Physiological optics 9 th lecture. Dr. Mohammad Shehadeh. Optical Prescriptions, Spectacle Lenses. Prescription of Lenses When prescribing a spectacle lens, the properties of the lens required are specified in the following way.
Dr. Mohammad Shehadeh
Prescription of Lenses
Thus, a cylinder of –2.0 dioptre power, placed with its axis (of no power) vertical is written as –2.0 DC axis 90° (DC = dioptre cylinder).
The cylindrical lens is usually placed in front of the spherical lens to allow the axis line to be seen.
(1) Let the cylindrical element be at axis 90°: the lens is now +2.0 DS/+1.0 DC axis 90°.
(2) Let the cylindrical element be of opposite power and at axis 180°: the lens is now +3.0 DS/–1.0 DC axis 180°.
This change in the description of the lens may be easily accomplished for any lens by performing the following steps.
The steps of toric transposition are now defined taking the following case as an example.
(2) Calculate the required power of the spherical surface (the numerator of the final formula). This is obtained by subtracting the base curve power from the spherical power given in (b) in step 1
(3) Specify the axis of the base curve. As this is the weaker principal meridian of the toric surface, its axis is at 90° to the axis of the required cylinder found in (b) in step 1. That is:
(4) Add the required cylinder to the base curve power with its axis as in (b) in step 1