Reference point placement, multi focal height and blank size determination,

1. Sahibzada Hakim Anjum Nadeem
Co-Incharge OTTC, Optician, Refractionist, COAVS
CEOAnjum Eye Care & Optical Company
Optometrist,Al-Khair Eye Hospital Lahore
Email: shanjum92@gmail.com
REFERENCE POINT PLACEMENT,
MULTIFOCAL HEIGHT,
BLANK SIZE DETERMINATION

2. Position of Frame
 Frame should be properly positioned for initial
measurements.
 With metal frames, nose pads adjust with correct angle.
 There should be correct bifocal height, and bridge size.
 With plastic frames, bridge size evaluation is simple.

3. Optical Centering for Single Vision
Lenses
 Optical center of lens should line up with the pupil of the
eye.
 Optical center is major point of reference for lenses.
 At the optical center there is no prismatic effect.
 Prismatic effect can be induced away from the OC.

4. Prismatic Effect
 The optical center of lens has no prism.
 A lens prescription can require a certain amount of prism.
 Major Reference Point( MRP ):The point on the lens
where the prism is equal to that called for by the prescription
is called the Major Reference Point

5. Major Reference Point
• When there is no prism in prescription, OC and MRP are
exactly the same point.
• When there is prism in prescription, OC and MRP are on
different locations.
• The MRP is in front of the line of sight of eye.Whereas the
optical center is some where else.
• MRP should be placed according to monocular PDs than the
binocular PDs if the lenses are of different power.

6. Prentice’s rule
• Prism power is the amount light is displaced in centimeter at
a distance 1m away from the lens or prism.
• If lens decentration in centimeter (c) and lens focal length
(f) are known, the prismatic effect may be calculated
= c x F (cm)
• Where
=Prismatic effect
F =Power of the lens
c =Amount of displacement in cm
• The equation is commonly known as Prentice’s rule.

7. Prentice’s rule
 = c x f (cm)
 Prismatic effect due to decentration increases with:
- increase in power of the lens
- increase in distance from the optical centre of the lens.

8. Face Form
• The curve in the frame front is referred as Face Form.
• Frame front more closely conforms the curve of the face.
• If wearer’s PD is equal to frame PD, no face form is required.
• If wearer’s PD is less than frame PD then frame should be
bend from the bridge.
• If wearer’s PD is greater than frame eye size, bridge should
be bent opposite to the normal curve.

10. Amount of Face Form Required

14. Vertical displacement
• Pantoscopic tilt brings the optic axis of the lens in line with
the center of rotation of the eye—improving visual comfort
for the patient.
• With zero pantoscopic tilt, the lens optical center and optical
axis will pass through the center of rotation of the eye only if
the pupil is at the same height.
• However, the pupil is rarely vertically centered within the
lens—it is generally positioned approximately 5mm above
the datum line, or frame midline.

17. Pantoscopic Tilt
• For every 2 degrees of pantoscopic tilt added to the frame front,
the O.C. of the lens should be lowered 1mm.
• Most eyes sit about 5mm above the frame mid- line, so it is
important that the amount of pantoscopic tilt needed, usually 5
to 15 degrees, is applied to the frame prior to measurements
being taken.
• The O.C. height ordered must factor in the degree of tilt applied
to the frame.
• If the frame is not pre-fit, the relative placement of the segment,
or O.C., will be misaligned with the line of sight of the eye in
the finished lens.

18. Pantoscopic Tilt
For each millimeter the eyes are centered above
or below the optical center of the lenses two
degrees of lens tilt are required

21. Amount of Pantoscopic Tilt Required
If Then
1. Eyes at OC
2. Eyes above OC
3. Eyes below OC
No pantoscopic tilt
Pantoscopic tilt required
Retroscopic tilt required
• For each millimeter the eyes are centered above or below the optical
center of the lenses, two degrees of lens tilt are required

23. Steps in Measuring MRP Height
 Frames are adjusted to fit the wearer, giving attention to
nosepads, frame height, pantoscopic tilt and straight frame on
face.
 The fitter and patient should be at same level.
 Patient fixates on bridge of fitter nose.
 Fitter tilts wearer’s chin back until frame front is
perpendicular to the floor.
 Fitter marks the location of center of pupil with short
horizontal lines on the glazed lens.
 Measures MRP height as distance from the lowest portion of
inside of bevel of lower eyewire to the line on the glazed
lens.

25. Vertex Distance
 The vertex depth is the distance from the patient’s cornea to
the back side of the lens.
 The depth of the base curve affects the final vertex
distance, since each increase of 1 diopter in the depth of
the base curve increases the depth of the vertex distance
by approximately 0.6 mm.
 The exact amount of vertex distance increase will depend on
the size of the lens.
 If no vertex measurement is specified when ordering, then an
average value of 14 mm will be used.

26. Vertex Distance
 Vertex distance is important when fitting someone with
long lashes. Be sure to observe the person from the side
with the sample frame. Have the person blink to note
the lash clearance.
 Vertex distances becomes of more concern in high powered
prescriptions because a change in vertex distance induces a change
in both the spherical and cylindrical power of the lenses.

27. Measuring Vertex Distance With the Distometer
 The instrument used to measure vertex distance is the
distometer (Figure 5-13).
 The following technique is used to measure vertex distance:With
the spectacles in place,the subject is instructed to close the eyes.
 The flat side of the “scissors” end of the distometer is placed against
the closed lid.When the end of the distometer is pressed, the other
side of the “scissors” moves out to touch the back surface of the
spectacle lens.
 When the two parts of the distometer touch the lid and lens
simultaneously, vertex distance is read from the instrument.The
instrument takes average lid thickness into consideration so the
reading does not have to be compensated (Figure 5-14).

29. Measuring for Multifocal Segment
heights
 Bifocal height:
Most bifocal heights will be less than half of the frame B,
 Usually fall between 9 and 20 millimeters from the bottom
of the eyewire.

30. Segment height for bifocal
 Place yourself across from the patient and at the exact same
height as the patient.
 Ask the patient to put on his or her old pair of glasses, if
available, and note the position of the segment.
 Does it appear high, low, If it appears high, be sure to ask if
the patient must lower his or her head in order to see over it.

31. Segment height for bifocal
 If it appears low, ask them if they feel they have to tilt their
head back to read.
 Most people wearing lined multifocals have been wearing
them for years and will know exactly what you are asking.
 Adjust the new frame so that it sits correctly on the patient’s
nose and is positioned where he or she likes it.

32. Segment height for bifocal
 With the patient’s head in a relaxed position and looking
straight at you draw a line on the demo-lens that matches the
highest point of their lower eyelid, which is where the
top of the bifocal will be.
 Remove the glasses and measure from the lowest point in the
eyewire to the top of the line.

33.  Record the measurement as the segment height, or “seg”
height.
 Take the binocular PD measurement

35. Segment height for Trifocals
 Most trifocal heights will be near or less than half of the
frame B.
 It usually fall between 14 and 25 millimeters from the
bottom of the eyewire.
 Place yourself across from the patient and at the exact same
height as the patient.
 Ask the patient to put on his or her old pair of glasses and
note the position of the segment.

36. Lower Edge of Pupil Method
 Adjust the frame so that it sits correctly on the patient’s nose
and is positioned properly.
 With the patient’s head in a relaxed position and looking
straight at you,
 Draw a line on the demo-lens that matches the bottom of
their pupil, which is where the top of the trifocal.
 Measure from the bottom of the eyewire to the top of the
line as seg” height.

37. Segment height for Trifocals

39. Segment height for Progressive
 Most progressive fitting heights will be more than half of the
frame B.
 It will usually fall between 15 and 25 millimeters from the
bottom of the eyewire.

40. Segment height for Progressive
 Place yourself across from the patient and at the exact same
height as the patient.
 If the patient has worn progressive lenses before, ask them
specifically if they have had problems in the past with a
segment being too high or too low.
 Adjust the frame so that it sits correctly on the patient’s nose
is in the position

41.  With relaxed position and looking straight at you, on the
demo-lens dot the very center of their pupil.
 This is where the distance area of the progressive lens will be
placed.
 Measure from the lowest point of the eyewire to the dot.
Record the measurement on your lab order form as the
fitting height, or “fit” height.

43. Unequal Seg Heights
 Both eyes should be measured independently for bifocal or trifocal
heights (Figure 5-28). If one eye is higher than the other and the
segments are placed at equal heights, the wearer will have a blur area
considerably larger than normal; one eye sees the segment line first
as the person looks down, then as that eye begins to clear, the other
eye sees the line and begins to clear later.
 Before prescribing unequal segment heights, be sure to check (using
the actual frame to be worn) to be certain that the frame sits straight
on the face.
 A crooked frame obviously will result in unequal segment heights.
When unequal segment heights are used, they should be called to the
wearer’s attention. Otherwise they will be discovered as an “error.”

47. Determining Blank Size
 A lens blank is a lens before it is edged to fit into the frame.
 It may be either finished or semifinished.
 A finished lens blank has the correct powers and needs only
to finished.
 A semifinished lens has one side finished and other needs to
be ground and polished to the correct power.

48. Formula to find MBS
 Minimum blank size (MBS) for finished single vision lenses
can be find out as:
MBS = ED+2 (decentration per lens) +2
Where;
MBS is minimum blank size
ED is effective diameter
Decentration per lens is the difference between the
frame GCD and the wearer’s PD, all divided by 2

49.  Decentration per lens= (A+DBL)-PD/2
 Twice the decentration per lens equals total decentration.
 Total decentration= (A+DBL)-PD
 MBS= ED+total decentration+2mm