A technician wishes to produce the lens +2.00DS/+2.00DC x90 in toric form, in CR39(n=1.498)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 find out which CR39 tool must now be used to work the front surface, we need to take into consideration the desired lens prescription and the refractive index of the plastics material that was used in error.
The desired lens prescription is +2.00DS/+2.00DC x90, which represents a spherical power of +2.00 diopters, a cylindrical power of +2.00 diopters (in the x-axis), and a cylinder axis of 90 degrees.
The refractive index of the plastics material used in error is 1.56.
First, let's calculate the actual power of the toroidal surface that was produced with the incorrect material.
Actual Power = Desired Power / Index of Refraction
For the spherical power:
Actual Spherical Power = +2.00 DS / 1.56 = +1.28 DS
For the cylindrical power:
Actual Cylindrical Power = +2.00 DC / 1.56 = +1.28 DC
So, the actual lens prescription for the toroidal surface with the incorrect material is +1.28 DS/+1.28 DC x90.
To correct for this error, we need to determine the correct front surface power in the CR39 material.
Correct Front Surface Spherical Power = Desired Spherical Power - Actual Spherical Power
Correct Front Surface Cylindrical Power = Desired Cylindrical Power - Actual Cylindrical Power
Correct Front Surface Spherical Power = +2.00 DS - (+1.28 DS) = +0.72 DS
Correct Front Surface Cylindrical Power = +2.00 DC - (+1.28 DC) = +0.72 DC
Now, we can find the correct CR39 tool to work the front surface by considering the base curve and the corrected front surface power.
The base curve is -5.00, which represents a back surface power of -5.00 DS.
CR39 tools are usually labeled with their back surface power, so we need to find a tool with a base curve that corresponds to the back surface power of -5.00 DS.
Since the front surface power should be +0.72 DS, we need to find a CR39 tool with a base curve that, when combined with the back surface power of -5.00 DS, will result in a total lens power of +0.72 DS.
It is difficult to provide an exact answer without the specific tool options available, but a possible solution would be to use a CR39 tool with a base curve of approximately -5.72. However, it is recommended to consult with a lens manufacturer or supplier to obtain the accurate CR39 tool that matches the desired lens prescription and the refractive index of the plastics material used in error.