DISPENSING OPTICS - PART II

 SYSTEM OF OPTICAL DISPENSING

System of reference points to facilitate accurate lens placements into frame.

2 systems are very popular 

1. Datum system of Optical Dispensing

2. Boxing system of Optical Dispensing

1. DATUM SYSTEM OF OPTICAL DISPENSING

British Standard for spectacle frame measurements

It provides system of reference points for frames and lenses to facilitate accurate placement of optical center and bifocal segment height

Two horizontal lines tangent to top and bottom edges of lenses are drawn that runs parallel to each other.With reference to 2 lines various lens and frame measuring references can be explained.

1. DATUM LINE

Datum line lies midway between the horizontal tangents to lens shape at its top and bottom edges. 

Datum line of spectacle frame is continuous with datum line of each lens.

2. DATUM LENGTH

Datum length is horizontal measure of lens shape on its datum line.

3. DATUM CENTER

The midpoint lying on datum line denotes datum center.

4. DATUM CENTER DISTANCE

DCD is distance between datum centers of right and left lenses when the lenses fitted into frame.

2. BOXING SYSTEM OF OPTICAL DISPENSING

American Standard for spectacle frame measurements.

Two vertical lines tangent to either side of lens and run parallel to each other are added, forming a box around the lens as shown in figure. 

1.GEOMETRICAL CENTER

Halfway between two vertical lines of the box on datum line.

It corresponds to datum center of British standard.

GCD refers distance between geometric centers of right and left lens boxes.

It can be calculated by adding eyewire size to DBL.

Presumed to frame pupillary distance PD on assumption that pupils would correspond to this point.

If a frame marked as 52 ❒ 18 GCD of eyewire would be 70mm.

2.SIZE

The size of the lens is given by length and depth of box containing the lens.

Horizontal measure of the box is denoted by "A measurement" and Vertical measure of the box is denoted by "B measurement".

C measurement refers to width of the lens itself along the datum line.

Frame notation is 54 ❒  18 which means 

A measurement of the frame is 54mm

Distance Between lenses is 18mm

Geometrical center distance between two lenses is 72mm.

3.EFFECTIVE DIAMETER

Twice the longest radius from the geometric center of a lens to apex of the lens edge.

It can be calculated by measuring the distance from the geometrical center of lens to apex of lens bevel farthest from it and multiply it by 2.

4.FRAME DIFFERENCE

The difference between the horizontal and vertical measurement is called frame difference.

It is measured in millimeters.

If the frame difference is zero, it implies the frame eyewire shape is round.

If A measurement is larger than the B measurement, it implies the frame difference is larger and the shape of eyewire is rectangle.

5.DISTANCE BETWEEN LENSES

If a frame is marked with /18\, it implies that bridge width of frame is 18mm.

DBL is measure of distance between the nasal vertical tangents to lens at peak of the bevel.

This is usually signs with bridge size manufacturer follows boxing system of measurement.

6.SEGMENT HEIGHT

Datum line or lower line of box enclosing lens shape is used to specify multifocal segment height.

The reference is most commonly given as either distance below or above datum line or distance from the lower line of box enclosing the lens shape.

For eg, In a frame A measurement is 51 mm, B measurement is 47 mm and the segment top is placed 4 mm below datum line, the segment height would be 19.5 mm from the lower line enclosing the lens.

POINTS TO REMEMBER

GCD = eye size + bridge size.

When the frame difference is zero, the shape of the lens would be round.

The larger the frame difference the shape of the lens would be narrow rectangular.

Frame difference is difference between the horizontal and vertical measurements.