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Basic Optical Principle 5

The common form of spherometer normally available is the “Optician’s
lens measure” which is calibrated for a refractive index of 1.523 and gives
the dioptre power of the surface directly. More accurate version of this
simple lens measure is used in surfacing laboratory and is known as
“Sagometers” (Fig. 1.6).

Fig 1.6: Sagometer

FOCAL POWER
The total power of a lens or lens surface to bend light is referred to as its
focal power. Units of focal power are expressed as “dioptre” and are related
to the focal length of the lens. The relationship between focal length and
focal power can be expressed by the formula:

F =

Where F = Focal power in dioptre.
f = Focal length of the lens in metres.
n = Refractive index of the medium in which the light is traveling.
Since, the spectacle lenses are always in air, the relationship can be
simplified to:

1
F =
f
THIN LENS POWER

In optical dispensing while calculating the power of the lens, the effect of
the lens thickness is ruled out. The total thin lens power of the lens is the
sum of its surface powers. For example, if a lens has a front surface power
of + 8.00D and back surface power of – 4.00D, then its thin lens power is
+ 4.00D.