BLOC 3: DETERMINING THE PARAMETERS OF
OPHTHALMIC LENSES
•Subject 6.spherometer.
Study
and
functionality
of
the
• Subject 7.- Measuring and marking the posterior
vertex power of a lens.
Subject 6:Study and function of the
spherometer.
•
•
•
•
Description of the spherometer
Functional principals
Zero Error
Index Error
Description of the Spherometer
• Instrument that measures the surface power
(D) associated with the lens surfaces.
Functional Principle
• Supporting the legs on the lens surface, the
movement of the center leg, is translated to
the scale, giving the value of the surface
power of the surface.
Positive value in
convex surfaces.
Negative value in
concave surfaces.
Zero error
•Adjustment or Calibrating the instrument to zero:
•When placing the instrument on a flat polished
surface, the scale must mark 0.00 D.
•If not, all previous measurements will be affected by
this value.
•The measurement will be a result of the following
equation:
Pe  Pesf  P sup
Psur: Value given by spherometer when place on flat surface.
Psph: Value given by spherometer when placed on one of the curved surfaces
of the lens.
Index Error
• The spherometer reading in diopters is only valid for one
index of refraction
• The majority of spherometers are calibrated for the n of
crown glass (1,523)
n ' 1
P' E 
PE
n 1
P’E: real surface power of the lens
PE: reading given by the spherometer scale
n`:index of refraction of the problem lens
n: index to which the spherometer is calibrated