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?
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To determine which CR39 tool must be used to work the front surface after using a plastics material of refractive index 1.56 instead of CR39, we need to calculate the new required base curve.
First, let's understand the basic concept. When working with toric lenses, the power of the lens is described by two components: Sphere (S) and Cylinder (C). The sphere component represents the power in the vertical meridian of the lens, while the cylinder component represents the power in the horizontal meridian.
In this case, the lens prescription mentioned is +2.00DS/+2.00DC x90. "DS" indicates the sphere power, which is +2.00 diopters. "DC" indicates the cylinder power, which is +2.00 diopters, and "x90" indicates the axis of the cylinder.
The base curve of the lens is mentioned as -5.00, which means that the back surface of the lens needs to be worked with a -5.00 base curve. However, the wrong material with a refractive index of 1.56 was used, instead of the intended CR39 material.
To find the new required base curve, we can use the formula:
New Base Curve = (Cylinder Power / (Material's Refractive Index - 1)) + Existing Base Curve
Plugging in the values:
New Base Curve = (2.00 / (1.56 - 1)) + (-5.00)
Calculating the equation:
New Base Curve = (2.00 / 0.56) - 5.00
New Base Curve = 3.57 - 5.00
New Base Curve = -1.43
The new required base curve, after using the wrong material, is -1.43. Therefore, the CR39 tool with a -1.43 base curve should be used to work the front surface of the lens.
It's important to note that this answer assumes a spherical toric lens. If the lens has additional parameters, e.g., prism or multifocal, further calculations may be required. Additionally, it's always advisable to consult with an optical expert or technician for precise lens manufacturing instructions.