Polarized lens question...

[BMH]

Okay here is something I've never seen discussed on OB.

Is there an acceptable amount of off axis error for polarized lens blanks? I am refering to the 180 marks on the lens.

If you spot up a lens for edging and it is slightly off axis 180 marks vs rx axis, should you block the lens on the correct Rx axis or block the lens on the polarized axis?

Basically how much fudge room are in polarized blanks before the glare blocking features are compromised?

[Judy Canty]

If I remember correctly, more than 6 degress of axis will affect the polarization.

[eoptics]

If I remember correctly, more than 6 degress of axis will affect the polarization.

Can you explain it more... for example, based on what??? or according to what experimental result....???

[k12311997]

This is from a pdf comparing the manufacture of polarized lenses.




Industry standards allow a 3 degree tolerance for axis alignment beyond.

from here http://www.youngeroptics.com/PDF/mar...omparisons.pdf

[k12311997]

1. AXIS ALIGNMENT
COMPLAINT: Patient complains of distorted vision, headaches, dizziness, nausea. �Something�s just not right.�

PROBLEM: Axis is off by more than three degrees, compromising tolerance. Possible causes include: the manufacturer�s markings and tight tolerances on axis placement are off, the axis is misaligned during lab processing, or the axis is displaced through wear

from here
http://www.eyecarebiz.com/article.aspx?article=51289

[rbaker]

Lets not confuse "cylinder axis" with "axis of the plane of polarization."

[chip anderson]

Why not just take a marked plano, hold against a glare, off a shinny floor for example and tilt until glare is precieved in each direction . Should be simple enough for an "optician." Obviously the more it is off axis the sooner glare will be precieved from that direction and later from the oposite direction.
Even if 90 degrees off (which I once had a whole batch of Lux sunglasses) one will see the same just glare will be blocked from the side instead of blocked from above and below.

Chip

The best tolerence is always zero, on any specification.

[BMH]

If that were reality there would be no tolerances.

I posed this question to see what other people thought about the issue.

I always block the polarized axis with the above thought of no tolerance. If there was some concern with cylinder axis, I then check glasses after they are edged against ANSI tolerance.

Now I said always but I guess not always.

I also asked this axis question because I discovered my own sunglasses are off on one eye. I see fine but evidently I screwed up blocking my left lens, about 10-15 degrees off polarized axis but cylinder axis is right on the money.

Could the 180 marks on the blank have been off?

[Judy Canty]

Sounds like it was surfaced off axis.

[Barry Santini]

I think that ANSI agrees with the 3 degree polarized-axis tolerance.

I have a POLAROID brand lens axis checker, which allows a 5 degree shift for mounting.

Also: Lenses that are mounted too tight may show *zonal* polarizing axis differences,, not unlike a plastic lens that warps when mounted too tightly, or is made too thin for a metal frame.

FWIW

Barry

[Kyle]

In a wrapped frame, given the general concensus that frame wrap, tilt and vertex distance may potentially translate into modified axis and power on spherocylindrical lenses, shouldn't we also be considering the plane of polarization in the same way?

;)

[DoubleD]

Without getting into the complexities of polarization, off axis, rotational effects, simplist answer to polar axis is no more than 3 degrees off.
Polarized efficiency will make a difference to patient comfort. Higher efficiency requires tighter axis control. Past testing I have seen is at 98% efficiency, visual perception can sense beyond 3 degrees. Of course people also vary in sensitivity.