Ординатура / Офтальмология / Английские материалы / Essentials of Ophthalmic Lens Finishing, 2nd edition_Brooks_2003

Standardization for measuring frames and lenses is essential. The method used in most of the Englishspeaking world is based upon “boxed” lens size measurements and is called the boxing system. The oncepopular datum system seldom is used.

In the boxing system the eyesize or lens size is determined by the horizontal distance between two vertical tangents that enclose the lens on the left and right (Figure A-1). The point halfway between these vertical tangents and also halfway between horizontal tangents that enclose the lens in a box is the primary reference point. This is known as the boxing center because it is the center of the box enclosing the lens. It is referred to alternately as the geometrical center because after the lens is edged, it is the geometrical center point. (The term geometrical center usually must be qualified because the geometrical center of an uncut lens blank will not be at the geometrical center of the lens once it has been edged.)

The datum system (Figure A-2) defines the lens or eyesize as being the width of the lens along the datum line. The datum line is a horizontal line halfway between the two horizontal tangents that border the top and bottom of the lens. This measure corresponds to the socalled C dimension of the boxing system and is known as the datum length, or simply the eyesize. The datum eyesize is not necessarily the same size as the boxing eyesize. The central reference point in the datum system is halfway across the lens as measured along the datum line and is called the datum center.

The distance between lenses (DBL) is measured differently between the two standards as well. The datum system measures this along the datum line. As can be seen from Figure A-2, this distance between lenses may not necessarily be the smallest distance between lenses.

Therefore in the datum system the smallest measurable distance between the two lenses (regardless of the level at which this minimum occurs) is called the minimum between lenses (MBL). The datum MBL is the same as the boxing system’s DBL.

When patterns are being manufactured, the center of rotation is positioned to correspond to the central reference point of the lens shape. Because the boxing center is not located at the same point for a given shape as the datum center, a pattern drilled for the boxing system will not work if the datum system is used to calculate lens decentration, and vice versa.

Decentration calculations are made in exactly the same manner, regardless of the system being used. However, the results are not interchangeable.

For example, a frame has the following dimensions when measuring using the boxing system:

Boxing eyesize (A) = 52

Boxing DBL = 15

When the same frame is measured in the datum system, the dimensions are as follows:

Datum length = 49

Datum DBL = 19

Wearer’s PD = 63 mm

Decentration when using the boxing system is as follows:

(Eyesize + DBL) – PD = (52 +15) – 63 2 2

Decentration = 2 mm per lens

Decentration when using the datum system is as follows:

(Eyesize + DBL) – PD = (49 + 19) – 63 2 2

Decentration = 2.5 mm per lens

ED

(Same as DBC)

Distance between center (DBC) or

Geometrical center distance (GCD) or

Frame PD

0º

B

A P P E N D I X A S TA N D A R D S O F L E N S A N D F R A M E M E A S U R E M E N T

This underscores the importance of correct measurement of the eye and bridge sizes. Measuring the frame using the datum system but using a pattern made for the boxing system results in an incorrect decentration. The finished distance between the lens optical centers does not correspond to the wearer’s PD. In the example given, the distance between the two lens optical centers is 1 mm larger than ordered.

Differences and similarities in boxing and datum systems are summarized in Figure A-3. Corresponding boxing and datum system terms are listed in Table A-1. Similarities and differences between boxing and datum multifocal placement terminology are summarized in Figures A-4 and A-5.

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TABLE A-1

Comparison of Standard Terms

Datum

center

minimum between lenses; PD, inter-

Before the standards are outlined in this section, one point should be clarified. Unlike ANSI standards for safety eyewear, standards for dress prescription eyewear are not requirements, but recommendations. “The standard remains a recommendation. Therefore, it is the intent of the Z80 Committee that this standard not be used as a regulatory instrument.”*

The information summarized here in Tables B-1 through B-10 is not meant to be all-inclusive. For complete information the original document should be consulted: Z80.1-1999 American National Standard for Ophthalmics—Prescription Ophthalmic Lenses—Recommen- dations. This standard may be obtained from the following address:

*Z80.1-1999 American National Standard for ophthalmics–prescription ophthalmic lenses–recommendations, Merrifield, Va, 2000, Optical Laboratories Association, p ii.

Optical Laboratories Association

P.O. Box 2000

Merrifield, VA 22116-2000

Meridian of Highest Absolute

Power

To be able to understand the “meridian of highest absolute power” as referenced in Table B-1, the following must be considered:

•The power of one major meridian is the sphere power.

•The power of the other major meridian equals the sphere power plus the cylinder power.

•Of these two meridians, the meridian having the highest numerical value (plus or minus) is the meridian of highest absolute power.