Calculation of horizontal decentration of spectacle lens in spectacle frame
Introduction
Position of the spectacle lens centration point is equal to optical spectacle lens in lenses without prismatic effect (Rutrle, 2001). Position of the spectacle centration we can find out if we draw points (pupil position) with the pencil right on the demo-foil or we can calculate it without drawing on demo-foil (see exercise 5 in Spectacle lens technique I).
Goals
- Measure pupillary distance of the patient
- Measure size of the frame according the boxing system
- Measure size of the bridge according the boxing system
- Calculate the size of horizontal decentration in relation to centre of the eye frame for right and left side
Equipment
Digital and handy PD meter, writing equipment, spectacle frame
Methods
Measure pupillary distance of the patient
Use digital or handy PD meter and measure pupillary distance right and left.
Measure size of the frame according the boxing system
Measure size of the spectacle frame (from center of the right eye frame to the left eye frame).
Measure size of the bridge according the boxing system
Measure size of the spectacle bridge. It is size from the edge of right eye frame to the edge of the left eye frame. Use boxing system.
Calculate the size of horizontal decentration in relation to centre of the eye frame for right and left side
According the bellow formula, calculate size of the horizontal decentration for right and left spectacle lens (h – horizontal decentration right and left, c – frame size, PD is pupillary distance right and left)
Results
Measure pupillary distance of the patient
PDR =
PDL =
Measure size of the frame according the boxing system
c =
Measure size of the bridge according the boxing system
d =
Calculate the size of horizontal decentration in relation to centre of the eye frame for right and left side
hR =
hL =
Draw schematically spectacle frame according boxing system and measure and mark its parameters.
Discussion
The size of the horizontal decentration is important for proper grinding of the spectacle lens. If the grinding is manual we use TABO-schema for decentration of the spectacle lens. If we use automated grinding machine we put horizontal decentration data in to the centration instrument called tracer.
Conclusion, notes, comments
Which type of horizontal decentration calculation would you choose for your practice?
How horizontal decentration limits choice of the spectacle lens diameter?
If we get horizontal decentration as negative number, what does it means?