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Fig. 4.8 Examples of
(a) typical double vision
(horizontal diplopia) as
could arise from
horizontal strabismus.
(b) Double vision in the
case of aniseikonia
could appear as “front
and back” or one image
inside the other
be noted that the “double” reported by the patient
in anisometropia arises from aniseikonia. Hence
this “double” is not similar to that observed in
strabismus (Fig.4.8).
If the patient cannot tolerate it, then the difference in anisometropia should be reduced to a
tolerable level. This would involve reducing the
refractive correction in the more anisometropic
appropriate correction and not hesitate in
giving the correction, even if the acuity is
lower on the rst examination. The initial
blurred vision in the right eye would disappear if the patient could tolerate and adapt
well to the appropriate spectacle
correction.
eye. However, better solutions should aim at
reducing aniseikonia (contact lenses) or anisometropia (refractive surgery for adults). In
younger children, high anisometropic refractive
4.6 Presbyopia Correction
errors can lead to amblyopia. Thus, giving the
full refractive correction for both the dominant
and amblyopic eye would be important. This is
specically true in the case of hyperopic refractive error (see Case 2).
The most common practice of giving the near
addition power is by age. The easy-to-remember
formula is [age/10]−3. For example, a 55-yearold person will be given an addition of +2.5 DS
([55/10]−3). In a large majority of cases, this formula works well. The other methods to prescribe
the near addition depend on the accommodation
Case 2
A 12-year-old boy presented for eye testing
for the rst time. His visual acuity was
20/20 and 20/125in the right and left eye,
respectively. His refractive error was +0.50
DS and+5.00 DS in the right and left eye,
respectively. His subjective acceptance was
plano (20/20) and +4.00 DS (20/80). His
cycloplegic retinoscopy showed +2.50 DS
and +6.50 DS, respectively. In PCT, with
fogging, he accepted +1.25 DS (20/30)
and+5.00 DS (20/100). In a 1-month follow-up visit, the visual acuity improved to
20/20–2 and 20/50 in the right and left
eyes, respectively. The aim here is to give
amplitude, dynamic retinoscopy, fused cross cylinder, near duochrome, and measuring positive
and negative relative accommodation. It has been
shown that going by age gave a good estimation
for the near addition when all methods were compared [23].
A large majority of the patients adapt to their
new prescription by adjusting their working distances as well. The incipient presbyope, however,
tends to hold the reading material far away and
would expect the same working distance with the
new spectacles due to habit. In a few cases, where
they strongly prefer a particular working distance
or in cases of over-zealously corrected hypermetropia for distance, simply giving an addition by
the age criteria alone can be troublesome. Hence
P. Satgunam
4 Prescribing Spectacles
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55
prescribing near addition should be undertaken
with care. It is mandatory to check the range of
the near working distance with the near addition,
ensuring that it falls within the comfort range of
the patient’s habitual viewing distances.
Generally, a range from 35 to 60cm should be
very comfortable. The philosophy for near addition is the opposite of the prescribing pattern for
distance visual acuity; i.e., the lowest plus power
that gives the maximum comfortable near working distance is prescribed for near addition. Also,
remember that when increasing the plus power
for distance, it would be necessary to check the
near addition and range carefully and not simply
go by the age criteria (Case 3).
Case 3
A 50-year-old female, accountant by pro-
fession complained of blurring for near
vision. She had never worn any spectacles
before. Her dry retinoscopy showed mini-
mal hyperopic correction. She was given
+1.50 DS for near vision. The patient
returned after a week; she reported discom-
fort with reading but felt better with the
spectacles for distance viewing. Upon
cycloplegic retinoscopy, she was found to
have +1.50 DS for distance. She read 20/20
both with and without her distance correc-
tion. Duochrome was red better only with
her spectacle correction; without it, green
was better. Her spectacles were changed to
a distance correction of +1.50 DS and a
reading addition of +1.25 DS, which gave
her a comfortable range. The learning point
here is to do cycloplegic refraction even in
older individuals who could still have
active accommodation. Note this lady has
never worn any reading correction till
50years of age, in spite of having hyper-
opic correction for distance. In such indi-
viduals, near addition should not be based
on age, especially after fully correcting
their hyperopic error for distance.
4.7 Conclusion
Prescribing spectacles through subjective refraction is an art. With practice and experience, each
clinician carves out their practice pattern and
gets a sense of what would work best for their
patient. The end goal of subjective refraction is
to give a comfortable 20/20 vision. It is important to remember this, or else if the goal is only
to get the patient to read the last line, it could end
up in unnecessary over-minus or under-plus in
spectacle correction. Understanding the occupational visual needs of the person is equally
important to determine the appropriate spectacle
prescription. There are a few more specialty prescriptions that could incorporate relieving
prisms, size lenses, yoke prisms, etc., that are
beyond the scope of this book. The topics discussed here should help a general clinician
understand the fundamental concepts for arriving at a regular spectacle prescription encountered in a day-to-day clinic.
Funding Hyderabad Eye Research Foundation,
Hyderabad, India.
Disclosure None.
References
1. Strang NC, Gray LS, Winn B, etal. Clinical evaluation of patient tolerance to autorefractor prescriptions.
Clin Exp Optom. 1998;81(3):112–8.
2. Bist J, Kaphle D, Marasini S, et al. Spectacle nontolerance in clinical practice—a systematic review
with meta-analysis. Ophthalmic Physiol Opt.
2021;41(3):610–22.
3. Milder B, Rubin ML. The ne art of prescribing
glasses. Triad Publishing Company; 1991.
4. Wilkson ME. Sharpen your subjective refraction
technique. Rev Optom. 2016;58–65. https://www.
reviewofoptometry.com/CMSDocuments/2016/1/
SharpenYourSubjectiveRefractionTechnique.pdf.
5. Cantor LB, Rapuano CJ, George AC. Clinical
optics basic and clinical science course. Am Acad
Ophthalmol. 2019;61(713):385.
6. Leat SJ.To prescribe or not to prescribe? Guidelines
for spectacle prescribing in infants and children. Clin
Exp Optom. 2011;94(6):514–27.
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7. Roseneld M, Logan N. Optometry: science, techniques and clinical management. 2nd ed. Elsevier;
2009.
8. Collins N, Garratt S.Preferred practice pattern. Am
Acad Ophthalmol. 2012:49.
9. American Optometric Association. Comprehensive
pediatric eye and vision examination. 2017;10–39.
https://www.aoa.org/AOA/Documents/Practice/
Management/Clinical/Guidelines/EBO/Guidelines/
Comprehensive/Pediatric/Eye/and/Vision/Exam.pdf.
10. Freeman CE, Evans BJW.Investigation of the causes
of non-tolerance to optometric prescriptions for spectacles. Ophthalmic Physiol Opt. 2010;30(1):1–11.
11. Westheimer G. Scaling of visual acuity. Arch
Ophthalmol. 1979;97:327–30.
12. Atchison DA, Mathur A. Visual acuity with astigmatic blur. Optom Vis Sci. 2011;88(7):798–805.
13. Roy S, Bharadwaj SR, Patil-Chhablani P, etal. Spasm
of near reex: a comprehensive management protocol
and treatment outcomes. J AAPOS. 2021;25(3):162.
e1–6. https://doi.org/10.1016/j.jaapos.2021.02.010.
14. Kulp MT, Ciner E, Maguire M, et al. Uncorrected
hyperopia and preschool early literacy: results
of the vision in preschoolers-hyperopia in preschoolers (VIP–HIP) study. Ophthalmology.
2016;123(4):681–9.
15. Ciner EB, Kulp MT, Maguire MG, et al. Visual
function of moderately hyperopic 4- and 5-year-old
children in the vision in preschoolers—hyperopia in
preschoolers study. Am J Ophthalmol. 2016;170:143–
52. https://www.sciencedirect.com/science/article/pii/
S0002939416303634.
16. Mavi S, Chan VF, Virgili G, etal. The impact of hyperopia on academic performance among children: a systematic review. Asia Pac J Ophthalmol (Philadelphia,
Pa). 2022;11(1):36–51.
17. McClelland JF, Saunders KJ. Accommodative lag
using dynamic retinoscopy: age norms for school-age
children. Optom Vis Sci. 2004;81(12):929–33.
18. Satgunam P, Datta S, Sumalini R. Near vision in
individuals with down syndrome: a vision screening
study. Eye (Lond). 2019;33(8):1254–60.
19. Al-Bagdady M, Stewart RE, Watts P, etal. Bifocals
and Down’s syndrome: correction or treatment?
Ophthalmic Physiol Opt. 2009;29(4):416–21.
20. Borish IM.Comments on a “Delayed subjective” test.
Optom Vis Sci. 1945;22(9):433. https://journals.lww.
com/optvissci/Fulltext/1945/09000/COMMENTS_
ON_A__DELAYED_SUBJECTIVE__
TEST_.5.aspx.
21. Grosvenor TP.In: Falk K, editor. Primary care optometry. 5th ed. Butterworth Heinemann Elsevier; 2007.
22. Rutstein RP. Accommodative spasm in siblings: a unique nding. Indian J Ophthalmol.
2010;58(4):326–7.
23. Antona B, Barra F, Barrio A, etal. Comparing methods of determining addition in presbyopes. Clin Exp
Optom. 2008;91(3):313–8.
Optical Dispensing
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SrikanthMaseedupalli
5.1 Introduction
Optical dispensing is an essential component of
refractive error correction. In optical dispensing,
various tools and devices are used for precise eye
measurements to dispense accurate eyewear prescriptions. These measurements are crucial in
assessing visual needs, determining lens parameters, and ensuring optimal vision correction.
This chapter will look into the common tools
used for optical dispensing and describe their
usage.
5.2 Tools, Devices, Machines,
andInstruments inOptical
Dispensing
Tools are handheld or manually operated objects
to assist in performing specic tasks such as
frame adjustments, lens cleaning, and minor
repairs. Devices measure a particular parameter
(e.g., a lens clock). Machines are mechanical
devices to perform specic tasks, such as lens
edging, surfacing, etc. Instruments are specialized tools used to obtain precise measurements
for scientic, technical, or medical purposes
(Fig.5.1c). An automated lensometer is an example of an instrument.
S. Maseedupalli (*)
The Standard Chartered-LVPEI Academy for Eye
Care Education, L V Prasad Eye Institute,
Hyderabad, India
e-mail: srikanthm@lvpei.org
© 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_5
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a b
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S. Maseedupalli
c
Fig. 5.1 Top left: A half-padded plier used for adjusting
the spectacle frame temples. Top right: A thickness caliper
used for measuring the thickness of an ophthalmic lens.
5.3 Manual Lensometer
This measures spherical and cylindrical lens
power (Fig.5.2).
The different parts of the lensometer are as
follows: [1].
The eyepiece: Allows the user to view the lens
and measurements.
The lens holder: Holds the lens in position
during measurement.
Power drum: Allows for ne adjustments to
accurately measure the lens’s power. It is rotated
to bring the mires (green lines) into focus.
The axis wheel: Used to adjust the axis of
cylindrical lenses. It enables the alignment of the
mires with the appropriate axis markings on the
protractor.
Reticule or protractor: Provides a scale for
measuring the axis of cylindrical lenses.
Bottom: A semiautomated edging machine used for shaping the lens and xing it in a spectacle frame
5.3.1 Guide toUsing aLensometer
Step I: Focusing the eyepiece: [1] makes the
reticule, or protractor, appears clear with distinct lines. Start by turning the eyepiece anticlockwise, and while looking through it, rotate
clockwise until the reticule lines are sharply
visible.
Step II: Noting the instrument error: [2]. There
are two kinds of instrument errors.
(a) The power error of the instrument can affect
the accuracy of lens measurements. Begin by
rotating the power drum to the “zero” reading and observe whether the mires (green
lines) are clear and distinct. If they are, the
instrument has no power error, and you can
proceed to the next step (Step b). If the mires
are not clear, adjust the power drum until
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Fig. 5.2 Lensometer
59
they become sharp. Note the reading at
which the mires appear clear (e.g., mires
clear at +0.25). This indicates an error of
+0.25 DS.To obtain the correct lens power,
subtract 0.25 from the reading.
Example 1
If the reading is +1.50 DS, then after cor-
rection, it would be (+1.50 to −0.25)=+1.25
DS.
Example 2
For sphero-cylindrical values, only correct
the spherical component. For a reading of
+1.75 DS/−0.75 DC × 180, after correc-
tion, it would be +1.50 DS/−0.75 DC ×
180.
(b) The axis error is applicable only when the
lens has cylindrical power. Set the axis to
“zero” by rotating the axis wheel and check
whether the horizontal mires coincide with
the 180° on the protractor. If they align, proceed to Step 3. If the mires do not align,
rotate the axis wheel to align the mires with
the 180° line on the protractor and note the
axis reading.
Example 1
If the axis shows 6°, it means it is displaying 6° more than required. To correct this,
subtract 6° from the reading while determining the lens power.
Example 2
If the prescription reads +2.00
DS/−1.00 DC × 120°, the correct axis
would be 114°. Thus, the nal prescription
would be +2.00 DS/−1.00DC × 114°).
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S. Maseedupalli
Step III: Checking the Power of a SingleVision Lens (Fig.5.3a)
(a) Spherical power: For single-vision lenses,
position the lens with the back surface against
the lens stop without tilt. Rotate the power
drum until the mires are clear and perpendicular to each other. Once the mires appear
clear and distinct, write down the reading as
the spherical power.
Example: The mires are clear and distinct,
and the power reads as −3.00 D.Write this as
the spherical component.
(b) Cylindrical lenses: Example −3.00
DS/−1.50 DC × 130°. If you cannot focus
both the mires on a single go (Fig.5.3, bottom left), it indicates a cylinder lens. In this
case, focus the mires in one meridian and
align the axis to ensure clear, distinct, and
continuous lines, the power reads as −3.00 D
(Fig.5.3, bottom center). Note the reading as
the spherical power, I.
Next, rotate only the power drum to focus
the perpendicular mires.
The perpendicular mires focus at −4.50 D
(Fig.5.3, bottom right). Write this as II.
Calculate the cylindrical power by nding
the difference between the two powers (II—
I; −4.50 − (−3.00)=−1.50). The cylindrical
power is −1.50, labeled as III.
Note the angle at which the mires for
−4.50 D are aligned by looking through the
eyepiece, not relying on the axis wheel. If the
mires are aligned at 130°, write this as the
axis (IV).
Thus, the power of the lens should be
written as I/III × IV, i.e., −3.00
DS/−1.50DC × 130°.
5.3.2 Important Notes
1. The technique described here is one of many.
Optometrists may use alternative methods
based on their preferences and expertise.
2. Placing the lens back surface against the lens
stop is necessary to determine the back vertex
power, particularly for single-vision lenses [1].
Fig. 5.3 Top: the focused vertical and horizontal mires, power reads as +3.50 DS. Bottom: left: misaligned mires,
center; center: aligned mires at −3.00 D; and right: perpendicular mires aligned at −4.50 D
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3. For bifocals, the measurement of the add
power depends on whether it is altered on the
front or back surface [1]. If altered on the
front surface (glide your nger through the
surfaces, the addition is on the side where
you feel a bump or ledge in the transition
from distance to near zone; if smooth, it is a
fused bifocal that is always on the front surface), subtract the distance back vertex
power from the near front vertex power in
each meridian to determine the bifocal
addition.
4. Exercise caution when dealing with high
minus lenses, as they may appear as singlevision lenses but could be progressive addition lenses, especially when combined with a
low near addition. Check the power in the
upper and lower nasal portion of the lens.
Check if there is a difference of at least 0.75;
it could be a progressive addition lens. You
can also look for the engravings on the temporal side of the lens that would indicate it as a
progressive addition lens.
5. If you cannot bring the mires to the center of
the circle in the lensometer, the lens may
have a grounded prism incorporated during
lens manufacturing. The base is always
toward the direction of the displaced mires,
and the magnitude can be read from the graticule. The rst circle accounts for 0.5 prism
diopter and the next one is 1.0 prism diopter,
and each circle from there on accounts for
one prism diopter.
6. Similarly, mires may not be aligned at the
center when checking the near power for bifocal lenses when there is a signicant amount
of distance power, usually above 3.00 D.
power of lenses. This knowledge is essential for
providing accurate prescriptions and ensuring
effective vision correction.
5.4.1 GLM Calibration
The calibration of the GLM ensures accurate
power measurements (Fig. 5.4). Follow these
steps for calibration:
Step 1: Placement: place the GLM on a at
surface, ensuring that all three pegs touch the surface. Avoid applying pressure on the device during calibration.
Step 2: Calibration reading: if the GLM is
calibrated, the pointer will read zero. If the
pointer does not read zero, error compensation is
necessary. If it shows an error of +0.25 D, subtract 0.25 D from the nal reading or vice versa.
5.4.2 Power Determination
One must recognize the front and back surfaces
before determining the power. The front surface
refers to the surface facing away from the eye.
The back surface refers to the surface facing the
eye.
Step 1: Proper positioning: hold the GLM so
that the central peg aligns with the optic center of
the lens. Ensure that the peg is perpendicular to
the lens surface [1].
Step 2: Readings in different meridians: note
the GLM readings in at least four different merid-
5.4 Geneva Lens Measure
The Geneva Lens Measure (GLM) is a valuable
device used in optical dispensing to measure the
surface dioptric power of lenses, [1]. It is a
mechanical dial indicator; by understanding the
calibration process and following the power
determination techniques, optical professionals
can accurately measure the surface dioptric
Fig. 5.4 Geneva lens measure— measuring the front surface of an ophthalmic lens
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S. Maseedupalli
ians of the lens. Compare the readings to see if
there are any differences [1].
Step 3: Spherical surface determination: the
lens surface is spherical if the GLM readings
remain unchanged in all meridians. Spherical
lenses have the same power in all meridians.
Step 4: Toric surface determination: if the
GLM readings vary across meridians, the lens
surface is cylindrical (or toric). Note the highest
and lowest GLM readings and their corresponding meridians. Toric lenses have two principal
meridians with different powers.
Approximate Power Calculation. In some
cases, it is necessary to calculate the approximate
power of a lens. The following formula can be
used:
Approximate Power = Front Surface
Power+Back Surface Power.
5.5 Thickness Caliper—
Measuring Lens Thickness
A thickness caliper is used to measure the thickness of lenses (Fig.5.5). The thickness of a lens
is crucial for proper frame tting and lens
aesthetics.
5.5.1 Calibration oftheThickness
Caliper
Calibration ensures accurate measurements.
Follow these steps for calibration:
Step 1: Merge the end points: close the jaws of
the caliper until the two endpoints touch each
other. The caliper should read zero when the endpoints are merged.
Step 2: Error compensation: there is an error if
the caliper does not read zero after merging the
endpoints. To compensate for the error, adjust the
caliper screw (one of the tips) to bring the reading
to zero while both tips touch each other. Seek
assistance or consult a supervisor if needed.
5.5.2 Measuring Thickness
Follow these steps to measure:
Step 1: Positioning: hold the thickness caliper
perpendicular to the lens surface you wish to
measure. Ensure that the caliper is stable and
appropriately aligned.
Step 2: Reading: use the jaws (tips) of the caliper to measure the thickness. The outer short
lines typically have increments of 0.05mm. The
Fig. 5.5 The thickness caliper is used for measuring the central thickness of an ophthalmic lens. The top right insert is
the magnied view of the measuring clock
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inner short lines usually have increments of
0.1 mm, the longer 0.5 mm, and the longest,
1mm. Gently close the jaws around the lens edge
or center until they lightly touch.
Note the reading.
If the thickness of the lens is more than
9.95mm, the pointer would meet either at zero
point or go beyond one complete rotation, and the
readings are indicated in red color.
5.6 Polariscope—Detecting
Internal Stress
inTransparent Materials
The polariscope or colmascope is an optical
inspection device to detect internal stress in
transparent materials, including glass, plastics,
and synthetic resins (Fig.5.6). Stress can occur
due to external forces or manufacturing processes, and it can affect the structural integrity
and optical properties of the material [3].
5.6.1 Procedure forUsing
thePolariscope
To use the polariscope effectively, follow these
steps:
Step 1: Setup: position the material to be
examined between the two polarized lenses of the
polariscope. Ensure the material is placed against
a light source, such as a backlight, for optimal
visibility.
Step 2: Observation: look through the polariscope at the material against the light source.
Observe the patterns and colors displayed by the
material.
5.6.2 Interpreting Patterns
andColors
Different materials and types of stress will exhibit
distinct patterns and colors when viewed through
the polariscope. Here are some common
observations:
Birefringence: materials such as glass, plastics, and synthetic resins that are internally
stressed will show patches or bands of color
when viewed through the polariscope.
Injection molded lenses: color fringes may be
observed at the center of the lens.
Thermally toughened glass lenses: birefringence may be noted throughout the lens.
Polycarbonate lenses: birefringence may be
observed (Fig.5.6, right) [4].
Stress from tight frame t: color fringes can
be seen at the portion of the lens tightly tted into
the frame.
Chemically toughened and crown glass lenses:
no distinct pattern is typically noticed.
Fig. 5.6 Left: a polariscope; right: view of a polycarbonate lens through a polariscope
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