Slab Off Challenge

[rhondaboman]

Hey guys, any of ya got yer ears on?

Pete and I are having a problem. Our work ticket says -3.25 SLAB -2.00 HIGH with 4.25/100 in bold. I've been reading this incorr apparently because we've only achieved 2D of BU prism at near in our stt7x28 lens.

The rx is OD: -1.50 -1.00 X 100
OS: +1.50 -2.25 X 090 with 250 add

The resultant prism at near in the OS is 3D BU, so I expected the slab to provide us with 3D BU in the OD. It's a diopter weak. WHY? This is the 2nd attempt to achieve balance @ near. HELP!

Thanks, Rhonda Boman and Pete Doble- Hoya Vision Care Seattle

[Robert Martellaro]

Hi,

The VI or vertical imbalance looks like 3 prism diopters. The reading depth with a 7x28 trifocal will be about 14mm so I would want a 4^ BU slab for the RT eye.

Hope this helps

[rhondaboman]

The problem is that I can't figure out how to specify a prism amount to fix this, if indeed it does need fixing. DVI is the coolest thing since sliced bread, far be it for me to second guess it's calculations, I'm more inclined to think we've processed this incrr. somehow.

By the by, even if I do figure out how to increase the slab D, I'll need to add thickness to the lens as that measured correctly. If I don't, I'm going to thin out the bottom too much.

[Darryl Meister]

What is being sent to the generator in terms of prism to surface? This should tell you how much slab-off prism you're grinding into it. The generator will need to cut one curve to move the distance optical center from the geometric center to the correct position relative to the seg, and then a second curve (which may be surfaced prior to the first curve) with the additional base up prism added to this decentration prism. In this case, you should see mostly horizontal prism to move the optical centers since both lenses have nice looking cylinder axes and trifocal lens blanks generally place the trifocal relatively close to the vertical center of the blank.

I'm not familiar with DVI tickets, but that statement could mean something as simple as "3.25 PD Base Up Slab-off with the line 2 mm down, produced using 4.25 PD at Base 100." However, the 4.25 at Base 100 figure doesn't sound exactly right (at least not for our trifocal blanks, and not with a slab line 2 mm down). That's effectively 4.18 PD of base up prism, which would leave a residual value of almost a 1.0 PD once you subtract the slab-off prism -- and that would drop the distance optical center 6 mm (excessive for a trifocal lens blank). But it's a guess.

Also, are you using a focimeter (i.e., lensomter or vertometer or lensmeter) to measure your resultant prism? Have you tried using a lens clock to double-check? Are you certain the prism is wrong? Keep in mind that a 3.25 PD slab-off will only correct about half of the vertical imbalance that you would see in the bifocal segment with that prescription... (Even if the lens stop of the focimeter was right up at the bi/tri line, the center of the stop would still be roughly 15 mm below the distance optical center, which results in about 2.25 BD in the right and 2.25 BU in the left -- a difference of 1.25 PD once you subtract the 3.25 BU slab-off prism.) Consequently, you may in fact be getting the correct results.

Perhaps Robert can also offer some guidance; he's intimately familiar with job tickets.

Best regards,
Darryl

[shanbaum]

Perhaps Robert can also offer some guidance; he's intimately familiar with job tickets.

Yeah - my job tickets. I haven't really studied the DVI tickets; certainly not for slab-offs.

I can't really say anything based on the information provided (mainly because it's not clear to me where the distance center is being positioned), except that your guesses look pretty reasonable to me, Darryl.

My suggestion would be to call DVI.

[rhondaboman]

Also, are you using a focimeter (i.e., lensomter or vertometer or lensmeter) to measure your resultant prism? Have you tried using a lens clock to double-check? Are you certain the prism is wrong? Keep in mind that a 3.25 PD slab-off will only correct about half of the vertical imbalance that you would see in the bifocal segment with that prescription... (Even if the lens stop of the focimeter was right up at the bi/tri line, the center of the stop would still be roughly 15 mm below the distance optical center, which results in about 2.25 BD in the right and 2.25 BU in the left -- a difference of 1.25 PD once you subtract the 3.25 BU slab-off prism.) Consequently, you may in fact be getting the correct results.

Daryl, I'm not that great with paper calcs but how did you get the 2.25 BD OD, 2.25 BU OS? I ended up with 3.66 PD between the two. And if so, why is it requesting 4.25 PD slab. It just dawned on me that -2. HIGH refers to the distance between the dist. OC and the slab line. And if the 1PD in the add is correct, how much is within tolerance.

Frustrating that I'm ignorant on this, it's just never come up before. And worse, I have to know WHY WHY WHY , this is the way it works. It's the only way I'll remember.

Oh yeah, I'm using and old B & L for verification. Thanks for your patience, Rhonda

[Darryl Meister]

Daryl, I'm not that great with paper calcs but how did you get the 2.25 BD OD, 2.25 BU OS?

Using the distance of 15 mm I mentioned? Since the axis of each cylinder is close to 90 deg, I'll just ignore the cylinder power for now. If we assume a distance of 15 mm to get far enough into the bifocal segment for a power measurement, the prism at this point is,

Pod = -1.50 * 15 / 10 = -2.25 prism diopters (Base Down)
Pos = +1.50 * 15 / 10 = +2.25 prism diopters (Base Up)

The total difference in this case is +2.25 - (-2.25) = +4.50 prism diopters.

I have an online form for accurately calculating vertical prism imbalance on my OptiCampus site at:

http://www.opticampus.com/tools/vertical.php

It's hard to say how your lab software calculates slab-off with just this information. The software could just assume a reading level distance of 10 mm or could choose some point in the near zone that varies with the type of multifocal. The software could calculate it exactly or could use an approximation. In any case, you seldom need to provide the full prism correction for slab-off. I generally recommend undershooting it by 0.50 to 1.00 prism diopters, and letting the fusional reserves of the eyes do a little of the work, unless of course the doctor has specifically requested a certain amount of correction. Consequently, 3 out of 4 prism diopters might work well enough for you.

Best regards,
Darryl

[rhondaboman]

Mister Meister, (:) I've always wanted to say that)


Caught your site last week, verrry nice. An associate EM'd it to me, as I'd been off the board for a while. Wasn't smart enough to look there for my answers ahead of time.

I did contact John Thompson @ DVI. Digital Vision has plenty of answers within it's program, so long as you know where to look. There should not be any residual prism @ near once the slab has been applied.

Decided I may be comparing apples and oranges, so I sent the thing to the account to see if we'd improved any from the first time. I don't like remaking jobs when I don't have the original to compare it to.

Thanks to everyone for all your help. You may be hearing from me often, now. My surface manager moved to Arizona, Friday was his last day. Gonna miss you Pete. Keep in touch.

[Darryl Meister]

Caught your site last week, verrry nice.

Thanks, Rhonda. I plan to finish a bit more of it when I get around to taking a vacation this summer.

There should not be any residual prism @ near once the slab has been applied

Unfortunately, that's only strictly true at one point (i.e., the reading level distance) in the near zone. Above or below that point, the slab-off prism will produce either too much or not enough prism compensation. However, as long as you know where the point roughly is, your measurement should be close enough.

Best regards,
Darryl