topic 5E physical characteristics of ophthalmic lenses (minimum blank size)

Question 1

why is minimum blank size important?

Answer 1

Uncut lenses must be of sufficient size so that the lens required for the frame can be cut from it.

It is important to determine the size of the lens required for a particular prescription.

Otherwise there may be delays while an appropriately sized lens is ordered, additional cost of an oversized lens and possibly oversize lens.

Question 2

how do we calculate minimum blank size?

Answer 2

Practically, minimum lens diameter for edging will require additional 2mm for grooving.

MBS= longest axis + (frame PD – patient PD) + 2mm

*Decentration = difference between frame PD & patient PD

Question 3

what is the impact that Blank size can have on lens thickness

Answer 3

For a plus lens, the centre thickness will increase as the lens size increases.

For a minus lens, the edge thickness will increase as the lens size increases.

Question 4

what is one thing to note about diameter of plus lens?

Answer 4

Plus lens is always smaller in diameter

*because it becomes thick if made in large diameter