Lensometry of Higher Power &
Unusual Rx
Spectacles Lenses
ALBERTA ASSOCIATION OF OPTOMETRISTS
OCTOBER 2016
MARILYN SMITH RO
CLINICAL FACULTY LECTURER
UNIVERSITY OF WATERLOO
SCHOOL OF OPTOMETRY & VISION SCIENCE
Do Not Be Afraid
• Often the higher the Rx dioptre value, the
less lens choices
• Offer the best lens choice and best coatings
possible
• Be meticulous with measurements &
neutralization
• Know the proper techniques
Prism in Spectacles
 Prism is created and designed to be directly in front of the patients pupil
 Optical Centre (OC) is moved away from the pupil centre, but patient still
sees from that point, creating a prismatic effect
 Easy to miss…watch for:
- image jump
- unusual lens thicknesses
- Odd PD measurement if OC can be centred
- Unable to centre the target (= ground in prism)
- Jump of target between OD & OS at the same height on
lens table,
vertical prism
Simple Prism Rules
(that don’t always apply!)
• Horizontal prism - same amount &
direction in each eye
• Vertical prism is the same amount in
the opposite direction in each eye
• Prism can be prescribed in one eye
only
• Prism rules do not always apply
• YOU MUST KNOW THE PATIENTS’ PD
TO NEUTRALIZE PRISM!!
Prism in the Manual Lensometer
Prism dial up to 20∆
Up to 5∆ in manual lensometer
Add 3∆, 6∆ or 9∆ auxiliary prism
lenses when necessary
Oblique Prism
Oblique prism is a combination
of prism in two different
directions
Neutralize oblique prism in the
lensometer:
how far in or out from the 090
line and
how far up or down from the
180 line
Do not measure oblique prism as
a vector
Horizontal Prism
Read along the
180 line (in or out
From the 090 line)
Vertical Prism
Read along the
090 line (up or
down from the 180
line)
Example:
OD
+325 sph 2ΔBI 2ΔBU
Prism in a Progressive Addition Lens
The “dot” below the pupil cross from the PAL template must be placed
in front of the light source of the lensometer to check for prism.
The distance rx may not read the correct power but you
are only checking to see if the target is off centre
If only vertical prism, in the same direction = prism thinning
Measuring Slab Off Correction
 Slab-off/Bicentric Spectacles
 Prism ground onto lower lens area to solve diplopia at near due to
difference in Rx from OD to OS when eye is moved away from distance OC
(usually when a bifocal/PAL is prescribed)
 Measure prism value at 5mm below height of the slab off line in non-compensated
lens
 Move to slabbed off lens at same height. Prism amount should be the same value
(not necessarily 0∆) in the “slabbed off” lens
If prism is >050∆ difference,
Slab off is not correct
Vertical imbalance at near
will still affect vision
Measuring Slab Off Prism
• Difference in curve power is amount of prism in the slab off (not
always 100% accurate due to lens gauge index calibration & lens
index)
High Minus - Lenticular
• Lenticulated design for over approx. 15.00D
• Myodisc - scooped out minus aperture
with carrier
• Blended Myodisc (Optilenti) - aperture
edges blended
• Rx aperture from 28 – 42 mm available in
2mm increments
• Aperture size changes lens thickness
(smaller = thinner BUT less peripheral Noticeable edge
vision)
• Lensometry is the same as low RXs
Better cosmetics
Lenticular
High Plus Lenses
• Lenticular design an option to reduce weight & thickness
•
40 mm lens Rx zone with plano “carrier”
•
reduces weight & thickness
•
fits into frame edge better than
thick + lens edges
•
looks like a “fried egg”
•
high plus lenses over +12D
• no choices in aperture size
• Often worn by pre- IOL aphakes
40mm
65mm
ASPHERIC LENS DESIGN
• How can you tell if a lens is
aspheric?
• Centre 10mm are the “base curve”
• Gently place lens gauge across F1
(don’t pull across F1 – could scratch
lens surface!)
• If curvature changes from centre –
lens may be aspheric design
Why Do We Need to Flip the Glasses for a High Plus
Bifocal Rx?
• Light bends differently through a thick lens medium which alters the power
measurement (thick lens formula)
• Bifocal segments are created on the front surface (F1) of the lens but we
normally read the BACK VERTEX POWER from the F2 surface in the lensometer
(light comes from the back of the lensometer hitting F2 first, BVP)
• If we neutralize the segment Rx on F2 in a high + lens, it will be not be the TRUE
power in the bifocal segment
• Rather than using the thick lens formula calculation, we flip the glasses around
to determine the “True Add Power” from F1, Front Vertex Power (FVP)
Who Wears These Lenses
• Cataract removal with no IOL insertion
(surgery complications, allergies etc)
• Aphake before IOLs (pre 1980)
• Congenital cataracts
• Low Vision Rx
• Full Field is thick & can be heavy
• Lenticular has a plano “carrier” & 40mm
of Rx, fits into frame easier and is lighter
weight
• Segments may be odd shapes & sizes
• Add powers ay be higher than usual
• “Balance” may be in one lens
40 mm
Distance Rx – Same As Usual
• Centre target with light coming
through F2 (glasses facing you, as
usual), BVP
• Record sph, cyl & axis
• Dot on OC to measure distance
PD
To Determine True Add Power Step #1
• Flip glasses around so the light
is coming through F1 at the
distance OC (temples are facing
you)
• Reconnect axis lines (mirror
image of Rx axis)
• Rotate power drum to clear
most vertical lines in distance Rx
• Remember this power value
(will be less + than BVP on F2)
To Determine True Add Power Step #2
• Take off spring lens stop
• Move lens SLOWLY upward until light
source is just below segment line
• Target will be displaced far away from
the centre!!
• Clear same vertical lines as noted in
distance on F1
• Difference in power between distance
F1 and near F1 = True Add Power
This is ALL you may see
to neutralize the near power!
Trouble finding the Add Power?
1.53 or
1.49 only
1.49 AND
1.6 index
• Check the curvature of the distance
portion and the curvature of the segment
on F1
• The difference in curvature is the add
power!!
• Great for high power add or high plus
prescriptions
• Lens gauges are calibrated to indices, will
not be exact Rx if index of lens is different
Base Curve
(F1 of lens)
• Base Curve of lenses should stay consistent from
one pair of spectacles to the next
• Always record base curve in patient record
• Base curve cannot be duplicated if changing from
one lens index to another
• Counsel patient some adaptation may need to
occur when changing base curves
• Lens gauge readings can be incorrect if index is
unknown
Lens gauge vs lensometer
• Lensometer bends light = true
power
• Lens gauge measures surface
power BUT based on index of
lens
• If lensometer ≠ lens gauge
(total power), lens is a different
index from lens gauge
• Calculate to find true index!
How to Determine
Lens Index
• Lens gauge curves (Total Power = F1 + F2) will not be equal to power
from lensometer
• Lens gauge is calibrated to specific lens index
• Lensometer only bends light = true power of lens
• Some lens gauges have 1.49 AND 1.6 dials
How to Determine
Lens Index
Take lens curve from lens gauge with known index value & determine
power of lens in one meridian
Neutralize lens in lensometer for true power in same meridian
n
n
F
F
= index of the Lens
= index of the Lens Measure (lens gauge)
L
= power of the lens from vertometer (lensometer)
LM = power of the lens from lens measure (lens gauge)
V’
Will determine exact index of lens material
LM
Based on thin lens design
www.opticampus.com – good reference for optics/formulas etc
Example
Using 1.49 index dial, lens curve from lens gauge is = - 4.50
Lensometry power value in same meridian = - 5.75
n = (1.49 – 1) x ( – 5.75 ) + 1
- 4.50
n = (0.49) x (1.27) + 1
n = 0.62 + 1
n = 1.62
The index of the lens material is 1.6
High Add Powers
Straight Top Bifocals (CR-39)
- Adds up to +8.00D
Round Top Bifocals (CR-39)
- Adds up to +30.00D
- +0.25 steps up to +15.00D
- +0.50 steps +15.00 to +30.00D
Check add power with lens gauge
Progressive Addition Lenses
- Adds up to +300D in all designs
- Adds up to +5.00D in limited PALs
Vertex Distance
Distance from cornea to F2 of spectacle lens
Effective power can change noticeably depending
on Rx
Refraction with phoropter approx 13mm
Even 2mm shift in spectacle fitting can have
impact on rx’s over +/- 10D
Rule of thumb: take vertex distance for rx’s over
-8.00D or over +6.00D
Effective/compensated
power Changes
Minus Lenses
• Effective power decreases as the lens
moves away from the eye
Plus Lenses
• ↓ power away from the eye
• ↑ power closer to the eye
• To compensate the Rx required for shift in
vertex distance
• ↑ power if further from phoropter setting
• ↓ power if closer than phoropter setting
Effective power increases as the lens
moves away from the eye
• ↑ power away from the eye
• ↓ power closer to the eye
• To compensate the Rx required for
a shift in vertex distance
• ↓ power if further from phoropter
setting
• ↑ power if closer than phoropter
setting
Effective power calculation
• CAP – Closer Add Plus
• You can calculate the effective
power (what power the lens
will be when moved from the
phoropter vertex)
• OR
• You can calculate the
compensated power (what
power is needed to create the
same Rx at the new vertex
distance)
Dl
De = --------------1 + dDl
Dl
Dc = --------------1 - dDl
d = distance in metres
d is + if moved towards face
d is - if moved away from face
PAL Rx but No Template??
• Find manufacturers etchings
• Record add power
• They are always 34mm apart
• Prism dot is in the exact centre between the two
etching
• Check distance rx at least 10mm above prism dot
• Check near rx at lowest nasal placement where axis is
most accurate if necessary
The PAL Identifier Charts
• Type of PAL or invisible lens design is etched on the temporal
side of the laser engraving
• Each design has its own insignia
• Use OLA Progressive Identifier or AOQ chart to determine
lens design, lens material and manufacturer
• Includes lens indices, photochromics & minimum fitting
heights
• Progressive Ophthalmic Lenses Guide includes proper size
template designs
• New charts includes Low Add &
Computer Lenses
Clinic Patients
• All “invisible” lens designs are EASY TO MISS!!
• PALs have “Hourglass” shape in lens surface power change
• Enhanced Readers only have a drop in +power in the upper
half of the lens
• Look for change in power when viewing an object &
moving the lens vertically
• When neutralizing, lines may appear “fuzzy” or not able to
come to a sharp focus/power changes when lens moved
vertically
• Ask patients over 40 if their lenses look like SV!!
• Check in with 18 – 40 yr olds re:
Low Add PALS
Ordering One Lens Only
• Single vision – match optical centre height
• Line bifocal – specify how high optical centre is above
bifocal segment line (prism can occur if OC is at different
heights causing diplopia)
• PAL – check for prism at prism dot and specify BU or BD
if prism thinning has been used in previous pair (prism
not duplicated will cause diplopia)
When In Doubt…
• www.framesdata.com
• Quarterly book or Cdrom with all up
North American frames & Lenses
• Quarterly Lenses book or Cdrom
• All info Rx range, photochromics,
blank sizes, add range in EVERY lens
sold in North America!
Thank You!!
Any Questions?
Now or email me:
[email protected]
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