Centration Blocking Flashcards

1
Q

Where is the central point of rotation?

A

corresponds to geometric center of lens shape

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2
Q

What is the central point of rotation?

A

The point in which the lens rotates to be ground to a specific shape to fit the frame

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3
Q

What happens if a lens is not centered correctly? 4

A

-it ensure patient receives the full corrective power of the lens.
-visual distortion, discomfort
-headaches
-eye strain.

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4
Q

What is lens spotting?

A

The process of using a lensometer to apply locational marks
on a lens to be used for the decentration process prior to
edging

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5
Q

What is decentration?

A

The process of orienting a lens a specific amount from the
rotational center in preparation for the edging process

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6
Q

What are the 2 things a lensmeter does?

A

reads the power
spots the OC of a lens

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7
Q

What is the formula for decentration per lens?

A

(DBC – Wearers PD)/2
DBC = A + DBL

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8
Q

What formula is used to calculate vertical decentration?

A

MRP (OC) Height – (B/2)

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9
Q

How should MRP be calculated? 2

A

-from the bottom of the lens
-how far above or below (raised or dropped) it is from the horizontal midline in mm

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10
Q

What are the 5 methods of blocking a lens?

A

-Pressure
-Suction
-Metal Alloy
-Precast Free Block
-Adhesive Pad

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11
Q

How does the pressure blocking method work?

A

positioned using a centering fork and squeezed
between two felt pads

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12
Q

How does the pressure method get deblocked?

A

Deblocked by removing lens from machine

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13
Q

How does the suction blocking method work?

A

small suction cup constructed with an adapter on
the back surface allows it to fit into the spindle assembly of
a lens edger

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14
Q

How does the suction method get deblocked?

A

by breaking seal or lifting edge of suction cup

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15
Q

How does the Metal Alloy blocking method work?

A

Metal alloy with low melting point is heated and molded to
the lens which cools and forms a small metal block

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16
Q

How does the metal alloy method get deblocked? 2

A

-Glass – Pliers or hot water
-Plastic – Lens torquing or hot water

17
Q

How does the precast free block blocking method work?

A

A wax-like block that adheres to the surface of the lens when heated

18
Q

How does the precast free block method get deblocked?

A

by twisting block off

19
Q

How does the adhesive pad blocking method work?

A

A block made of metal or plastic is fastened to the lens by
means of a double-sided adhesive pad

20
Q

How does the adhesive pad method get deblocked?

A

by twisting or tilting off

21
Q

In a single vision lens what is a lens decentered in reference to? 2

A

-specified far PD measurement
-OC height measurements

22
Q

In a progressive lens what is a lens decentered in reference to?2

A

-specified far PD measurement
-location of the fitting cross

23
Q

What is a single vision lens spotted according to? 3

A

-sphere
-cyl
-axis

24
Q

In a segmented multifocal lens what is a lens decentered in reference to? 2

A

-Near PD measurement
-Segment center

25
Q

What 3 ways can we find the laser engravings in a progressive?

A

-Hold under a fluorescent light with a matte black background
-hold up to a light and look through
-use a hidden circle finding instrument