Progressive addition lenses

PROGRESSIVE ADDITION LENSES

BY, SANJANA CHOUHAN.S

OBJECTIVES• Details of progressive lenses.• Types of progressive lenses.• Progressive markings.• Fitting error and alignment and also the

types of bi-focals

INTRODUCTION • The very first Progressive lens was

invented by Bernard Maitenaz, from the French company Essel, in the year 1959 –known as Varilux lens .

• A progressive addition lens has more than one focal point or dioptric powers, without the need to switch lenses.

• Progressive lenses can be called progressive addition lenses (PAL), progressive power lenses (PPL).

• Based on the number of focal points, ophthalmic lenses are divided into four types

Single vision lenses Bifocals Trifocals Progressive addition lens

• SINGLE VISION LENSES - Corrects only one vision at a time . (Single focal point)

• BIFOCALS – Corrects two vision, Distance and near.(Two focal points)

• TRIFOCALS – Corrects three vision, distance, near and intermediate.(Three focal points)

• PROGESSIVE – Corrects different distances.(Many focal points)

FEATURE OF A PROGRESSIVE LENS

• A progressive lens provides visual compensation at all distances namely Far, Distances and Near.

• It has a lens that changes its dioptric power continuously, starting at its geometric centre and gradually progressing its dioptre value.

• The progression of power occurs in an optically pure progression corridor.

• By “Pure” it is meant that there are no astigmatic aberrations in the peripheral zones of the lens .

• The progression corridor starts few millimetre (mm) below the fitting cross and keeps progressing until it reaches the near visual field .

• At the lateral zones the astigmatic aberration zones are present, where vision is not totally clear .

• The aspheric lens design is responsible for the power progression of PALS and has different radii of curvature.

• It is made up of multiple focal points due to change in its radius of curvature.

PROGRESSIVE LENS DESIGNS

• Hard design • Soft design

TEMPORARY MARKINGSDistance reference circle:• This is the identification circle used to

check the distance lens power. • It should be crossed checked with the

lensometer or focimetry. Fitting cross:• It should be located exactly at the center

of the patients pupil. • It is used to measure the MPD(monocular

pupillary distance) and fitting height.

Prism reference circle:• This is the point which is used to check the

amount of prism in the lens.Near reference circle:• This marking is used to check the near

vision power. • It should be verified with the lensometer.

Small horizontal locater:• This indicator should be level with the

prism reference point. • This mainly helps the surfacing lab

markers to mount the lenses and check the position of horizontality.

• The horizontality precision can even be spotted easily with the naked eye.

TEMPORARY MARKINGS

PERMANENT MARKINGS

PERMANENT MARKING Lateral circles:• There are two small lateral circles that are used

to relocate the temporary markings and they are positioned exactly 34 mm apart one each on the nasal and temporal sides.

• Lens logo and material code:• The lens type is identified with the help of lens

logo.• The material coat helps us to identify the type of

lens material.

• E.g: In case of 1.5 index the lens logo is followed by no.6 and in case of polycarbonate lens, a letter P is seen.

• This logo is nasally located. Addition power:• This is located below the temporal locator • We can read the addition power directly

from the lens by using the lensometer.

DETAILS OF LENS MARKINGCentral fitting cross:• The central fitting cross is marked with the

ink• Mounted in the frame as per the center of

the customers pupil.• It is a temporary marking.• The marking can be wiped off with

isopropyl alcohol.

RELOCATING THE FITTING CROSS

• A template is provided by the manufacturer for the progressive lenses.

• By placing the lens on the template marking sure to align the 2 lateral traces in the horizontal direction and mark the fitting cross.

• The wearer should be looking straight ahead with the head in the straight up while the measurements are being taken.

• The fitting cross is placed exactly at the pupillary center.

• If these precautions are not taken it will lead to inaccurate measurements.

ADVANTAGES • Improved appearance • Complete range of working distance• NO JUMP effect• Thinner and lighter• No need to switch lenses• No dividing line

DISADVANTAGES• Peripheral aberrations• Adaptation problems• More critical fitting required• More expensive

Guess the markings ???

THANK YOU