Centration Blocking Flashcards
Where is the central point of rotation?
corresponds to geometric center of lens shape
What is the central point of rotation?
The point in which the lens rotates to be ground to a specific shape to fit the frame
What happens if a lens is not centered correctly? 4
-it ensure patient receives the full corrective power of the lens.
-visual distortion, discomfort
-headaches
-eye strain.
What is lens spotting?
The process of using a lensometer to apply locational marks
on a lens to be used for the decentration process prior to
edging
What is decentration?
The process of orienting a lens a specific amount from the
rotational center in preparation for the edging process
What are the 2 things a lensmeter does?
reads the power
spots the OC of a lens
What is the formula for decentration per lens?
(DBC – Wearers PD)/2
DBC = A + DBL
What formula is used to calculate vertical decentration?
MRP (OC) Height – (B/2)
How should MRP be calculated? 2
-from the bottom of the lens
-how far above or below (raised or dropped) it is from the horizontal midline in mm
What are the 5 methods of blocking a lens?
-Pressure
-Suction
-Metal Alloy
-Precast Free Block
-Adhesive Pad
How does the pressure blocking method work?
positioned using a centering fork and squeezed
between two felt pads
How does the pressure method get deblocked?
Deblocked by removing lens from machine
How does the suction blocking method work?
small suction cup constructed with an adapter on
the back surface allows it to fit into the spindle assembly of
a lens edger
How does the suction method get deblocked?
by breaking seal or lifting edge of suction cup
How does the Metal Alloy blocking method work?
Metal alloy with low melting point is heated and molded to
the lens which cools and forms a small metal block
How does the metal alloy method get deblocked? 2
-Glass – Pliers or hot water
-Plastic – Lens torquing or hot water
How does the precast free block blocking method work?
A wax-like block that adheres to the surface of the lens when heated
How does the precast free block method get deblocked?
by twisting block off
How does the adhesive pad blocking method work?
A block made of metal or plastic is fastened to the lens by
means of a double-sided adhesive pad
How does the adhesive pad method get deblocked?
by twisting or tilting off
In a single vision lens what is a lens decentered in reference to? 2
-specified far PD measurement
-OC height measurements
In a progressive lens what is a lens decentered in reference to?2
-specified far PD measurement
-location of the fitting cross
What is a single vision lens spotted according to? 3
-sphere
-cyl
-axis
In a segmented multifocal lens what is a lens decentered in reference to? 2
-Near PD measurement
-Segment center
What 3 ways can we find the laser engravings in a progressive?
-Hold under a fluorescent light with a matte black background
-hold up to a light and look through
-use a hidden circle finding instrument
