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1. BY:SWATI PANARA
FROM:BHARTIMAIYA
COLLEGE OF OPTOMETRY
2nd YEAR 3rd SEMESTER
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2. • 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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3. 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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4. • (10)MAJOR REFERENCE POINT
• (11)SEGMENT SIZE
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5. •WHY DO WE USE
SEGMENT?
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6. • 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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7. 7

8. Terminology
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9. 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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10. 10

11. 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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13. 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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14. • 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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15. 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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16. 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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17. 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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18. 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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19. • GIVEN A CHOICE WIDER SEGMENT WIDTHS
ARE PREFERRED.
• SEGMENT WIDTH IS MEASURE IN
MILLIMETER.
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20. Executive bifocal
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21. Straight top bifocal
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22. 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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24. 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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26. • 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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27. • 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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29. FORMULA
• SEGMENT INSET=DISTANCE IPD-NEAR IPD/2
• TOTAL SEGMENT INSET=
GEOMETRIC CENTRAL DISTANCE-
NEAR CENTRAL DISTANCE/2
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30. SEGMENT DROP
• THE VERTICAL DISTANCE FROM THE SEGMENT
TOP TO THE DISTANCE OPTICAL CENTER.
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31. Terminology
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32. 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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34. • 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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35. 35

36. 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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37. 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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38. TYPES OF SEGMENT SHAPE
• (1)D-SHAPE BIFOCAL
• (2)C-SHAPE BIFOCAL
• (3)EXECUTIVE BIFOCAL
• (4)B-SHAPE BIFOCAL
• (5)P-SHAPE BIFOCAL
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39. D SHAPE BIFOCAL
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40. Executive bifocal
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41. • 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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42. • 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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43. 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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47. 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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48. • THE SIZE OF THE BIFOCAL SEGMENT CAN
GREATLY INFLUENCE ITS PRISMATIC EFFECT.
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49. 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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