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Describe the thickness of positive and negative lenses
-ve lenses: thicker on edge- thickness is determined by power
+ve lenses: thicker in centre
How can lenses be made?
Either by/ combo of:
Moulding: moulded from liquid (monomer + catalyst)
OR
Grinding:
a- Generating/ rough cut via generating wheel
Rc = y/ sin0
(Surface radius= semi diameter / angle of presentation)
b- Smoothing: abrasive pad for smooth surface
c- Polishing: finer abrasive pad
What does ‘blocking’ mean?
Attaching a block to the lens so it can be held for edging/ surfacing
What does ‘slumping’ mean?
Heating glass until it ‘slumps’ into the mould (PPLs/ multifocals)
What does glazing mean?
Cutting the lens to the correct shape and size, then edge profiling so it can fit into the frame
How do you work out MSU?
MSU= Longest frame length + (2x decen) + 2mm
How do you work out decentration?
((Longest frame length + DBL) - PD)/ 2 = the decentration per lens