GRINDING OF LENSES IN OPHTHALMIC PRACTICE
It is a procedure by which we convert a slab of glass (blank) into a spectacle lens with specific power. It is also known as surfacing.
This is done by grinding the blank in such a way that curvatures are produced on both front and back surfaces.
The difference of curvatures on both surfaces gives us the dioptric power of the lens. A wide range of tools calibrated for a specific power are required.
These tools are made from cast iron and should be checked from time to time for their accuracy.
Various processes involved in the making of lenses are as under.
1. Blocking:
It is a process by which a blank is fixed to the grinding machine. Gluing material is heated and applied between the blank and the platform of grinding machine.
Once the material cools down; it holds the lens very tightly and grinding can be carried out easily.
2. Roughing:
It means generating the desired curvature. Surface of the blank is rotated against tool of specific curvature using sand or carborundum as abrasive powder. Thus a blank is converted into a curved piece of glass with rough surface having approximate desired power.
3. Smoothening:
Here a proper tool with specific power is used to give accurate power to the blank. Fine abrasive material like aluminous oxide is used to grind the blank. Two grades of abrasive 302 and 303 are used one by one. Surface of the blank is trued with 302 and then with 303 to make it more smooth. The end product of this process is the lens with correct power. It is ready for polishing.
4. Polishing:
A soft pad of felt cloth or wool cloth or specially designed polishing pads are attached to the tool of rough use.
5. Deblocking:
Polished lens is examined under incandescent bulb and if found satisfactory, the lens is deblocked from the metal block.
The lens and the block are immersed in cold water and the block is tapped by a wooden mallet. Ice cubes may also be used to facilitate the deblocking process. Lens is cleaned with a thinner to remove any undesired particle over it.
This process is repeated on the other surface of the lens. After each step lens and the block are cleaned properly.
Thickness of the lens is controlled during grinding of other surface.
OPTOMETRY-SHARP VISION
Optometrist
Tags:
Optics and refraction