Segment architecture

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SEGMENT ARCHITECTURE

BY:SWATI PANARA FROM:BHARTIMAIYA COLLEGE

OF OPTOMETRY2nd YEAR 3rd SEMESTER

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• SEGMENT IS A SOMETHING IN WHICH OF THE PART IN TO WHICH THE SPECTACLE ARE DIVIDED.

• ARCHITECTURE IS A DESIGN AND CONSTRUCTION OF THE SPECTACLE.

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TYPES

• (1)CENTRATION POINT• (2)SEGMENT SHAPE• (3)SEGMENT TOP• (4)SEGMENT HEIGHT• (5)SEGMENT WIDTH• (6)SEGMENT INSET• (7)SEGMENT DIAMETER• (8)SEGMENT DROP• (9)SEGMENT DEPTH

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• (10)MAJOR REFERENCE POINT• (11)SEGMENT SIZE

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•WHY DO WE USE SEGMENT?

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• THE SEGMENT USED FOR ORDINARY READING CAN BE SMALL,FOR WHEN READING MOST PEOPLE DO NOT MOVE THEIR EYES THROUGH AND ARC MORE THAN 12 DEGREES FROM SIDE TO SIDE.

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Terminology

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CENTRATION POINT

• THE NEAR ADDITION IS USUALLY PLACED ON INFERONASAL PART OF THE SPECTACLE BECAUSE THE EYES ARE LOWERED AND CONVERGED WHILE READING .

• SO NEAR ADDITION MUST HAVE DIFFERENT CENTRATION POINT THAN THE DISTANCE CORRECTION.

• THE NEAR CENTRATION POINT LIES 2mm NASAL AND 8mm BELOW THE DISTANCE CENTRATION POINT.

• THE CENTRATION POINT IS CALLED CP.

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OPTICAL CENTRE

• IN DISTANCE VISION THE VISUAL AXIS OF THE EYE PASSES THROUGH THE SPECTACLE LENS AT A POINT REFERRED TO AS THE DISTANCE VISUAL POINT,WHICH SHOULD COINSIDE WITH THE OPTICAL CENTRE.

• ON READING,EACH EYE MOVES SO THAT ITS VISUAL AXIS NOW PASSES THROUGH THE LENS AT A POINT APPROXIMATELY 8mm BELOW AND 2mm NASAL TO THE DISTANCE VISUAL POINT AT A KNOWN AS THE NEAR VISUAL POINT.

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SEGMENT TOP

• IT IS THE HIGHEST POINT OF THE SEGMENT IN CASES OF CURVED TOPS.

• IN CASE OF FLAT TOP SEGMENT,IT IS THE MID POINT OF THE UPPER STRAIGHT LINE.

• SEGMENT TOP SHOULD LIE AT THE LEVEL OF LOWER EYELID.

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• THE TOP OF THE SEGMENT IS USUALLY PLACED 1.5-2.0mm BELOW THE DISTANCE VISUAL POINT OF THE MAIN LENS.

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SEGMENT DIAMETER

• THE DIAMETER OF THE OF WHICH THE BOUNDRY OF THE FINISHING SEGMENT FORMS A PART.

• EXECUTIVE LENS HAVE NOT A SEGMENT DIAMETER.

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MAJOR REFERENCE POINT

• THE MAJOR REFERENCE POINT IS USUALLY ALSO THE OPTICAL CENTRE OF THE DISTANCE LENS WHEN NO PRISM HAS BEEN PRESCRIBED.

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SEGMENT DEPTH

• THE LONGEST VERTICAL MEASUREMENT OF THE SEGMENT IS KNOWN AS THE SEGMENT DEPTH.

• E.X.B-SHAPED ARE AVAILABLE IN TWO DIFFERENT SEG DEPTH:THE ‘B’SEGMENT IS 9mm DEEP AND THE ‘R’SEGMENT IS 14mm DEEP.

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SEGMENT WIDTH

• DIFFERENT SEGMENT WIDTHS ARE USUALLY AVAILABLE FOR STRAIGHT TOP SHAPED SEGMENTS IN 22,25,28 AND 35 mm WIDTHS.

• SOME MANUFACTURER AISO PROVIDE THIS OPTION IN ROUND SHAPED SEGMENTS.

• E-STYLE BIFOCAL ENCOMPASSES THE FULL WIDTH OF THE LENS,SO IT IS OBVIOUS NOT TO SPECIFY SEGMENT WIDTH WHEN THIS LENS IS PRESCRIBED.

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• GIVEN A CHOICE WIDER SEGMENT WIDTHS ARE PREFERRED.

• SEGMENT WIDTH IS MEASURE IN MILLIMETER.

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Executive bifocal

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Straight top bifocal

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INSET

• THE HORIZONTAL DISTANCE FROM THE GEOMETRIC CENTER OF THE LENS TO THE MAJOR REFERENCE POINT IS USUALLY KNOWN AS THE INSET,BECAUSE THE MAJOR REFERENCE POINT IS ALWAYS DECENTERED INWARD.

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SEGMENT INSET

• SEGMENT INSET IS SPECIFIED AS THE DIFFERENCE BETWEEN THE SUBJECTS DISTANCE PD AND NEAR PD.

• IN THE NORMAL RANGE OF PDs,THE NEAR PD FOR A READING DISTANCE OF 40cm IS 4mm LESS THAN THE DISTANCE PD.

• SEGMENT INSET,THEREFORE,IS USUALLY SPECIFIED AS 2mm FOR EACH LENS.

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• THE HORIZONTAL DISTANCE BETWEEN THE MAJOR REFERENCE POINT AND THE SEGMENT CENTRE IS KNOWN AS THE SEGMENT INSET.

• THE TOTAL LATERL DISTANCE FROM THE GEOMETRIC CENTRE TO THE CENTRE OF THE SEG IS CALLED TOTAL INSET.

• THE SUM OF THE INSET AND THE SEG INSET EQUALS THE TOTAL INSET.

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• THERE ARE TWO REASONS FOR INSETTING BIFOCAL SEGMENT-TO ENSURE THAT THE SUBJECT’S LINE OF SIGHT WILL GO THROUGH THE SEGMENT AT ITS OPTICAL CENTRE AND TO ENSURE THAT THE READING FIELDS FOR THE TWO SEGMENTS WILL COINSIDE WITH ONE ANOTHER.

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FORMULA

• SEGMENT INSET=DISTANCE IPD-NEAR IPD/2• TOTAL SEGMENT INSET=GEOMETRIC CENTRAL DISTANCE-NEAR CENTRAL DISTANCE/2

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SEGMENT DROP

• THE VERTICAL DISTANCE FROM THE SEGMENT TOP TO THE DISTANCE OPTICAL CENTER.

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Terminology

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SEGMENT HEIGHT

• IT IS THE DISTANCE OF SEGMENT TOP FROM THE LOWERMOST POINT OF THE SPECTACLE LENS.

• IN THE VERTICAL MERIDIAN BIFOCAL LENSES PRESCRIBED FOR GENERAL PURPOSE USE ARE USUALLY MOUNTED BEFORE THE EYES SO THAT THE SEGMENT TOP IS TANGENTIAL TO THE LOWER EDGE OF THE IRIS.

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• REQUIRED SEGMENT HEIGHT DEPENDS ON THE PATIENT’S VOCATION.

• FOR A PATIENT LIKE ACCOUNTANT AND OFFICE WORKER WHO NEED LARGE READING FIELD, SEGMENT HEIGHT COULD BE REDUCED.

• SEG HEIGHT IS MEASURE IN MILLIMETER.

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SEGMENT SHAPE

• PREVIOUSLY,CIRCULAR SHAPED ADDITIONS WERE EMPLOYED.

• THEY CREATED PROBLEMS BECAUSE THEIR WIDEST PART LAY FAR BELOW READING LEVEL,SO THE READING FIELD WAS RESTRICTED AND IMAGE JUMP OCCURRED.

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IMAGE JUMP

• SUDDEN UPWARD DISPLACEMENT OF IMAGE WHEN AN OBJECT IS FIRST VIEW THROUGH, DISTANCE AND THEN THROUGH TOP OF READING ADDITION.

• IT IS ALSO KNOWN AS IMAGE DISPLACEMENT.

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TYPES OF SEGMENT SHAPE

• (1)D-SHAPE BIFOCAL• (2)C-SHAPE BIFOCAL• (3)EXECUTIVE BIFOCAL• (4)B-SHAPE BIFOCAL• (5)P-SHAPE BIFOCAL

D SHAPE BIFOCAL

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Executive bifocal

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• EXECUTIVE BIFOCALS ARE OF THE MOST MODERN DESIGN.

• THEY ARE MADE IN SUCH A WAY THAT LOWER HALF OF THE SPECTACLE HAS NEAR ADDITION AND UPPER HALF HAS THE DISTANCE CORRECTION.

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• THEIR DIVIDING LINE CENTRE OF CURVATURE FOR DISTANCE AND NEAR, ALL LIE AT THE SAME LEVEL,THEREBY REDUCING THE IMAJE JUMP AND CHROMATIC ABERRATION AND OFFERING WIDEST READING FIELD.

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C-SHAPED BIFOCAL

• IN C-SHAPED BIFOCAL ,THE TOP OF THE SEGMENT WAS MADE SEMICIRCULAR WHICH WAS COSMETICALLY MORE ATTRACTIVE AND REDUCED REFLECTIONS FROM DIVIDING LINE.

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C shaped bifocal

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P shaped bifocal

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B shaped bifocal

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SEGMENT SIZE

• THE FLAT TOP HAS A SEGMENT SIZE OF 22,25,28mm THAT TIME ITS OPTICAL CENTRE LOCATION IS 5.0mm BELOW SEGMENT LINE.

• THE FLAT TOP HAS A SIZE OF 35mm THAT TIME ITS OPTICAL CENTRE LOCATION IS 4.5mm BELOW SEG LINE.

• THE SIZE OF THE SEGMENT INFLUENCE THE IMAJE JUMP BECAUSE THE LOCATION OF THE OPTICAL CENTRE VARIES WITH RESPECT TO THE TOP OF THE SEGMENT.

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• THE SIZE OF THE BIFOCAL SEGMENT CAN GREATLY INFLUENCE ITS PRISMATIC EFFECT.

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REFFERENCE

• (1)A.K.KHURANA• (2)DUKE-ELDER• (3)OPHTHALMIC LENSES• (4)L.P.AGRAWAL• (5)CLINICAL REFRACTION• (6)SIR’S NOTE• (7)INTERNET SITE

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THANK YOU