The formula to calculate the power of a cross cylinder lens is:
\(P = \frac{C1 - C2}{2}\)
Where:
A Cross Cylinder is a type of lens used primarily in optometry and ophthalmology for refining a patient’s eyeglass prescription. It is characterized by having one meridian with plus power and the other with an equal amount of minus power, oriented 90 degrees apart. This unique design allows the lens to simultaneously correct for both astigmatism and spherical refractive errors.
During an eye examination, the optometrist or ophthalmologist places the cross cylinder lens in front of the patient’s eye and rotates it to different orientations, asking the patient to compare the clarity of their vision with each change. This process helps determine the precise degree and axis of any astigmatism present, enabling the prescription of the most accurate corrective lenses for the patient.
Example:
Calculation:
\(P = \frac{2.00 - (-1.00)}{2} = \frac{2.00 + 1.00}{2} = \frac{3.00}{2} = 1.50 \text{ D}\)