4 dispensingoptics February 2014

Measuring that unknown

RGP in practice by Keith Cavaye
FBDO(Hons)CL FBCLA

CompetencIes covered:
Dispensing opticians: Contact lenses
Contact lens optician: Verification and identification
Optometrists: Contact lenses

Most contact lens opticians will and would like a duplicate pair. Their allowing the verification or replication
recognise this situation. A patient visits previous practitioner has retired and to take place with a good level of
for the first time, wearing very old rigid of course, they have no contact lens confidence.
lenses saying that they fit perfectly specifications! In these
circumstances, it would be really Before any measurements are
useful to have full laboratory attempted, a full knowledge of rigid
equipment so that the majority of lens lens design is necessary. Although
IMPORTANT: CLO
parameters could be measured and individual laboratories may make
competency requirement
recorded for replication to take almost any configuration of back
Contact lens opticians will notice
that this article is approved for the place. In most practices, this simply is surface design, the majority fall into
CL competency of Verification and not the case. the following groups:
identification. This competency is 1. Spherical, with two concentric
rarely offered by CET Providers, so Other occasions where such curves (bicurve or C2) or three
will remain active until the end of information would be valuable concentric curves (tricurve or C3)
2014. CLO ABDO members are include checking the laboratories’ or four or more concentric curves
strongly recommended to complete accuracy as you would with (multicurve or C4). These spherical
the article and answer the multiple- spectacles before final dispensing, to curves usually flatten in radius
choice questions so that they can confirm that the lenses are being towards the lens edge.
ensure that their CLO competency
worn in the correct eyes or to check if 2. Aspheric, with varying options of
requirements are met.
lens parameters have altered with aspheric curve designs, such as
The expiry date for all other CET time and wear. ellipse or parabola.
articles in Dispensing Optics in 2014 3. A combination of the above.
will be approximately 14 weeks However, in practice, there are things Possibly a spherical central portion
following publication. n that we can do to obtain the most combined with an aspheric
relevant parts of the specification, so periphery.

This article has been approved for 1 CET point by the GOC. It is open to all FBDO members,
including associate member optometrists. The multiple-choice questions (MCQs) for this month's
CET are available on page 10 and online. Insert your answers to the six MCQs on the inserted
sheet or online at www.abdo.org.uk. After log-in, go to 'CET Online'. Please ensure that your email
address and GOC number are up-to-date. The pass mark is 60 per cent. The answers will appear
C-33206
in the January 2015 issue of Dispensing Optics. The closing date is 16 December 2014.
 Continuing Education and Training

For the purpose of this article, we will lens, which equates to the radius of
assume a tricurve lens design is being curvature of that lens or surface. The C
assessed. A typical lens will have the instrument is not difficult to use, but
following attributes, written in this does require practice and dexterity.
style1:
BOZR:BOZD/BPZR1:BPZD1/BPZR2:TD The lens must be thoroughly cleaned
BVP Tint CT Marks Material with sterile water or saline and then
dried. Any deposits or liquid on the B
An explanation of these terms lens surface will distort the images. It is
appears in Table 1. then placed, concave side up on the
lens holder, which must also have a
When receiving new contact lenses drop of sterile liquid placed in it,
from a laboratory, the tolerances which will help to neutralise
should comply with those in Table 22. unwanted reflections from the front
surface of the lens. Positioning the
For our purpose, assume the lens to lens and holder centrally is critical.
be verified or measured was made to This can be achieved by turning the
A
the following specification: target light on as bright as possible
C3 7.80:7.70/8.60:8.50/11.25:9.50 and observing externally the
-5.00DS, Boston XO, Blue tint, ct 0.20, reflection on the lens surface, moving Figure 1: A binocular radiuscope
with dot marking for right eye the holder to obtain the most central
position possible.
In practice, the author would suggest can be seen between the two target
that we can have a realistic attempt Before measuring takes place, it is images.
to measure or check all of this helpful to confirm that both of the
specification with the exception of two images are visible. By racking the Having found these images, the
the two peripheral radii, the 1st instrument target up and down with gauge needs to be zeroed before
peripheral zone diameter and the the black circular dial (Figure 1A) this recording can take place. It should
material. can be achieved. Normally, the be noted that instruments may zero
microscope itself moves, but with on either of the two images. The
Measuring BOZR some instruments, it is the stage implication of this is that instead of
Usually, this is the most important or holding the lens and holder. It may be reading from perhaps zero to 7.80,
useful parameter to establish. Ideally, necessary to further move the holder the gauge may move in the opposite
the practice will have a radiuscope, horizontally to improve centration direction, ie, from zero (or 10.00)
which may be monocular or before both images are available. backwards through 9.00, 8.00 etc to
binocular. Figure 1 shows a standard Any distortion of image at this point stop at 2.20. The actual reading of
binocular version with mechanical suggests poor centration, a dirty or course would not be 2.20, but
measuring gauge. Some instruments wet lens or back surface lens 10.00 – 2.20, ie, 7.80.
have a digital scale or even an distortion. The images are usually a
internal one viewed via the eye circle of dots similar to a focimeter Having established the zero position,
pieces. Using Drysdale’s method, it image. Elongation of one image may the gauge must be reset to record
measures the distance between two suggest that the surface is toroidal. A this, usually achieved by turning the
images, formed at the lens surface clue to correct positioning is that an horizontal metal dial as seen in (Figure
and the centre of curvature of the additional image of the bulb filament 1B). The black plastic dial is then

Table 1 Table 2

Abbreviation Tolerance
Abbreviation Definition BOZR +/- 0.05mm
BOZR Back Optic Zone Radius Any BPZR +/- 0.10mm
BPZR1 1st Back Peripheral Zone Radius BOZD +/- 0.20mm where blending allows
BPZR2 2nd Back Peripheral Zone Radius measurement
BOZD Back Optic Zone Diameter Any BPZD +/- 0.20mm where blending allows
BPZD1 1st Back Peripheral Zone Diameter measurement
TD Total Diameter TD +/- 0.10mm
BVP Back Vertex Power BVP Up to 5.00DS, +/- 0.12DS
Tint Any tint of material, eg, blue 5.00DS to 10.00DS, +/- 0.18DS
CT Geometric Centre Thickness 10.00DS to 15.00DS +/- 0.25DS
Marks Any identifying marks, such a ‘R’ for 15.00DS to 20.00DS +/- 0.37DS
right lens Over 20.00DS +/- 0.50DS
CT +/- 0.02mm

Continued overleaf
6 dispensingoptics February 2014

Figure 2: Contact lens holder for a keratometer Figure 3: Band magnifier Figure 4: V-gauge

turned again to find the next image. from its dial. The process is quicker generally viewed to assess condition
The gauge will record this movement and easier than using a radiuscope. and check for any identification
and needs to be read to supply the Without the special holder, a mock- marks that may be engraved on the
actual BOZR in mm. Viewing the up version can easily be made using front surface, such as a dot or
gauge carefully, readers will note that a rule to sit on the chin rest and lean lettering.
there is an outer pointer giving the against the forehead strap. Blu-tack
decimal places and a small inner dial or similar, with a concave depression Alternatively, a ‘V’ gauge may be
(Figure 1C) supplying the whole in it, holds the lens in place at the used, but this will not measure any
millimetres. Care must be taken when correct height for the machine. This peripheral diameters, only TD. The lens
the larger pointer is near vertical, works perfectly satisfactorily, but care is placed in a wide area of the
which implies that the whole number should be taken over hygiene and reducing width groove and gently slid
on the small dial is about to or has just possible damage to the lens. As along the narrowing channel until it
altered. Mistakes are frequently made keratometers are calibrated to touches both sides with no further
in this case, so particular note should measure convex surfaces rather than movement possible. Care must be
be made of exactly where this concave, errors due to aberrations taken not to damage the lens by
pointer is indicating to avoid a whole do occur. Some manufacturers exerting too much pressure. (Figure 4)
digit of error. produce a conversion table to use, shows a lens in this position with a TD
but as the errors are small, it is reading of 9.1mm.
These instruments are very sensitive, suggested3 that adding just 0.03mm
requiring a number of readings to be to the recorded reading will give an Back Vertex Power (BVP)
taken and an average final reading accurate result. The author concurs As with any optical lens, the BVP must
recorded. with this. be established. Any standard
practice focimeter may be used. If it
In theory, by tilting the lens holder, it is Total Diameter (TD) is possible to align the focimeter
possible to measure peripheral TD can be measured using a band vertically, this will help keep the
curves, but this is very dependent on magnifier or ‘V’ gauge. contact lens in position. Using a small
levels of blending and width of the stop, less than the total diameter of
peripheral band. A band magnifier (Figure 3) is a the contact lens, prevents the lens
simple magnifying device (usually 7X) from falling into the machine. A lens
Many practices, however, do not with a scale or graticule in millimetres holder (Figure 5) is often supplied with
have a radiuscope. An alternative and 10ths of millimetres engraved on focimeters to further help safe
method of measuring BOZR can be the flat viewing surface. A lens is positioning.
performed using a keratometer. The placed convex side out on this
lens needs to be held in front of the surface and viewed through the The clean, dry lens should be placed
keratometer in a similar position to magnifier towards a background light concave surface down to measure
where the eye is normally placed. source. It should be gently moved to BVP, centred as well as possible to
Special holders are available for this line up with the graticule and a direct avoid introducing any prismatic
purpose (Figure 2). The post is inserted reading of TD in mm, correct to one effect. If the lens is not placed on or
into the chin rest where the securing decimal place is taken. In exactly the very close to the stop, it will not be in
pins for the disposable paper wipes same manner, BOZD and any the correct plane to give an
are usually located and the clean peripheral curve diameters may be accurate reading. The error will be to
lens placed into the concave plastic recorded if the blending on the lens show a more positive or less negative
holder with sterile liquid. The allows them to be seen. reading. This can also occur if the lens
keratometer then is used in the has a very steep back curve, forcing
normal way to give a direct reading At the same time, the lens can be the centre away from the stop.
 Continuing Education and Training

Figure 5: Contact lens holder for a focimeter

Once positioned, a reading is taken advance of gas permeable materials,

in exactly the same way as for the choice reduced considerably,
spectacle lenses. Should a cylindrical but still exists. The best way to check
component be present, this will of a lens for tint is to place it against a
course show in the normal way, white background, a clean tissue is Figure 6: Contact lens thickness gauge
although the axis shown will depend ideal, and observe it in good natural
on the rotational position of the lens light. Recording is personal rather
on the stop, rather than the true axis. than scientific, using terms such as to supply the desired BOZR, TD and
‘blue’, ‘light blue’, etc. Older PMMA BVP (plus material and tint), leaving
Geometric Centre Thickness (CT) lenses used numbers to identify their their programmed computer systems
CT is very easy to measure with an tint, such as 912 for light grey or 512 to calculate all other parameters.
appropriate thickness gauge. As seen for dark grey, but this practice is little So perhaps you can obtain more
in Figure 6, it consists of a simple dial used today. information than you expected with
gauge, very similar to a radiuscope, your RGP contact lens wearers!
which is directly connected to a Material
spring loaded probe. The movable There is now a large range of gas References
leg of the probe needs to be raised, permeable materials available with 1. British Standards Institution BS EN ISO
the lens inserted concave side down varying properties of DK, wettability, 18369-1:2006
centrally on the fixed part and the hardness etc. In a practice setting, 2. British Standards Institution BS EN ISO
leg gently repositioned onto convex the author suggests that it is not 18369-2:2006
surface of the lens. The lens position is possible to detect which material has 3. Phillips AJ and Stone J, Hard lens
shown as (Figure 6A). A reading is been used to make a lens. verification procedures. In AJ
directly taken. Because CT is Phillips and J Stone, editors.
generally very small, say between In summary Contact Lenses, 3rd ed. London:
0.10 and 0.40mm, errors do not We started with a lens to be checked Butterworths 1989; pp 440 – 504
normally occur in taking the reading or measured with the following
as they may with the radiuscope. specification: Keith Cavaye is currently a locum
Only the large pointer is observed as C3 7.80:7.70/8.60:8.50/11.25:9.50 contact lens optician and consultant,
the reading is always less than -5.00DS, Boston XO, Blue tint, ct 0.20, an ABDO practical CL examiner, an
1.00mm, unless a scleral lens is being with dot marking for right eye ABDO theory dispensing marker, and
measured! sits on the GOC Investigation
With some measuring devices in Committee. He is chairman of the
The thickness gauge can also be practice, it should be possible to ABDO CET Committee and member of
used to determine edge thickness or obtain realistic readings as below, the Contact Lens Committee, ABDO
indeed any thickness across the lens. with the exception of the scored out Advice and Guidelines working group,
It is helpful in identifying prism details: College of Optometrists Membership
incorporated, with the edge being C3 7.80:7.70/8.60:8.50/11.25:9.50 and Standards Committee. Past
considerably thicker in one meridian. -5.00DS, Boston XO, Blue tint, ct 0.20, Council member BCLA and GOC. He
with dot marking for right eye has had various articles published on
Tint contact lenses. Previously he was
Most rigid contact lenses have a tint. This is enough information to make a professional services manager for
When PMMA was popular, replacement lens with reasonable Indigolighthouse Group, contact lens
laboratories issued tint charts or confidence. Indeed, most product manager for Dollond &
samples as found today with manufacturers today make ‘system’ Aitchison, and contact lens services
spectacle lens tints. With the lenses, which require the practitioner manager for Boots Opticians. n
MCQs Continued overleaf
8 dispensingoptics February 2014

Multiple choice questions (MCQs):

Measuring that unknown RGP in practice
1. What does not form a rigid contact lens back surface 4. The geometric centre thickness of a rigid corneal lens is
design? likely to be . . .
a. Paraboloidal or elliptical aspherical curves a. Reduced if the back curve is very steep
b. A series of spherical concentric curves b. Rejected if 0.01mm different from the specification
c. Convex concentric curves to flatten the peripheral radii ordered
d. Aspherical and spherical curves c. Over 1mm
d. Between 0.1mm and 0.4mm
2. Which abbreviation is incorrectly written according to
British Standards? 5. If the contact lens is not placed very close to the
a. BPZD1 b. BOZR focimeter stop . . .
c. BPZR3 d. BZOD a. A cylindrical element will be introduced
b. The reading will be more positive than the true value
3. Which statement is true regarding rigid contact lenses? c.The reading will be more negative than the true value
a. It is not practical to verify the first peripheral zone d. A prismatic effect will distort the image
diameter
b. The tolerance on a back vertex power of -6.00D is +/-0.25D 6. On which set of measurements from the practitioner do
c. Back optic zone radius cannot be measured with a contact lens manufacturers mostly rely?
keratometer a. BVP, TD, BOZR b. BPZR1, BVP, TD
d. Peripheral radii are best checked using a V-gauge c. BOZD, BVP, CT d. BPZD1, CT, TD, BVP

The deadline for posted or faxed response is 16 December 2014. The module code is C-33206
Online completion - www.abdo.org.uk - after member log-in go to ‘CET online’
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