3. DEFINITION
The base curve of a lens is the surface curve that serves
as the basis or starting point from which the remaining
curve will be calculated.
4. CHARACTERISTICS
The selection of base curve is made when the lens is
the semi finished state, in which case the base curve is
always on the finished side of the lens.
In toric surfaces, the flatter of the two curvatures is
considered the base curve
5. IMPORTANCE
Base curve will determine the thickness of the lens
The choice for base curve will control the aberrations
associated with the lens
If the dispenser combines proper base curve along
with good fitting and perfect optical center the result
is good cosmetics and great optics
6. WOLLASTON FORM OF LENSES
William Wollaston in 1804 studied image formation
experimentally by using different form of lenses.
According to Wollaston, best image for oblique rays is
produced by the use of very steep surface powers.
Wollaston form not only eliminates oblique
astigmatism, but also minimizes distortion.
Wollaston form is more expensive, difficult to
manufacture and cosmetically is not good due to it’s
bulbous apperance.
7. OSWALT FORM OF LENSES
F. Ostwalt(1898) of France presented calculation for
lenses that were free of oblique astigmatism- a shallow
form & the deeper form
The Ostwalt form is used as a basis of the design of
modern ophthalmic lenses
8. TSCHERNING’S ELLIPSE
Within the range of lens powers over which oblique
astigmatism is zero,both the Oswalt and Wollaston
forms are plotted and an ellipse is generated.This
ellipse is known as Tscherning’s ellipse.
This is the point between Wollaston and Oswalt form)
10. BASE CURVE SELECTION
There are two formulae for selecting base curve-
(a) Vogel’s formula
(b) Manufacturer’s table
11. VOGEL’S FORMULA
This formula helps in determining the approximately
what base curve might be expected for a given lens
power.
For plus lenses-
Base Curve=spherical equivalent+6.00D
For minus lenses-
Base Curve=spherical equivalent/2 + 6.00D
13. MANUFACTURER’S TABLE
Manufacturers typically produce a series of semi-
finished lens blanks,each with it’s own base curve.
This base curve series is a system of lens blanks that
increases incrementallay in surface power
(eg.+0.50D,+2.00D and so on)
The more base curves available in the series,the
broader the prescription range of the lens.
15. FORM OF THE LENS
The term form of the lens refers to the relationship
between the front and back surface curvature of a lens.
For a lens of given power,an infinite number of forms
are possible.
The simplest lens forms-
PLANO-CONVEX in which one surface is flat and
other surface is convex
PLANO-CONCAVE in which one surface is flat and
other surface is concave
16. Another simple lens form is the BI-CONVEX or BI-
CONCAVE having half the power on the front surface
and half on the back surface.
Another type of lens is MENISCUS lens,which have a -
6.00D back surface power(for plus lens) or a front
surface power of +6.00D(for minus lens)
17.
18.
19. BEST FORM OF LENSES
A lens is said to be in best form when it follows
following criterias-
(1) Mechanical criteria
(2) Optical criteria
20. MECHANICAL CRITERIA
Flatter lens forms are slightly thinner than steeper lens
forms, and vice versa.
Since the lenses are thinner, they also have less mass
making them lighter in weight as well.
Due to variation in the lens thickness,the lens form
will also produce significant differences in the plate
height, or overall bulge, between lenses of the same
power. Essentially, plate height is the height of a lens
as measured from a flat plane
21.
22. This table represents a range of +4.00 D lenses in hard
resin plastic, edged to a 70-mm diameter and a 1-mm
minimum edge thickness.
Base curve Center Plate Weight
10.00D 6.9mm 15.3mm 21.7g
8.00D 6.3mm 11.7mm 19.5g
6.00D 6.0 mm 8.7mm 18.3g
4.OOD 5.9 mm 6.0mm 17.7g
23. The table, below, represents a range of -4.00 D lenses in
hard resin, edged to a 70-mm diameter and a 2-mm
minimum center thickness
Base Curve Edge Plate Weight
6.00D 8.7mm 16.4mm 25.4g
4.00D 7.8mm 12.8mm 24.0g
2.00D 7.3mm 9.7mm 23.2g
0.00D 7.0mm 7.0mm 22.8g
24. Flatter (less "bulge")
Plus lenses with flatter plate heights do not fall out of
frames as easily, which is especially important with
large or exotic frame shapes.
In addition, flatter plate heights are also more
cosmetically pleasing than steeper, bulbous ones—
particularly in plus powers.
25. Less magnification (or minification)
A reduction in plate height will also provide a
significant reduction in the magnification associated
with plus lenses.
Since a flatter plate height brings the back surface
closer to the eye, the minification associated
with minus lenses is also reduced slightly.
This gives the wearer's eyes a more natural appearance
through the lenses
26. Thinner center thickness (plus) or edge thickness
(minus)
Lighter in weight
Better frame retention (in plus powers)
27. OPTICAL CRITERIA
Lens aberrations act as errors in power from the desired
prescription, and can degrade the image quality produced
by the lens as the wearer gazes away from—or obliquely
to—its optical axis.
There are six different lens aberrations that can affect the
quality of peripheral vision through a spectacle lens:
Oblique Astigmatism
Power Error
Spherical Aberration
Coma
Distortion
Chromatic Aberration
28. Chromatic aberration is caused by the material from
which the lens is made and therefore it can be
controlled to some extent by the judicious selection of
the material.
Monochromatic aberration on the other hand caused
by factors such as size of lens aperture and position of
the lens in relation to the eye.
These are controlled by varrying the front and back
surface powers and the thickness of the lens and also
by varrying vertex distance
