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 pupilOptical Centre (OC) is moved away from the pupil centre, but patient still sees from that point, creating a prismatic effectEasy 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

Up to 5∆ in manual lensometer

Add 3∆, 6∆ or 9∆ auxiliary prismlenses when necessary

Prism dial up to 20∆

Oblique Prism

Oblique prism is a combination of prism in two different directionsNeutralize oblique prism in the lensometer:how far in or out from the 090 line andhow far up or down from the 180 lineDo not measure oblique prism as a vector

Example:OD +325 sph 2ΔBI 2ΔBU

Horizontal PrismRead along the180 line (in or outFrom the 090 line)

Vertical PrismRead along the090 line (up or down from the 180 line)

Prism in a Progressive Addition Lens

The “dot” below the pupil cross from the PAL template must be placedin front of the light source of the lensometer to check for prism.The distance rx may not read the correct power but youare only checking to see if the target is off centreIf 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 correctVertical imbalance at nearwill 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 vision)

• Lensometry is the same as low RXs

Noticeable edge Better cosmetics

LenticularHigh 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 (mirrorimage 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?

• 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

1.53 or1.49 only

1.49 AND1.6 index

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 DetermineLens 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 DetermineLens Index

Take lens curve from lens gauge with known index value & determine power of lens in one meridianNeutralize lens in lensometer for true power in same meridian

n = index of the Lensn = index of the Lens Measure (lens gauge)F = power of the lens from vertometer (lensometer)F = power of the lens from lens measure (lens gauge)

Will determine exact index of lens materialBased on thin lens designwww.opticampus.com – good reference for optics/formulas etc

L

LM

V’

LM

Example

Using 1.49 index dial, lens curve from lens gauge is = - 4.50Lensometry power value in same meridian = - 5.75

n = (1.49 – 1) x ( – 5.75 ) + 1- 4.50

n = (0.49) x (1.27) + 1n = 0.62 + 1n = 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/compensatedpower Changes

Minus Lenses

• Effective power decreases as the lens moves away from the eye• ↓ 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

Plus LensesEffective 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 phoroptersetting

• ↑ power if closer than phoroptersetting

Effective power calculation

• CAP – Closer Add Plus

• You can calculate the effectivepower (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 metresd is + if moved towards faced 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:

marilyn.smith@uwaterloo.caAll slides are the property of M Smith and may not be duplicated without specific permissions