MRP/Optical Center

[April_01]

Hi guys/girls!!

I haven't given you all a stupid question lately so here goes. What is the difference between MRP and Optical center?!! I've just been looking at my new book and the definitions sound similar.

[shanbaum]

Hi guys/girls!!

I haven't given you all a stupid question lately so here goes. What is the difference between MRP and Optical center?!! I've just been looking at my new book and the definitions sound similar.

I'm not sure which book you're using, but "MRP" stands for "Major Reference Point" and while it's still in common use, there's an effort amongst the standards-making groups (the VCA, ANSI, and ISO) to obsolete it, as it has always been ambiguous, having been used to refer variously to the "optical center" (as qualified below) and to the "seg" or "fitting cross".

The reason that we need a term other than "optical center" is this: the optical center is defined as that point on a lens through which a passing ray is not deviated. Normally, one expects a patient to be gazing through that point, especially at distance. If a lens has prescribed prism, however, there may not be any such point on the lens at all, and even if there is, it's not of particular interest. The point we're interested in is that point at which the prescribed prism is obtained (keeping in mind that the prescribed prism could be zero). Hence the name: "Prism Reference Point."

The other point of interest on a lens is that point by which the lens is laid out, the "Layout Reference Point". Which point that is depends on the kind of lens in question: for single vision lenses, it's the PRP; for segmented bifocals, it's the segment; for progressives, it's the Fitting Cross. That's the point to which you're measuring when you take a "PRP height," "seg height," or "fitting height," and it corresponds directly to the reference point used in aligning the lens for blocking and edging.

The short answer to your question was implicit in my description of the PRP: "optical center" doesn't take into account prescribed prism.

[April_01]

Thanks so much Shanbaum!!!! The book I was reading (one of my many new selections thanks to optiboard:) ) is Ophthalmic Dispensing by brooks and borrish. It does say "synonym Prism reference point" but that still didn't make sense to me. So what you are saying is that the PRP is where the prescribed prism is measured and the OC is where the light is undeviated?

[shanbaum]

Thanks so much Shanbaum!!!! The book I was reading (one of my many new selections thanks to optiboard:) ) is Ophthalmic Dispensing by brooks and borrish. It does say "synonym Prism reference point" but that still didn't make sense to me. So what you are saying is that the PRP is where the prescribed prism is measured and the OC is where the light is undeviated?

Yup - keeping in mind that a very weak-powered lens with lots of prism may not have an optical center. That is, no matter how you position the lens in a lensmeter, you won't be able to get the mires in the center of the reticle.

[Oculista Carioca]

I have only seen MRP on Michigan Medicaid order form which prompted me to search about MRP. Thanks Shanbaum! And thanks April for the good question.

[tmorse]

This is a 13-year old thread.

[Barry Santini]

I diagree with the assumption that the wearer should (always?) be viewing through the optical center in distance gaze position.
B

[shanbaum]

Old, but...

This is a 13-year old thread.

I'm still alive.

[shanbaum]

I diagree with the assumption that the wearer should (always?) be viewing through the optical center in distance gaze position.
B

How about, if a ray normal to the PRP is coincident with the wearer's optical axis?

[Barry Santini]

Nope. Not that eithet

[shanbaum]

Nope. Not that eithet

Why not?

[tmorse]

I'm still alive.

Hey, congrats!!

[Robert Martellaro]

I'm still alive.

We need you. Keep us on the straight and narrow.

I diagree with the assumption that the wearer should (always?) be viewing through the optical center in distance gaze position.
B

How about, if a ray normal to the PRP is coincident with the wearer's optical axis?

Nope.

I can think of one example- when there's a power differential in the vertical median, using a frame that introduces pantoscopic tilt, one might misalign the optical axis of the lens with the center of rotation of the eye by raising the OC into a compromise position that reduces vertical prismatic imbalance to an acceptable level, albeit increasing rms power and astigmatic error, but not as much error if the OC was placed level with the primary gaze. The lessor of two evils so to speak.

Best regards,

Robert Martellaro

[Randle Tibbs, ABOM]

How about, if a ray normal to the PRP is coincident with the wearer's optical axis?

Am i correct you are assuming in this particular scenario there is 0 degree panto?

[shanbaum]

Am i correct you are assuming in this particular scenario there is 0 degree panto?

Yes - I think it would have to be zero, as would Rx prism (or a ray couldn't be normal to the PRP). Now, I am thinking that "normal to the PRP" implies "normal, cx and cc."