A technician wishes to produce the lens +2.00DS/+2.00DC x90 in toric form, in CR39 with a -5.00 base curve. After finishing the toroidal surface with the CR39 tool it is discovered that, in error, they have used a plastics material of refractive index 1.56. Which CR39 tool must now be used to work the front surface?
To determine the appropriate CR39 tool that should be used to work the front surface, we need to calculate the required base curve for the toric lens.
First, let's understand the terms used in the lens prescription:
- DS: Spherical Power (diopters): The correction for nearsightedness or farsightedness.
- DC: Cylinder Power (diopters): The correction for astigmatism.
- x: Axis (in degrees): The orientation of the cylinder power.
- x90: Axis value of 90 degrees.
Now, let's calculate the required base curve for the toric lens:
1. Determine the overall power of the lens:
Total Power = Spherical Power (DS) + Cylinder Power (DC)
Total Power = +2.00DS + +2.00DC
Total Power = +4.00 diopters
2. Calculate the power for the toroidal surface:
Toroidal Power = Spherical Power (DS) – Cylinder Power (DC)
Toroidal Power = +2.00 diopters
3. Calculate the sagittal depth (d) for a spherical surface:
d = (n - 1) / R
Where:
- d: Sagittal Depth
- n: Refractive index of the material
- R: Required base curve
Given that the refractive index of the plastics material is 1.56 and the toroidal power is +2.00 diopters, we can calculate the required base curve as follows:
Using the sagittal depth formula:
d = (1.56 - 1) / R
2 / R = 1.56 - 1
2 / R = 0.56
Cross-multiplying, we get:
2R = 1 / 0.56
R = 1 / (2 * 0.56)
R ≈ 0.892
Therefore, the required base curve for the toric lens is approximately -0.892.
Now, considering the error that was made, where a plastics material with a refractive index of 1.56 was used instead of CR39, we need to find the CR39 tool that corresponds to the required base curve of -0.892.
You can consult the manufacturer's specifications for the CR39 material to find the appropriate tool for the given base curve. The manufacturer should provide a reference guide or a table that matches base curve values with the corresponding CR39 tool number. By using this reference guide, you can identify the specific CR39 tool required for the -0.892 base curve.
Please note that the manufacturer's specifications may vary, so it is essential to refer to the specific documentation provided by the manufacturer of the CR39 material to ensure accurate information.