefractor designs is quite sensitive to patient
alignment and typically requires them to be positioned in a chinrest for reliable measurements.
This tends to make the design bulky and demanding, thus limiting its portability and utility in
infants, children, and individuals with special
needs. Further, autorefractors within this domain
are monocular, cannot measure the vergence sta-
tus of the eye, and do not come with algorithms
that can measure the pupil size (although such a
measurement is theoretically possible). While
open-eld autorefractors were traditionally used
in research settings that required accurate control
over the accommodative demand [18], more
recently, they have also gained popularity in measuring the peripheral refraction of the eye [19], a
critical component in the management of myopia
and its progression [20].
2.3 Clinical Applications
2.3.1 Accuracy andPrecision
Considerations
The aforementioned advantages of any autorefractor technology are contingent on the technology producing accurate and repeatable
measurements. Accuracy is the closeness of the
technique’s estimate of the parameter of interest
(refractive error, in this case) to a gold standard
value [21, 22]. Simultaneously, repeatability
(also called reliability or precision) is the variability in estimating the given parameter over
repeated measurements [21, 22]. Repeatability
also reects the measurement technique’s ability
to discriminate one value of the given parameter
from another—the higher the precision, the better
the discriminant capability [21, 22]. For screening purposes, an allowance for measurement
accuracy can be made so that the device’s sensitivity and specicity in identifying a predetermined level of refractive error are not
compromised. For diagnostic settings, the accuracy of measurements is more critical as these
become the starting point for subjective refraction or, in some challenging cases where subjective refraction is not possible, and the refractive
error correction is based entirely on the objective
refraction value (e.g., young children or mentally
challenged individuals). For both settings, high
measurement repeatability is desirable to increase
faith in the outcome measures.
Comparative analyses of accuracy and repeatability across different autorefraction techniques
obtained from the same cohort of subjects pro-
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vide critical input on one technique’s performance against others, enabling evidence-based
justication of their utility as a starting point for
subjective refraction in the clinic and/or as a
screening tool for refractive errors in public
health settings. Two recent studies—Padhy etal.
[7] and Venkataraman etal. [8]—systematically
addressed this issue and found similar results. On
an average, none of the autorefractors showed
larger biases in the estimates of spherical and
cylindrical refraction relative to retinoscopy, but
there was signicant inter-technique variability
in the results (Fig.2.2). The estimate of refractive
error by an autorefractor could differ from retinoscopy by as much as ±2.5 D for the spherical
component and by as much as ±1 D for the astigmatic component (Fig.2.2) [7]. Surprisingly, this
difference does not seem to be impacted by the
patient’s age [7], as one would expect from
younger children with stronger accommodative
tendencies [23]. The inter-session repeatability of
refractive error estimates was within ±1 D
and±0.75 D in the spherical and astigmatic components, respectively, across all measurement
techniques [7]. Photorefraction showed marginally poorer repeatability than the other techniques
(Fig. 2.3) [8]. Open-eld autorefractors and
closed-eld autorefractors with an inbuilt fogging mechanism showed better accuracy and precision than those that did not incorporate such a
mechanism [8]. However, the inter-session
repeatability of the measurement techniques was
smaller than the inter-instrument repeatability,
indicating that some of the variability shown
Fig. 2.2 Bland–Altman type plots of the agreement in M
(top panels) and J0 (bottom panels) power vector values
between the three autorefractor designs and retinoscopy
evaluations in this study (see Sect. 2.3.2 for details of
power vector analysis). The solid dashed lines in each
panel indicate the mean difference (MD) and the 95% limits of agreement (LOA). The numerical values of these
parameters are also indicated in each panel. The negative
values along the abscissa of each panel indicate myopic
refraction. The positive and negative values along the
ordinate of each panel indicate an overestimation and
underestimation bias by a given autorefractor relative to
retinoscopy. (Figure adapted from Padhy etal. [7])
c
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Fig. 2.3 Repeatability of M, J0, and J45 power vector
components obtained across different autorefractor
designs (see Sect. 2.3.2 for details of power vector analysis). SPH sphere, M spherical equivalent, J0, and J45 cylindrical vector components, VX eye refract, WA
WaveAnalyzer 700, NIII TONOREF™ III, NH handheld
ref./keratometer Handy Ref-K, PO PlusoptiX A12C, SN
NVision-K 5001. (Figure adapted from Venkataraman
etal. [8])
above in Fig. 2.2 occurs genuinely due to the
autorefractor technology mis-estimating the
eye’s spherocylindrical refractive error. Since the
cohort in both studies had regular refractive
errors, the accuracy and precision in estimating
the oblique and irregular astigmatism components remain to be fully evaluated. Such an investigation will offer important insights into the
utility of autorefractor technology for estimating
the refractive errors of eyes with complicated
optics (e.g., keratoconus).
2.3.2 Legitimate Representation
ofSphero-Cylindrical
Refraction
This section is not unique to the estimation of the
eye’s refractive error by autorefractors; it applies
to the representation of sphero-cylindrical refraction in general, as estimated by retinoscopy,
autorefractors, and subjective refraction or corrections applied to the eye using spectacles or
contact lenses. This section is motivated by the
routine errors observed in the representation of
21
sphero-cylindrical refraction while comparing
between instruments, describing longitudinal
data obtained from patients, describing the
impact of optical/surgical interventions on the
eye’s optical power, or while comparing the
sphero-cylindrical refractions of two cohorts of
patients (e.g., keratoconus vs. healthy controls).
Consider a scenario where the patient’s refractive
error is +2.00 DS/−1.00 DC × 70° on the rst
visit and +2.50 DS/−2.00DC × 30° on the second visit. Clearly, the spherical equivalent of
refraction (1.50 D) remains the same between the
two visits. However, making inferences about a
change in the astigmatic component of the two
refractions is less straightforward because there
is an alteration in both the magnitude and axis of
the astigmatism. Often, an inference about the
change in astigmatism is arrived at by simply
comparing their magnitudes while ignoring their
axes. This is incorrect because astigmatism is a
vector inherently dened by its magnitude and
axis. It is more legitimate to compare the magnitudes of astigmatism when they are referenced to
the same axis. Several mathematical formulations have been derived to achieve this, notably
including the power vector analysis by Thibos
etal. [24] and vector decompensation analysis by
Alpins etal. [25] and Harris etal. [26]. The former analysis will be briey described here; readers interested in a deeper understanding of this
topic are urged to read the abovementioned
citations.
The power vector analysis breaks down any
sphero-cylindrical refraction into three terms: M,
J0, and J45, the equations for which are noted
below (Eqs. 2.1–2.3) [24]. The term M
corresponds to the traditional spherical equivalent of refraction that represents the overall
refractive error of the eye, the correction of which
will move the Conoid of Sturm onto the retina.
The terms J0 and J45 correspond to the crosscylinder astigmatism terms of Jackson’s crosscylinder lens, the correction of which will
collapse the Conoid of Sturm into a point image
on the retina. The J0 component represents the
equivalent astigmatic power along the 90° and
180° axes, while the J45 component represents the
same along the 45° and 135° axes. Eyes with
regular with- or against-the-rule astigmatism will
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