Aspheric Lenses - Optometry Today

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Aspheric Lenses
DISPENSE WITH CONFIDENCE PART 2 C-19559 O/D
Dr Eirian Hughes, BSc (Hons), PhD, FBDO
An aspheric lens is classified as one where either one, or both, surfaces are
non-spherical, which means it could be toroidal or cylindrical. Such a surface
is one which has been produced by rotating an ellipse about its major
diameter, producing a conic section (oblate ellipse, circle, prolate ellipse,
parabola or hyperbola). Aspheric lenses utilise a non-spherical surface with
surface astigmatism to neutralise oblique astigmatism produced by off-centre
refraction. They provide both the visual advantages of best form lenses and the
cosmetic advantages of thinner, flatter and lighter lenses. This article describes
the various types of aspheric lenses and the main dispensing considerations.
Circle
Parabola
Oblate
Ellipse
Figure 1
The origins of an aspheric lens surface are based on the rotation of an elliptical shape forming a
conic shape. The various sections that can be produced are a circle, oblate ellipse, prolate ellipse,
parabola and hyperbola. Reproduced with permission from Walsh 1
Hyperbola
Prolate
What are aspheric lenses?
When a circle is rotated about its diameter,
the solid form which results is a sphere and
therefore, any lens produced from this will have
a surface of a spherical nature. In contrast, any
shape of variable diameter (eg an ellipse) which
is rotated in a similar fashion will not produce
a spherical solid – a lens produced from such
a solid will have a surface that is classed as
being aspherical. The latter encompasses both
cylindrical and toroidal surfaces and these
are collectively known as conic sections since
they are curved forms which originate from
sections of a cone (Figure 1). BS EN ISO 13666
(1999) states that aspheric lenses must be
rotationally symmetrical. This excludes both
aspherised astigmatic (atoral) and progressive
addition lens surfaces. 1 When an ellipse is
rotated about its x-axis, an ellipsoid is formed. If
the major axis is horizontal then this is referred
to as a prolate ellipsoid. However, if the minor
axis is horizontal, it is referred to as an oblate
ellipsoid. The parabola is in a plane parallel to
the side of the cone. When a parabola is rotated
about its x-axis, a paraboloid is formed. Should
the plane at which the section is made be
tilted beyond that of the parabola it is called
a hyperbola. When a hyperbola is rotated
about its x-axis, a hyperboloid is formed.
Mathematically, aspheric surfaces are derived
using the formula: y 2 =2r 0
x – px 2 where r0 is
the radius of curvature of the surface at the
vertex and p is the eccentricity of the curve
(where the type of curve is determined by
the value given). 2 Practically, an aspheric lens
surface can be identified using a lens measure.
If the sag of a spherical lens is read using a
lens measure, the measurement obtained
will be the same, no matter where the lens
measure is placed on the surface. If this process
is repeated for an aspheric lens, however,
a change of curvature across the surface
will be seen as the lens measure is moved.
Therefore an aspheric surface is spherical
at the very centre but becomes astigmatic
away from the optical centre. The gradual
change in curvature differs between positive
and negative powered lenses such that the
curvature of a positive powered lens gradually
flattens from the centre to the periphery, whilst
that of a negatively powered lens steepens
from the centre to the periphery. Accordingly,
this represents a change in tangential power,
45
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to which quiet means activities that, at each (eg, point puzzles on the surface and
colouring) (apart from the rather centre than of the allowing lens) there children is surface
to astigmatism, become which excited is utilised by playing to counteract with
any oblique aberrational astigmatism. This
active items. Remember that children
in turn results in an improvement in optical
may resent being dragged away from a
quality of the retinal image formed. The other
toy corner to have an eye examination.
corresponding feature of aspheric lenses is
Think about the equipment on display in
that the sag of the lens is smaller than that of
your a spherical examination lens of the room. same Is diameter. it unfamiliar As such,
and this allows scary? thinner Is it computerised lenses to be manufactured,
and highly
attractive which also makes to inquisitive lenses correspondingly fingers? Can lighter. you
place unnecessary equipment behind a
Evolution of aspheric lenses
screen? Can you replace your information
posters Aspheric lenses with were children’s first described pictures? in the
Arrange early 1900s your and Zeiss appointment subsequently book produced to
avoid
a famous
unnecessary
range called
waiting
the Katral
and
lenses,
to
which
give
utilised concave ellipsoid surfaces to eliminate
flexibility. Some practitioners like to
oblique astigmatism and reduce pincushion
reserve one whole session per week for
distortion. 2 These lenses were usually dispensed
children. For older children, reserving
to patients post-cataract extraction, with
after-school
resulting high
appointments
hyperopic refractive
can be
errors
useful.
that
When exceeded the the appointment +7.00DS limit is made, of Tscherning’s it may be
helpful ellipse; lenses to discuss of such parental power cannot concerns be made and
determine free of oblique whether astigmatism an extended if made or second in the
appointment spherical form might (Figure be needed. 2). 1 It was Remember not until
to the allow 1960s, time however, during when the changes examination occurred to
explain following what the introduction is going to happen of CR39 and as a after lens
the
material.
examination
Then, the
to
most
discuss
widely
the
used
outcome
aspheric
lenses employed convex ellipsoid surfaces to
with both the child and the parent(s).
eradicate oblique astigmatism and diminish
Some children, particularly those
distortion and transverse chromatic aberration.
with special needs, may be nervous.
Consider allowing children to visit the
practice before their first appointment,
to meet the staff and see the room
WOLLASTON
where the examination will take place. In order to successfully test + 20 children,
There are children’s books about eye it may be necessary to purchase tests
tests available so consider loaning a specifically designed for the age group.
BEST FORM
+ 15
book at the time the parent makes the +2.00D Of LENSES equal importance is the attitude
appointment. Some practitioners create of all staff, the practice environment
their own leaflets especially for children. and the approaches taken
+ 10
to make
Staff training may be OSTWALT needed as not the entire experience child-friendly.
everyone has a natural rapport with
In this case leave the door open or
have another member of staff join you.
Conclusion
About the author
+ 25
children. Ensure that all members of staff
are familiar with child protection issues Maggie Woodhouse is senior lecturer
PL
and local protocols (see the College of at the School of Optometry and Vision
Optometrists’ -25 -20guidelines -15 on -10 examining -5 Sciences, PL Cardiff +5 University, +10 where she
the younger child and consider LENS studying POWERspecialises in paediatric optometry.
the e-learning module on safeguarding She runs the Special Assessment
Figure 2
children provided by DOCET). It is Clinic, which caters for patients of all
Tscherning’s ellipse. The focal power of the ellipse is about +7.00DS and lenses above this power
good practice to ensure that a child is ages with disabilities. Her particular
will experience aberrations such as oblique astigmatism that need aspheric lens designs to
never eliminate. alone with a member of staff. interests are visual development in
This extends to the examination; ensure children with Down’s syndrome and the
that Mo Jalie a parent patented or a design guardian with comes the aspheric into
the surface examination the front room surface with of positive the child. lenses
previously impact of described, visual defects and on their education. surfaces
are denoted by a complex mathematical
This (convex may hyperboloid) not always and be on the possible back surface eg, a polynomial References equation. They combine the
parent
of negative
may
lenses
need to
(concave
take a
hyperboloid).
distracting
4 advantages
See www.optometry.co.uk/
of lenticular and full aperture lens
The reduction in thickness is possible through designs and so result in thinner and lighter
sibling outside, or an older child may clinical. Click on the article title and
the use of hyperboloid surfaces, which means lenses with no distinct dividing line, no annular
not want the parent to accompany them. then on ‘references’ to download.
that the lens is flatter, which reduces the lens scotoma and improved optical performance. 4
This trend continued in the 1970s, whereby sag, which in turn reduces lens thickness
flatter Module ellipsoid curves questions were used to eliminate Course (see the first code: article of this C-18705 series, OT August O/D Current aspheric lens designs
aberrations, but resultant uncut diameters of 17, 2012). Flattening of the lens also reduces Modern aspheric spectacle lens forms are now
PLEASE NOTE There is only one correct answer. All CET is now FREE. Enter online. Please complete online by midnight on May 18, 2012 – You will be unable to submit
the exams lenses after were this very date. large Answers and, to in the fact, module using will this be published spectacle on www.optometry.co.uk/cet/exam-archive. magnification whilst the aspheric CET points made for according these exams to will far be more uploaded complex to Vantage criteria.
flatter on May form 28, resulted 2012. Find in out overcorrection when CET points of will oblique be uploaded surface to Vantage serves at to www.optometry.co.uk/cet/vantage-dates
eliminate the high levels of They are produced by taking into account
astigmatism at the lens periphery, inducing oblique astigmatism. Such lenses are used parameters such as refractive power, final
1. Diagnosing a congenital colour vision defect in early childhood c) Uses a flashing colourful target
large has amounts the following of mean benefits oblique EXCEPT: error and giving in today’s low power d) Uses aspherics a large for target hyperopic lens thickness, face-form (dihedral) angle,
poor a) The peripheral defect can visual be treated acuity. Lenses using such prescriptions, but not for myopic corrections pantoscopic angle and back vertex distance.
b) Teachers can understand a child’s colour choice in artwork
3. Success in eye health examination of a young child may be improved if
flatter c) Inappropriate curves were career subsequently plans can be avoided introduced due to the lack of
the
toroidal
practitioner:
surfacing tools. Consequently aspheric surfaces comprising
to d) the Alternatives market to as colour blended coding lenticular can be used lenses. 2 In the latter 1970s a) and Asks early the child 1980s, to sit the as still use as possible polynomial for as long surfaces as it takes have been produced and
In 1976, Davis and Fernald were granted US of computer-aided
b)
design
Asks the child
enabled
to keep
more
looking at
this
an interesting
has been
picture
made
on
possible
the wall,
with
no matter
computer
2. Measuring eye movements in children is likely to be more what
patents successful for a if series the practitioner:
of aspheric lenses based on complex polynomial c) aspheric Uses a slit-lamp lens surfaces and a Volk to lens numerical control (CNC) technology. The ways
improved a) Moves the cosmetic target very appearance slowly and minimum be manufactured. These d) Examines surfaces sections are generally of the eyes in in separate which intervals polynomials differ from blended
b) Avoids distracting the child by speaking
tangential error best form principles. 3 4. Practice preparation may include all of the following EXCEPT:
In 1981, of a higher order than the conic surfaces lenticulars are that they have excellent
+ 5
BASE CURVE
a Novartis company
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Sponsored by
Sponsored by
a Novartis company
optical properties in their aspheric zones and
(Abbott Medical Optics Inc., Santa were compared before instillation of not been statistically validated at the
the blending which occurs is concave. Like
Ana, California, USA) for the duration Blink’n’Clean eye drops, five minutes time of this publication, the results
blended lenticulars, polynomials are thinner
of the study. They were instructed to after the first eye drop instillation, are preliminary but not conclusive.
and lighter than full-aperture lenses of the
maintain their regular correct cleaning and 14 days after twice daily use.
same prescription and there is no dividing line
regime between zones. (rub The and patient rinse) has as a wide well field as of Pre-lens non-invasive tear break-up Analysis and results
their view and normal ring replacement scotoma. Aspheric schedule. designs
Types for myopes of utilise contact convex lenses oblate used ellipsoid in
this or convex investigation polynomial surfaces were in Biofinity order to
(CooperVision, produce thinner and Fairport, flatter point New focal York, lenses.
time (NIBUT) was evaluated without
application of fluorescein dye with
different types of mires, such as placido
disc of the videokeratograph or mires
Subject’s gender is not a parameter to
be included in the analysis since the
distribution of values is not balanced
sufficiently (nine men and 42 women). The
USA), Atoric Acuvue lenses differ Oasys, from Acuvue standard Oasys aspheric for of the keratometer. This allowed age distribution in the sample was normal
Presbyopia,
surfaces in
Acuvue
that the
Advance
oblique
(all
astigmatic
Johnson calculation of the NIBUT without (Kolmogorov-Smirnov test, p=0.66).
aberrations are controlled along both meridians
& Johnson Visioncare Inc., Jacksonville, interfering with tear film stability. The distribution of NIBUT values
and so the p-value (eccentricity of the curve) of the
Florida, USA) Air Optix, Air Optix Corneal staining, which was analysed was normal (Kolmogorov-Smirnov
aspheric surface differs from a minimum along
Night & Day (both CIBA Vision, Duluth, with the application of fluorescein dye,
one meridian to a maximum along the other.
test, p>0.05 on all occasions), and
Georgia, USA) and PureVision (Bausch and hyperaemia were assessed according therefore parametric tests were applied
& Low Lomb, powered Rochester, aspheric New lensesYork, USA). to Figure the Cornea 3 and Contact Lens Research (ANOVA with post hoc analysis and
Subjects It is in more were recent wearing years that lenses aspheric for at designs least Unit
Benefits
(CCLRU)
of aspheric
grading
lenses. The
scale
spectacles
(School
shown have lenses of +6.00DS power. The lens on the
Pearson’s correlation coefficient). The
left of the image is of spherical form whilst that on the right of the image is of aspherical form.
five have hours been used per to day correct and prescriptions were examined of low of
The
Optometry
aspheric lens
and
is clearly
Vision
flatter
Science,
and thinner.
The
Other remaining differences parameters in properties follow are also categorical
shown.
at positive the power. following The use intervals: of such lenses before again University of New South Wales, Sydney, scaling and therefore non-parametric
administration
confers the benefits
of Blink’n’Clean
of making
eye
the
drops,
lenses
Australia).
tilt, the vertical
LIPCOF
centres
were
should
also
be
counted,
decentred
tests Case scenario were applied 1: myope(Chi-square,
thinner and lighter. The reduction in thickness 1mm down (by applying the dispenser’s rule)
five minutes after the first instillation of vertically below the temporal limbus. Friedman
Prescription -7.00/-1.00x90
ANOVA, and
R+L
Wilcoxon
is as a result of initially making the lens flatter • The dihedral (face-form) angle of the frame
Blink’n’Clean eye drops, and 14 days A subjective questionnaire (Table 1) When dispensing aspheric lenses for such a
matched-pairs signed-ranks tests).
in form by utilising a shallower base curve, front should be 5° for right and left sides. myopic patient, the index of the material needs
after first use of Blink’n’Clean eye drops. was also given to wearers five minutes
whilst also having a sag which is smaller than This ensures that there is no horizontal to be discussed with the patient. The patient is
The authors used the modified Rudko after first instillation of Blink’n’Clean Non-invasive break-up time
a spherical surface of the same vertex radius prismatic effect at the near centration points likely to be concerned by the edge thickness
validated (and for any scale lens 10,11 diameter) (Figure (Figure 1) 3). This for and • Keep again the back after vertex distance 14 days to a minimum of twice as
Results showed significantly higher
and weight of the finished lens and these
assessment flatter aspheric of deposits lens form on contact also neutralises lenses. daily the back use. surface It of is the necessary lens will be flatter to point than NIBUT values after 14 days (13.4±6.7
can naturally be reduced with a higher index
Heaviness, oblique aberrational extent and astigmatism. type of The deposits same out in a spherical that, since lens, which the questionnaire enables the lenses has to seconds)
material. Combining
of twice-daily
this with an anti-reflection
use of
principles apply to negative lenses with regard be fitted closer to the eye. However, owing to coating will provide the best lens of choice. Such
to flattening them to make them thinner. Here the flatter back surface, consider the patient’s
1. During a normal day within the last week, how often was the wearing comfort of your contact lenses lenses give a unpleasant?
reasonable degree of control over
one surface is made aspheric to restore the eyelashes as they should not touch the lens the edge substance while providing good offaxis
off-axis
Never
performance of the
Rarely
flatter lens form. • Unlike
Sometimes
spherical lens forms, prism
Often
should
performance Always in oblique gaze, whilst serving
It is generally accepted that convex oblate not be provided by decentration in aspheric to control the effects of distortion. In addition,
ellipsoid
2.When exactly
surfaces
did
are
you
used
note
to
this
reduce
unpleasant
edge
wearing
lenses. If
comfort?
an aspheric lens were to be there is less minification of the eyes too. For
thickness Never of negatively powered Early morning lenses. This decentred, Noon the vertex of the aspheric Evening surface this prescription, All 1.74 dayaspheric hard and MARcoated
lenses would be a good option. It is also
lens form enables positive surface astigmatism no longer occupies the position assumed
to 3. neutralise If you felt the this negative unpleasant oblique wearing astigmatism comfort, when how unpleasant originally designing was this the feeling lens at and, the as end of the contact lens wearing time?
important to be sensible with frame selection
which results from the flat-form lens. 2 such, the aspheric surface will no longer – consider shape and size carefully and fit the
No problem Slightly unpleasant Unpleasant Annoying Extremely annoying
occupy a position in which symmetry occurs spectacle frame with as small a vertex distance as
Dispensing 4. During a normal of aspheric day within the lenses last week - how for ocular often rotations did you have away the from feeling the optical that your contact possible. lenses If considering are dirty? glass lenses, it is important
When dispensing aspheric lenses, there are some axis. Therefore prismatic correction must be to remember the refractive index of the lens
rules Never by which to abide. These Rarely listed below: incorporated Sometimes during working of the Often lens 5
increases with density
Always
(mass divided by volume).
• Ensure correct horizontal centration of the • The flatter curves of aspheric lenses result
5. How was the wearing comfort after using Blink’n’Clean?
If weight is the patient’s priority, then a plastic lens
lenses by taking monocular pupillary distances in unwanted reflections, which are inevitably has to be the material of choice. It is, of course,
• Much Ensure worse correct vertical centration Worse of the lenses by noticed by Same the patient. For this reason, aspheric Better important to ensure Much that better the material best suited
taking heights of the optical centres. The vertical lenses should always be dispensed with a to the patient’s requirements is always dispensed.
6. How was your vision after using Blink’n’Clean?
centration should then be compensated for the multi anti-reflection (MAR) coating. This also
pantoscopic Much worsetilt of the spectacle Worse frame to ensure improves Same the cosmetic appearance of the Better lenses Case scenario Much 2: better hyperope
that the optical axis of the lens passes through • Distortion may be greater in aspheric lenses Prescription +7.00/-1.00x90 R+L
Table the eye’s 1 centre of rotation. The pantoscopic tilt compared to spherical lenses and so the When dispensing aspheric lenses for a hyperopic
angle should be measured and for every 2° of patient may require some time to adapt 6
patient, nasal edge thickness, centre thickness,
Subjective questionnaire used to evaluate the effect of Blink’n’Clean eye drop use on contact lens comfort and vision
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21/09/12 20/04/12 CET
to weight quiet and activities overall cosmetic (eg, result puzzles will be and the with polynomial special lenses needs, would may be a be good nervous. option. lenses In this which case provide leave the visual door and open cosmetic or
colouring) areas of most rather concern than to allowing the patient. children Unlike Consider allowing children to visit the benefits have another for patients. member Unlike of spherical staff join lenses, you.
minus lenses, the finished blank size of a In summary
to become excited by playing with practice before their first appointment, aspheric lenses flatten progressively from
positive lens plays an important role in dictating Aspheric lenses do not necessarily provide better the centre to the lens margin. This results
active items. Remember that children to meet the staff and see the room Conclusion
the thickness of the lens when glazed. The optical performance than best form lenses, in lenses of reduced thickness, which are
may resent being dragged away from a where the examination will take place. In order to successfully test children,
avoidance of unwanted decentration is vital but simply provide comparable performance consequently more lightweight and confer
toy corner to have an eye examination. There are children’s books about eye it may be necessary to purchase tests
and by combining minimum substance uncuts without the restrictions imposed by best form decreased peripheral visual distortion. Aspheric
Think about the equipment on display in tests
along with using aspherical surfaces, optimum base curve available selection. so However, consider the loaning advantages a specifically designed for the age group.
lenses can be ordered in high index materials
your results examination can be obtained. room. Only Is plastics it unfamiliar should be book of aspheric at the lenses time over the spherical parent makes lenses are: the Of equal importance is the attitude
7
for the ultimate in attractive thin lenses.
and considered scary? for Is high it computerised hyperopes due to and the highly volume
attractive of material to involved; inquisitive glass lenses fingers? would Can prove you
place very heavy unnecessary and unsafe. equipment A hard and MAR behind coating a
screen? should be Can dispensed you replace on the your lenses information
order to
posters prevent unwanted with reflections. children’s Lenticular pictures? lenses
appointment. • Flatter than the Some best practitioners form spherical surface create
their • Elimination own leaflets of large especially amounts of oblique for children.
Staff astigmatism training may be needed as not
everyone • Decreased has spectacle a natural magnification/ rapport with
children.
minification
Ensure
as shape
that
factor
all
is
members
reduced
of staff
of all staff, the practice environment
About
and the
the
approaches
author
taken to make
Eirian the entire Hughes experience is the dispensing child-friendly.
optician in
charge of teaching dispensing to final year BSc
optometry About the students author at the School of Optometry
can be considered, not only standard lenticulars • Increased field of view
Arrange your appointment book to are familiar with child protection issues and Maggie Vision Woodhouse Sciences, Cardiff is senior University. lecturer She
but also blended lenticulars, or even polynomial • Reduced lens thickness
avoid unnecessary waiting and to give and local protocols (see the College of is at a the module School tutor of Optometry for paediatric and optometry Vision
aspheric lenticular lenses too. Polynomial lenses • Reduced edge substance in negative lenses and eye care for people with learning
flexibility. Some practitioners like to Optometrists’ guidelines on examining Sciences, Cardiff University, where she
give the main advantage of an absence of a • More lightweight
disabilities for the MSc in clinical optometry.
reserve one whole session per week for the
ring scotoma and Jack-in-the-box effect which • Good
younger
off-axis
child
optical
and
performance
consider studying specialises in paediatric optometry.
Her BSc and PhD are in medical biochemistry.
children.
normally occurs
For
at
older
the edge
children,
of a strong
reserving
positive
the • Controlled e-learning distortion module on safeguarding She runs the Special Assessment
after-school lens. Frame selection appointments is also important can be to useful. ensure children • Better cosmesis provided by DOCET). It is Clinic, which caters for patients of all
When the shape the and appointment the size does is made, not lead it to may overly be
helpful great edge to discuss and/or centre parental thickness. concerns For and this
determine prescription, whether 1.5 Omega an extended hard and MAR or second coated
good practice to ensure that a child is
never Conclusion alone with a member of staff.
This Aspheric extends lenses to are the a examination; highly useful group ensure of
References
ages with disabilities. Her particular
See interests www.optometry.co.uk/clinical. are visual development Click on the in
article children title and with then Down’s on ‘references’ syndrome to download. and the
appointment might be needed. Remember that a parent or guardian comes into impact of visual defects on education.
to allow time during the examination to the examination room with the child.
Module questions Course code: C-19559 O/D
explain what is going to happen and after This may not always be possible eg, a References
PLEASE NOTE There is only one correct answer. All CET is now FREE. Enter online. Please complete online by midnight on October 19, 2012 – You will be unable to
the submit examination exams after to this discuss date. Answers the to outcome
the module will parent be published may on need www.optometry.co.uk/cet/exam-archive. to take a distracting See CET points www.optometry.co.uk/
for these exams will be uploaded to
with Vantage both on the October child 29, 2012. and Find the out parent(s).
when CET points sibling will be uploaded outside, to Vantage or an at www.optometry.co.uk/cet/vantage-date
older child may clinical. Click on the article title and
Some 1. Which children, of the following particularly is NOT a conic those section? not want the parent to 4. accompany Which of the following them. measurements then on ‘references’ is NOT required to download. when
a) Oblate ellipse
dispensing aspheric lenses?
b) Asymptote
a) Horizontal centration
c) Circle
b) Vertical centration
d) Module Hyperbola questions Course code: C-18705 O/D
d) Inset
PLEASE 2. Which NOTE of the There following is only one is NOT correct an answer. advantage All CET of is aspheric now FREE. lenses? Enter online. Please complete online by midnight on May 18, 2012 – You will be unable to submit
exams a) Lighter after in this weight date. Answers to the module will be published on www.optometry.co.uk/cet/exam-archive. 5. Which British Standard CET points defines for aspheric these exams lenses? will be uploaded to Vantage
on b) Reduced May 28, 2012. spectacle Find out magnification
when CET points will be uploaded to Vantage at www.optometry.co.uk/cet/vantage-dates
a) BS EN ISO 13666
c) More curved lens form
b) BS EN 166
1. d) Diagnosing Cosmetically a more congenital appealing colour vision defect in early childhood c) Uses c) BS a flashing 2738 colourful target
has the following benefits EXCEPT:
d) Uses d) BS a large EN ISO target 7998
a) 3. The Which defect of the can following be treated is the dispenser’s rule for aspheric lenses?
b) a) Teachers For every can 5° of understand pantoscopic a child’s tilt, vertical colour centres choice should in artwork be decentred 2mm 3. Success 6. Which in eye of the health following examination is MOST of likely a young to improve child may the be optical improved and if
c) down Inappropriate career plans can be avoided
the practitioner:
cosmetic quality of aspheric lenses?
d) b) Alternatives For every 1° to of colour pantoscopic coding tilt, can vertical be used centres should be decentred 1mm a) Asks a) A the photochromic child sit as lens still as possible for as long as it takes
down
b) Asks b) A the hard child coat to keep looking at an interesting picture on the wall, no matter
2. c) Measuring For every 2° eye of pantoscopic movements tilt, in vertical children centres is likely should to be decentred more 1mm whatc) A mirrored coating
successful down if the practitioner:
c) Uses d) An a slit-lamp anti-reflection and a Volk coating lens
a) d) Moves For every the 2° target of pantoscopic very slowly tilt, vertical centres should be decentred 2mm d) Examines sections of the eyes in separate intervals
b) down Avoids distracting the child by speaking
4. Practice preparation may include all of the following EXCEPT:
a Novartis company
Find Find out out when when CET CET points will will be be uploaded to to Vantage at at www.optometry.co.uk/cet/vantage-dates