optical centers to high

**midohioeye** · 04-01-2008, 10:11 AM

typically oc are 4 mm above the seg, I will occasionally receive jobs from the lab where the oc are 10mm or so above the seg, and the powers of the lenses are close enough or equal to each other that there is no verticle imbalance. its kinda of like prism thinning for the pals except in lined lenses and in the oppsit direction. is this rejectable?

any thoughts anyone?

**drk** · 04-01-2008, 10:47 AM

Nah.

Lab question: how does this occur?

In my younger days, I used to specify level MRP/vertical OCs on my segmented MF jobs.

This prompted an exasperated call from the lab owner: "Please don't", he implored.

I guess if you fit a segmented MF the typical distance from pupil center (about 8 mm), you're getting about a 4 mm drop in the OC (quite acceptable), and another 4 mm to the seg line.

I guess the real issue would occur if you fit segs especially low, such as a sunglass or golf seg, and the powers were high or signficantly aniso.

So, "labsters" how do you surface a MF job with a specified level MRP other than default? It must be a hassle to some degree, or my lab guy wouldn't have called. Is it indeed surfaced prism?

**DragonLensmanWV** · 04-01-2008, 10:57 AM

Originally Posted by drk

Nah.

Lab question: how does this occur?

In my younger days, I used to specify level MRP/vertical OCs on my segmented MF jobs.

This prompted an exasperated call from the lab owner: "Please don't", he implored.

I guess if you fit a segmented MF the typical distance from pupil center (about 8 mm), you're getting about a 4 mm drop in the OC (quite acceptable), and another 4 mm to the seg line.

I guess the real issue would occur if you fit segs especially low, such as a sunglass or golf seg, and the powers were high or signficantly aniso.

So, "labsters" how do you surface a MF job with a specified level MRP other than default? It must be a hassle to some degree, or my lab guy wouldn't have called. Is it indeed surfaced prism?

We can put the distance OC anywhere we want to. By (our) default, the only time you will have OC really high above the seg is if you have a very low seg, so the OC stays more in the center of the frame.

**midohioeye** · 04-01-2008, 11:00 AM

I would of specfied an oc height but I was lazzy and the b measurement was only 26mm and the seg hgt was requested at 10mm high.
:cheers:

**drk** · 04-01-2008, 11:46 AM

Hmm, VV...
If the "B" is only 26, wouldn't that put the datum line at 13? 10mm high would "default" the OC to about 1mm above the datum line... (am I using the term "datum line" correctly?).

How high up are these OCs, anyway? You would only have 12 mm of lens above a "default" OC in the first place.

Dragon: Is it achieved by vertical prism, then?

**midohioeye** · 04-01-2008, 01:05 PM

datum line is a correct term, aka the 180 line etc. the current oc is 22mm high or 9 above the 180 line

**DragonLensmanWV** · 04-01-2008, 01:12 PM

Originally Posted by drk

Hmm, VV...
If the "B" is only 26, wouldn't that put the datum line at 13? 10mm high would "default" the OC to about 1mm above the datum line... (am I using the term "datum line" correctly?).

How high up are these OCs, anyway? You would only have 12 mm of lens above a "default" OC in the first place.

Dragon: Is it achieved by vertical prism, then?

Not per se, but the program will figure a smaller amount of prism to not move the OC as much. Since most FT segs are anywhere from 5 in and 5 down to 7 in and 6 down, when you block on geometric center you will grind varying amounts of prism to move the OC over and down to it's normal position. If you specify an OC higher than default, it will figure a different angle to grind the prism on so the OC will end up higher.

**drk** · 04-01-2008, 01:23 PM

I take back my cavalier "nah" and replace it with a "?".

Dragon, so, to dumb it down:

typically, blocking is done on the geometric center
typically, then, the OC has to be routinely moved about 5 in and 5 down, on average, so there's always some angle on the lens as it surfaces
If you want a different OC placement, just ask, and yes, it involves surfacing the lens at an angle
"Prism" per se is what exists at the OC, and of course it's zero, so the OC is not really moved by "prism", but "tilted surfacing" for lack of a better term.

If that is essentially correct, how was it done in the "old days"? Prism rings?

**Jacqui** · 04-01-2008, 01:33 PM

Waaay back in the Dark Ages, when I was taught to surface, we blocked on optical center so the lenses would come out right. No one stops to think what blocking on geometric center and using prism does to the OC of the seg. I have a beautiful looking pair of D-45's that someone blocked on the GC and have so much prism on the segs that they are unuseable.

The OC should in most cases be 3 - 4 above the seg for easiest use.

**DragonLensmanWV** · 04-01-2008, 01:45 PM

Originally Posted by drk

I take back my cavalier "nah" and replace it with a "?".

Dragon, so, to dumb it down:

typically, blocking is done on the geometric center
typically, then, the OC has to be routinely moved about 5 in and 5 down, on average, so there's always some angle on the lens as it surfaces
If you want a different OC placement, just ask, and yes, it involves surfacing the lens at an angle
"Prism" per se is what exists at the OC, and of course it's zero, so the OC is not really moved by "prism", but "tilted surfacing" for lack of a better term.

If that is essentially correct, how was it done in the "old days"? Prism rings?

In the old days you would use a prism ring around the outside of the block. With the advent of 3-axis generating, the precise placement of the OC is with generated prism. Different blocking and generating systems work to the same end but differently, a la blocking on OC instead of GC.

Perhaps I should have said the program will figure out a different prism axis rather than angle.

**mullo** · 04-01-2008, 05:47 PM

What is the approximate Rx in the 090°/vertical meridian when you notice this occurring?

RT · 04-01-2008, 06:42 PM

Lab layout software frequently would interpret a Seg Height of 10mm as meaning "10mm below the datum line". There's typically a break point at 10mm in terms of how the lab software interprets Seg Height data. That's so that you can specify that you want the seg to be a specific height below the OC and datum line. That's a not uncommon way of specifying height for an uncut.

If you specify a Seg Height of 10mm or below, it is recommended that you explicitly specify that you mean Seg Height above the bottom of the frame box to avoid confusion.

It is likely that in this case, the lab produced the job exactly as their software interpreted the data that it was given as input.

**Cherry Optical** · 04-02-2008, 10:21 PM

Originally Posted by midohioeye

typically oc are 4 mm above the seg, I will occasionally receive jobs from the lab where the oc are 10mm or so above the seg, and the powers of the lenses are close enough or equal to each other that there is no verticle imbalance. its kinda of like prism thinning for the pals except in lined lenses and in the oppsit direction. is this rejectable?

any thoughts anyone?

If you are ordering uncuts be sure to provide the B and ED measurements. Some lab management software will place OCs based upon B and segment location. If you are getting finished lenses that way someone is probably using an automated lensometer to final inspect your lenses.

Adam

**HarryChiling** · 04-02-2008, 10:30 PM

Originally Posted by Cherry Optical

If you are ordering uncuts be sure to provide the B and ED measurements. Some lab management software will place OCs based upon B and segment location. If you are getting finished lenses that way someone is probably using an automated lensometer to final inspect your lenses.

Adam

Great point Adam, I get scripts at times from pediatric OMD's where the Rx calls for split the pupil and children often have very small frame. If I were to make a high Rx with a DRP at 4mm above the seg they would look horrible and perform horribly, often in this case I will split the B measure for the DRP.

Originally Posted by RT

Lab layout software frequently would interpret a Seg Height of 10mm as meaning "10mm below the datum line". There's typically a break point at 10mm in terms of how the lab software interprets Seg Height data. That's so that you can specify that you want the seg to be a specific height below the OC and datum line. That's a not uncommon way of specifying height for an uncut.

If you specify a Seg Height of 10mm or below, it is recommended that you explicitly specify that you mean Seg Height above the bottom of the frame box to avoid confusion.

It is likely that in this case, the lab produced the job exactly as their software interpreted the data that it was given as input.

This is also a geat point, oru office uses Innovations and if I entered a seg height of 10mm the software will switch to the decentration at times instead of the height in which case I have to then enter the vertical decetration. I have worked on Visual Lab Pro as well and it did not do this so dependign on the software some of them interpret the data differently. So in cases where the measures are different thant the average or could be miscontrued it is important to send along notes with the job to specify exactly what's needed.