The document defines and describes different types of lenses, including spherical and cylindrical lenses. It explains that a spherical lens can be either convex or concave, and discusses their optical properties such as focal length and power. Measurement techniques for lenses are also outlined, including trial lens neutralization, movement tests, and instruments like the focimeter and lensometer.

2.  A lens is defined as a portion of a refracting
medium bordered by two curved surfaces
which have a common axis.
 When each surface forms a part of sphere, the
lens is called a spherical lens.

3. SPHERICAL LENS

4.  SPHERICAL LENS
- CONVEX LENS
- CONCAVE LENS
 CYLINDRICAL LENS

5.  CONVEX LENS
 CONCAVE LENS

8.  A convex lens causes convergence of
incident light, whereas a concave lens
causes divergence of incident light.

9. CONVEX LENS

10. CONCAVE LENS

11. Center of curvature (C)
The centre of the sphere of which the
refracting lens surface is a part.
Radius of curvature (r)
It is the radius of the sphere of which the
refracting surface is a part.

12. The principal axis (AB)
It is the line joining the centers of
curvature of its surfaces.
Optical centre (N) of the lens
corresponds to the nodal point of a thick
lens.

13. The principle focus (F) of a lens is that
point on the principal axis where parallel
rays of light, after passing through the
lens ,converge (in convex lens) or appear
to diverge (in concave lens)
The Focal length of a lens is the
distance b/w the optical centre and the
principal focus.

14. Power of a lens (P)
The ability of the lens to converge
a beam of light falling on the lens.
Converging (convex) – positive
Diverging (concave) – Negative
 Unit is dioptre (D)

15. Lenses of shorter focal length are more
powerful than lenses of longer focal
length.
F = 1/ f2
Where F = Vergence power of lens in
dioptres.
f2 = Second focal length in metres

17. So, in the above vergence at the lens
is:
F = 1 / 0.25 = 4.00 D.
vergence at the lens is:
F = 1/ 0.10 = 10.00 D.

18.  Vergence is the term that determines the
direction and power of light transmission.
 The vergence of light is defined by:
V = n/L
Where n = refractive index of the material.
L = the distance in accordance

19.  Since the distance L1 is measured from the
wavefront and the light is traveling from left
to right, it is a negative distance and the
vergence is negative (divergent).
 L2 is positive since it is directed to the right
from the wavefront (convergent)

21.  Parallel light rays are said to have zero
vergence.
 The unit to measure vergence is “dioptre”.
The change in vergence when the light
encounters a refracting surface is equal to the
power of the surface.

23.  Most of the optical lenses have curved
surfaces; the surface may be described as
convex, if it bulges out of the material, and
concave if it is depressed into the material.
 The measure of the shape of a curved surface
is known as curvature.

25.  The curvature of a spherical surface can be
read with a simple instrument called
“spherometer”.

26.  The common form of spherometer
normally available is the “Optician’s
lens measure”.
 More accurate version of this simple
lens measure is used in surfacing
laboratory and is known as
“Sagometers”.

29.  Trial lens hand neutralizationTrial lens hand neutralization
 Lensometer / FocimeterLensometer / Focimeter
 Geneva lens measureGeneva lens measure oror Lens clockLens clock

30.  It is possible to determine the spherical lens by
studying the image formed when two lines crossed at
90º, are viewed through the lens.
 Spherical lens causes no distortion of the cross.
However, when the lens is moved from side to side
and up and down along the axis of the cross, the cross
also appears to move.

31.  Against movement:- In the case of a convex
lens, the cross appears to move in the opposite
direction to the lens .
 With movement :- a movement in the same
direction, is observed if the lens is concave.
 Rotation of a spherical lens has no effect upon
the image of the crossed lines.

32. Against movement:-

33. With movement

34. Two lenses neutralize each other when placedTwo lenses neutralize each other when placed
in contact with each other so that the combinedin contact with each other so that the combined
power of the two lenses is equal to zero.power of the two lenses is equal to zero.
An unknown lens is neutralized by a knownAn unknown lens is neutralized by a known
trial lens of equal power but opposite in sign.trial lens of equal power but opposite in sign.

36. This is performed in the absence of a lensometerThis is performed in the absence of a lensometer
It more accurately estimates low plus and minusIt more accurately estimates low plus and minus
power lenses than toric lenses.power lenses than toric lenses.
It is used to measure theIt is used to measure the front vertex powerfront vertex power ofof
the lensthe lens

37. View a large distant cross target through theView a large distant cross target through the
lens.lens.
Hold lens on visual axis, at arm’s distance.Hold lens on visual axis, at arm’s distance.
Align lens such that the cross target isAlign lens such that the cross target is
continuous.continuous.
Move lens vertically, observe movement ofMove lens vertically, observe movement of
horizontal line.horizontal line.
Move lens horizontally, observe movement ofMove lens horizontally, observe movement of
vertical line.vertical line.

38. For a plus or a minus lens, movement is usedFor a plus or a minus lens, movement is used
to neutralize powerto neutralize power
 If observe “If observe “against movementagainst movement - use- use MINUSMINUS lenslens
 If observeIf observe “with movement“with movement -use-use PlusPlus lenslens

39. Place known trial lens against front surface ofPlace known trial lens against front surface of
unknown lensunknown lens
No movement indicatesNo movement indicates neutrality.neutrality.
The dioptric power of the unknown lens must
equal that of the trial lens of opposite sign, e.g,
a + 2.00 D lens neutralizes a – 2.00 D.

40.  To measure this accurately, the neutralizing lens must
be placed in contact with the back surface of the
spectacle lens.
 with highly curved lenses, this is not possible and an
air space intervenes. It is, therefore, better to place the
neutralizing lens against the front surface of the
spectacle lens.

41.  Neutralization is, thus, somewhat inaccurate
for curved lenses of more than about 2.00D

42.  Large distant cross targetLarge distant cross target

43. Plus lensPlus lens

44. Minus lensMinus lens

45.  Focimeter is an optical instrument for determining
the vertex power, axis direction and optical center
of an ophthalmic lens.
 It is used to measure theIt is used to measure the back vertex powerback vertex power oror frontfront
vertex powervertex power of the lensof the lens
 The image of the target is seen as a ring of dots
when a spherical lens is tested

46.  Target as seen in
Focimeter
Lensometer

47.  Can be used to find the surface powers of a
lens by measuring the surface curvature.
 The total power of a thin lens equals the sum
of its surface powers.

48.  It is called as Geneva Lens Gauge, Lens
measure or Lens Clock