Module 5, part 5
Correcting Vertical Imbalance with Unlike Segment Styles. 

Unlike Segment Styles

Segment Induced Prism

round segment OD and reading position diagram
~   ~  ~ ~~~~ {|} ~~~~ ~  ~   ~
  1. Subtract the drop (1. above) from the reading depth (2. above).  This tells you how far from the top of the segment the reading level is.
  2. Take the difference between a. and the segment OC (3. above).  Ignore which is larger.  Just look at the difference. 
  3. Use b. and the power of the ADD (4. above) to determine the prism induced by the segment power. 
  4. If the segment OC is below the reading level the prism is BD.  If the segment OC is above the reading level the prism is BU.
In the round 22 segment pictured to the right above,
  1. the drop was 3 mm. 
  2. The reading depth was 8 mm. 
  3. The segment OC is 11 mm below the top of the segment. 
So, the reading level in the segment is
  1. (reading level) - (drop) = 8 - 3 = 5.
  2. (seg OC) - a. = 11 - 5 = 6.
  3. Prentice rule tells us that the prism is then (6)(ADD power)/10.
  4. Seg OC is below reading level so the base is BD.
So in this instance the prism will be found using a distance of 6 mm from the segment OC and the base direction will be BD.  If the ADD is +1.50 the segment induced prism at the reading level is (6)(1.50)/10 = 0.9^ BD.  If the ADD is +2.00 the prism is 1.2^ BD.  If the ADD power is +2.50 the seg-induced prism is 1.5^ BD.  Yes?

Here we have a Franklin (Executive) style bifocal.  The segment drop is 4 mm.  The reading level is 8 mm below distance OC.  The ADD is +2.50.
  1. Reading drop - segment drop is 8 - 4 = 4 mm.
  2. Segment OC is on the line, so seg OC is 0 mm form the top of the segment.  4 - 0 = 4 mm.  The reading level is 4 mm from the segment OC.
  3. The ADD is +2.50 so the prism is (2.50)(4)/10 = 1.0^
  4. The segment OC is above the reading level so the prism is BU.
Yes?

So what?  We are going to use different segment styles to decrease the induced prism in a presbyopic anisometropic Rx. 
Vertical Imbalance continued . . .
EXAMPLE 1:  Given the Rx: OD  -1.50 -1.00 x 090      ADD +2.00 OU
OS   -3.00 -1.00 x 045
reading level is 8 mm below OC. 
There is 2.00 D difference between the lenses on the 90th meridian, so at 8 mm below the distance OC the wearer will experience 1.6^ of vertical imbalance.  The wearer comes back to you, the dispenser, and says that this new glasses Rx (the Dr. changed the left lens, it used to be -2.00 -1.00 x 045)  is working well in the distance, and the reading is OK if she covers one eye (does not matter which, she likes the reading vision in each eye) but she is very uncomfortable trying to read with the glasses with both eyes open. 
Let's say that for some reason you decide to try unlike segments.  The Rx has a little BD prism in the reading for the right lens (1.2^ BD) and more in the left lens (2.8^ BD).  So what you want is segments where the right segment induces more BD than the left does, or the left induces more BU than the right does. 

The textbook says that the formula for the difference between the segment OCs is (induced prism)(10)/(ADD power).  In this example the induced prism is 1.6^, the ADD power is +2.00, so the difference between segment OC's  would ideally be (1.6)(10)/(2) = 8 mm.  We want segment styles that differ by about 8 mm.  ] Ultex and round 22 differ by 8 mm, but Ultex is difficult to get, and the major advantage to doing this technique is using segments that are available and inexpensive.  FT and round 22 will give us 6 mm difference, which will not completely correct the problem but will decrease it to the point where the wearer can use the glasses with both of her eyes open.

Now look at the "rule" on page 116 under table 4-1.  Of the two segment styles, the segment with the lowest OC is the round 22.  The lens with the most plus or least minus power is the OD.  So, the OC gets the round 22, which induces a lot of BD prism, and the OS gets the FT, which at that reading depth will induce almost no prism.

Let's see what we actually did.  Suppose the below for the segment is 4 mm.  This is a reasonable assumption if the reading level is 8 mm, and would be given to you in a problem of this type, if needed. 

On the right lens:
  1. Reading drop - segment drop is 8 - 4 = 4 mm.
  2. Seg OC - a. is 11 - 4 = 7 mm.
  3. Add is +2.00, so at 7 mm from the segment OC the segment is inducing 1.4^ prism.
  4. Seg OC is below reading level, so the base is BD.

On the left lens:
  1. Reading drop - segment drop is 8 - 4 = 4 mm.
  2. Seg OC - a. is 5 - 4 = 1 mm.  (If a FT25.  If a FT28 or FT35 it might be 4 - 4 = 0)
  3. Add is +2.00, so at 1 mm from the segment OC the segment is inducing 0.2^ prism.  (or 0, for the FT28 or 35)
  4. Seg OC is below reading level, so the base is BD.
The result:  The OD had 1.2^ BD, the round 22 added 1.4^ BU, for a total of 2.6^ BD.  The OS had 2.8^ BD, the FT25 added 0.2^ BD, for a total of 3.0^ BD.  The imbalance is now 0.4^, and the wearer will probably have no problem with this. 

EXAMPLE 2
OD +5.75 +2.00 x 110   ADD +3.50 OU
OS +8.50 +3.25 x 080
Current seg style FT28, set down 4 mm
Reading level 7 mm below distance OC.

Power on the 90th is:  OD +5.98, OS +8.60.  Vertical imbalance is 1.8^.
The formula on page 115 gives us a difference between seg OC's of 5 mm.
The table on page 116 tells us we can use a round 22 with a FT25/28/35, or an executive with a FT25/28/35.  I don't like the round/FT combination.  I don't like the exec/anything combination, particularly with a +3.50 add.  But what the table seems to be missing is the FT40, which has the same optics as the exec.  So, which combination I use will depend on what I can get in a high plus with a high add. 

Let's assume that we can get a FT28 and a FT40.  The segment with the lower OC is the FT28.  Based on the rule on page 116 the lens with the most + power on the 90th is the OS.  So the FT28 goes on the OS and the FT40 goes on the OD.
Why might we do this? 
  1. The bifocal styles are standard styles, so at most it will take one day to order the lenses in.
  2. The bifocal styles are standard styles, so there is no unusual cost involved.
  3. . . . can't think of any other excuse for it.
Why might we not do this?
  1. It is ugly.
If you need a pair in a hurry while you are waiting for the slaboff to be ground you might do the dissimilar seg for temporary glasses.  If you want to find out if the problem the wearer is having is due to the vertical imbalance you might do the dissimilar seg, and then order the slaboff if the problem is solved.  If there is a problem with the added cost of the slaboff, which tends to add around $100-150 to the cost of the glasses, you might do this as a permanent solution.

Regardless of the reason to do it,  I would point out to the wearer what it will look like before ordering the lens to be made, so that there is no surprise when it comes time to dispense the glasses.  There are ways of making it not as ugly:
I have done this for patients on public assistance, when I was in a state where public assistance barely covered the cost of the least expensive glasses and would not pay for anything "extra".  Careful choice of segments and frame shape, plus a light tint, makes the glasses perfectly acceptable without necessitating costing the practice an unusual amount of money while providing a patient (who is usually told to just get used to it) with the best vision I can provide.

Your turn.  In each case tell me what combination of seg styles you would order, and which lens would get which style.  You do not have to do the justification to show that the solution worked.  That is in the exercise above to convince you that the solution works.

1.   OD  -15.00                    ADD +1.75 OU
      OS  -17.50 -1.50 x 180
    Reading position 7 mm below distance OC.  Seg drop 3 mm.

2.  OD pl -2.00 x 055     ADD +2.50
      OS +2.00 sph           ADD +2.50
     Reading position 10 mm below distance OC.  Seg drop 6 mm.


The answers are here. Call or e-mail your instructor if you have questions.