its about optics of optometry

1. Optometric Optics II.

2. The History of Spectacles
• The Persian, Alhazen, wrote about
using lenses to correct visual
defects one thousand years ago.
While this laid the foundations for
spectacles, the lenses were hand
held.
• The two most often credited with
their invention are Roger Bacon, the
English Philosopher, and Salvino
Armati.

3. • The two main developments since
the invention of spectacles
are, arguably, the invention of
bifocals and the invention of
progressives.

4. The History of Spectacles
• In 1784 Benjamin Franklin,
American inventor and
statesman, tired of having to
change between his distance and
near spectacles, cut the lenses
of both in half and re-attached
them to form bifocals.
• Franklin’s original design is
still in use today for prism
control.

5. The History of Spectacles
• In 1959 French scientist,
Bernard Maitenaz, developed the
first successful progressive
lens, Varilux.
• This lens type is rapidly
becoming the lens of choice for
presbyopes.

6. Spectacle Correction
• Spectacle lenses are
used to provide
optical correction of
the eye’s refractive
error, and to
compensate for loss
of accommodation in
presbyopes.
• They can also be used
to treat binocular
vision anomalies.
Distan
ce
Intermed
iate
Near

7. Spectacle Correction
• Spectacle lenses are made in
two main designs: single vision
lenses, or multifocal and
progressive lenses.

8. Single vision lenses
• Single vision lenses consist of
a single optical power, and are
prescribed to correct vision
for a specified viewing
distance. Examples of single
vision glasses would be to
correct:
– A myopic eye for distance viewing.
– The same myopic eye for reading.
– An emmetrope who is presbyopic.

9. Single vision lenses
• Single vision lenses have a
large field of view but can be
inconvenient because its range
of clear vision depends on the
eye’s physiological factors
such as accommodation.

10. Multifocal and progressive
lenses
• These lenses consist of two or
more optical power areas in the
same lens.
• The three main types are
bifocals, trifocals, and
progressive addition lenses

11. Lens Types - Single Vision
•Single vision lenses
– Spherical and toroidal lenses
– Aspheric lenses

12. Spherical and toroidal
lenses.
• These are the most common
single vision lenses.
• They are simple meniscus or
toric lenses with, usually, a
spherical positive front
surface and a spherical or
toroidal negative back surface.
• The lenses are available in
most materials.

13. Aspheric lenses
• Aspheric lenses use conical
curves, notably ellipsoids, to
produces flatter and thinner
lenses.
• The asphericity allows for a flat
lens design without increasing
marginal astigmatism.

14. Aspheric lenses
• While it is possible to produce
aspheric lenses in any material
they are most often made in the
new high index plastics which
further improve the reduction
in lens thickness.

15. Aspheric lenses
• Until recently, aspheric lenses
were confined to high powered
plus lenses used for aphakia.
• They have since evolved into
the low to medium power lenses
where the emphasis is on the
cosmetic effect rather than the
optics.

16. Aspheric lenses
• The aspheric curves allow a
flatter and, therefore more
attractive lens to be made
without creating unacceptable
aberrations, particularly
marginal astigmatism.
• The main focus of the earlier
high powered aspherics was
improved field of view.

17. Aspheric lenses
• In addition to improved
cosmetics and reduction of
aberrations the reduced power
at the edge of an aspheric lens
will reduce the ring scotoma in
plus lenses

18. Loss of
visual field
at edge of
plus power
lens
(scotoma)
Image is
deviated
so that
the field
of view
in the
lens is
smaller
than the
original

19. Lens Types - Multifocals
1 Bifocals
1 Trifocals
3 Occupational Multifocals

20. Bifocals
• Round seg
• Flat-top
• Curved-top
• Executive

21. Round seg
• The round seg bifocal was the first of
the modern bifocals to be developed.
• In CR-39 it can be felt as a small
elevation on the front surface of the
lens and is formed by moulding the
lens.
• In glass it is formed by fusing a
higher index glass into a crown glass
depression.
• Once finished the seg cannot be felt.

23. Round seg
• The round seg is often considered a
general purpose bifocal, suitable
for people who do not do much
reading.
• It is, though, a very easy bifocal
to produce and its round shape
allows for easier production of
astigmatic prescriptions since the
axis does not have to correspond
with a flat seg top.

24. Round seg
• Despite its attractive simplicity the
round seg has two significant
disadvantages.
• First, and most important, it has an
area of wasted space at the top of the
seg, where the seg is not wide enough
for reading & therefore, requires the
lines of sight to drop further into
the seg for comfortable reading.
• It is not a good bifocal for patients
involved in a significant amount of
close work.

25. Round seg
• Secondly, since the optical
centre of the segment is a
considerable distance from the
segment top (it is at the
geometric centre of the
segment) there is also
noticeable image jump, with the
image appearing to jump up as
the lines of sight enter the
top of the seg.

26. Flat-top
• The flat-top bifocal solved the main
problem of the round seg, that is, it
removed the useless area at the top of
the seg.
• In doing so, though, the seg became, by
necessity, more obvious, particularly in
the CR-39 version.
• Like the round seg, the CR-39 flat-top
can be felt but rather than a small
elevation, there is a definite ledge on
the front surface which is not only more
easily felt but also more easily seen.

27. • In glass it is formed by fusing
a higher index glass into a
crown glass depression but with
a crown carrier seg at the top
of the high index seg to
produce the D shape.
• Once finished the glass seg
cannot be felt.

28. Flat-top
• In addition to its more useful
reading width, the flat-top
also has considerably less
image jump than the round seg
since the seg’s optical centre
is close to the top of the
segment.

29. Flat-top
• When compared to the round seg,
the flat-top is less
attractive.
• The production of astigmatic
prescriptions is also more
critical since when the seg top
is perfectly horizontal the
lens must also be on axis.

30. Curved-top
• The curved-top bifocal is a
variation on the flat-top.
• Rather than a perfectly flat seg top
the curved-top has a slight curve.
• Like the round seg and the flat-top,
the CR-39 curved-top can be felt.
• Its ledge is less noticeable than
the flat-top but more obvious than
the round seg.

31. Curved-top
• It can also be produced in
glass, again formed by fusing a
higher index glass into a crown
glass depression but with a
curved crown carrier seg at the
top of the high index seg to
produce the curved shape.
• Again, once finished the glass
seg cannot be felt.

32. Curved-top

33. Executive
• The Executive bifocal is the most
obvious of all bifocals and has a
large downward facing ledge across
the whole lens.
• The size of the ledge and the
overall thickness of the lens are
determined by the power of the
addition.
• The Executive was designed largely
as an occupation bifocal for people
whose main concern was close work
(such as an architect).

35. Executive
• The Executive’s main advantage
is its very large reading zone.
• It also has no image jump, with
the seg optical centre being on
the seg line.
• It does, however, have numerous
disadvantages.

36. Executive
• It is a thick, heavy and quite
ugly lens.
• Also, like the flat-top the
Executive requires careful
production of astigmatic
prescriptions, since when the
seg top is perfectly horizontal
the lens must also be on axis.

37. Trifocals
Flat-Top Executive E-D Line

38. Trifocals
• Trifocals are designed for advanced
presbyopes to see comfortably at
intermediate distances.
• They have an intermediate segment of
a lower add (usually 50% of the
reading add) which enables them to
view objects at the other side of
the desk, on a noticeboard etc.
• Like bifocals, trifocals are
available in different designs of
which the three most popular are the
flat-top, the E-D Line and the
Executive

39. Trifocals
• The characteristics of trifocals are the same
as the equivalent bifocal design.
• So, for example, the Executive trifocal is an
unattractive lens but offers a wide reading
and intermediate area.
• The E-D Line trifocal is a combination of an
Executive design and a flat-top.
• It is designed for people who require a large
intermediate zone but not necessarily a wide
reading zone.

40. Occupational Multifocals
Librarian Executive Occupational

41. Occupational Multifocals
• While any bifocal can be designed for
occupational purposes and large seg
bifocals and trifocals are often
classified as occupational multifocals,
there are a few multifocals that are only
suitable for specific uses.
• It is these specific purpose multifocals
that we will consider as occupational
multifocals.

42. • The best example of this category is the
librarian style trifocal.
• It appears to be a lens with two flat-top segs,
one in the normal position and the other upside
down at the top of the lens.
• The top segment is the intermediate power and
the bottom seg is the reading add.
• It is designed for people who need to see
objects at an intermediate distance above their
heads, for example librarians looking at books
on high shelves, pilots looking at controls
above them etc.

43. • The Executive occupational
trifocal is of similar design,
with the distance power in the
middle band.

44. Field of View
Positive lens (hyperopic
correction)
The prismatic effect deviates
light towards the prism base.
This means that for a given ray of
light entering the eye, the object
actually seen is not at the
undeviated angle, but at a smaller
angle.

45. Field of View
This will reduce the FOV compared
with a plano lens (zero power)
placed in the same aperture at
the same distance from the eye
(i.e., RFOV < AFOV).

47. Field of View
The difference between the two
is equivalent to a “blind spot”;
the complete circumferential
zone is referred to as a ring
scotoma.

48. As the picture shows, a plus powered
lens deviates light in such a way that
the field of view is contracted (left
edge of frame), and some details in
the visual field can disappear (right
edge of frame). Loss of
visual
field at
edge of
plus
power
lens
(scotom
a)
Image
is
deviat
ed so
that
the
field
of
view
in the
lens
is
small

50. Negative lens (myopic
correction)
A minus lens acts like two base
out prisms at the optical centre.
This effectively expands the FOV
compared with a plano lens (RFOV >
AFOV).

51. In the area between the larger RFOV and
the smaller AFOV, more peripheral
objects will be brought into view.
However, they will not be properly
focused and the overlap with focused
objects may cause a confusing diplopia.
The complete circumferential zone is
referred to as a ring of diplopia.

53. Field of View
• Because these effects occur at the lens
periphery, they are usually not noticeable
when the eye is looking straight ahead.
(This is due to the lower visual resolution in
the periphery of the retina which is unable
to detect fine detail.)
• However, the ring scotoma (hyperope) or ring
of diplopia (myope) becomes obvious when the
eye is rotated towards the extremes of the
lens.

54. Deviation of images at
the edge of a minus lens
can create confusion in
the peripheral visual
field (diplopia).
Image through the
lens is deviated so
that the real field
of view in the lens
is larger than the
apparent field (as
predicted by
geometry)

55. Thank you.