Help me understand polarization axis!

[drk]

I have a patient with odd symptoms that I will not bore you with.

What I would like to know is how polarization axis works.

Let me take a few uneducated guesses, and please confirm or correct them! Here goes:

1.) Semi-finished polarized lenses come with markings on the front, like progressives do. All spherocyls must be surfaced because of the need to orient the polarization axis, disallowing the lens to be turned in the lensometer and blocked at the correct cylindrical axis, like a clear lens normally is.

2.) When blocking up the lens for the generator, the markings must be at 180 (or whatever reference point).

3.) If there is a mismatch between the right and left eye, there will be a reduction in efficiency of the polarization effect: the right lens, e.g., could be "off axis" and letting in more reflected light at some specified angle than the left lens would be, resulting in a "luster" effect (reflection in one eye only).

4.) Polarization axis seems to be at the same angle OD vs. OS: say, 45 degrees in both eyes.

How am I doing on this?

[Leo Hadley Jr]

Not bad, but I dont get #4. Is that the result of the lenses?

Polar axis has to be at 180 no matter what. The lens is blocked and generated at 180 but you can enter the rx prescription to whatever you want. When you finish the lenses you inspect the rx in the lensometer. Your rx should be correct and your polar axis is still at 180.

There are normally notches at the edges of the blanks to determine the 180. An experienced eye could look through the lens at the glare on the tile lab floor and twist the lens until the glare disappears.

If your finished pair has one lens twisted off the 180 polar axis, it will be very disorienting to the user.

[shanbaum]

1.) Semi-finished polarized lenses come with markings on the front, like progressives do. All spherocyls must be surfaced because of the need to orient the polarization axis, disallowing the lens to be turned in the lensometer and blocked at the correct cylindrical axis, like a clear lens normally is.

The "marking" usually consists of two notches in the polarizing film, one on each side of the equator of the lens (in other words, at "0" and "180").

2.) When blocking up the lens for the generator, the markings must be at 180 (or whatever reference point).

Using an old-fashioned marker/blocker system, the cylinder axis would be marked with the notches along the horizontal, and then the lens would be blocked on the mark. If the cylinder axis is other than 180, the notches are rotated relative to the block.

3.) If there is a mismatch between the right and left eye, there will be a reduction in efficiency of the polarization effect: the right lens, e.g., could be "off axis" and letting in more reflected light at some specified angle than the left lens would be, resulting in a "luster" effect (reflection in one eye only).

What do you mean by "mismatch"? If you mean different cylinder axes on the two sides, then, no. The orientation of the polarizing film is independent of cylinder axis.

4.) Polarization axis seems to be at the same angle OD vs. OS: say, 45 degrees in both eyes.

Not sure I'm getting this... are you talking about the rotation of the polarizing film, or the angle of incidence at which light is optimally absorbed by it (which is less than 45 degrees, as I recall)?

[drk]

Thanks to the expert responders.

In #3, I meant to ask if it were not uncommon that the "notches" at 180 could be "off axis" on one of the lenses, so that when the cylinder was surfaced and the lens were glazed, the polarization axis is rotated with respect to the 180, but the cylinder axis looks correct.

That would mean that the polarization effect in that eye would be dampened with your typical horizontally vibrating (I believe) reflected light wave, since the filter would be misaligned in that eye?

In #4, I'm wondering this, essentially: It seems that when you are wearing polarized lenses, and you notice a minimal reflection off an object, and then you tilt your head approx. 45 degrees, the object has a maximal reflection.

Somehow, I expected to have to turn the lens 90 degrees to get a maximum and minimum, and not only 45 degrees.

If you are wearing polarized lenses and look at an LCD, if you tilt your head 45 degrees left of vertical and then 45 degrees right of vertical, then you will see an absolutely dark monitor in one position and a bright one in the other, with an intermediate transmission with normal head position. This may be an effect of the LCD wave vibration orientation and not the sunglasses, though. I think this holds true for looking at any specular reflection. Any insight on this?

[fjpod]

DrK
Some reflected glare is horizontal in nature as off a roadbed or a lake. In a city environment, some will be vertically oriented as along the side of a tall glass building (or diagonally) oriented...but still a lot is horizontal.

Polarized lens makers make the assumption that for the most part, in the real world, horizontally reflected glare is the enemy. That is why the polarization marks on the front of the lens should both always be oriented as close to 180 as possible...no matter what the cylinder axis is.

Polarized lenses oriented at 180 block almost all horizontally reflected glare, some diagonally reflected glare, and almost none of the vertically reflected glare.

I suppose one could specially request the polarization axes to be placed elsewhere for a specific need, but then these glasses would not be best for general use, as most glare is reflected off the horizon.

Is this what you are asking?

[drk]

I was asking what it would seem like when the polarization axis is incorrect in one eye.

Let me get down to the problem prompting this post. Guy in Polar Poly Brown wants a new pair just like them. I hadn't specified the brand the first pair, and didn't specify the second time, either.

On dispense, the guy reported seeing light streaks out of one of the lenses. Trying them on and looking at a light source, I saw obvious, vertically oriented light streaks emanating from the light source. Other eye fine. Guy thinks I'm incompetent (I know I am :P)

Called the lab, but didn't get to the guru at the time. I sent it back to be remade, since I believed it to be probably a film defect. I specified that they look up the last brand used and use that again, for good measure.

Get 'em back, and they look good to me. I try them on. I dispense. Guy stares into the sun (!) as his test light source, and says "still streaky". I clean the lenses: "still there". I assume some type of astigmatic blur, and sure enough they are glazed off axis about 2-3 degrees on a -2.00 cyl...Finally the guy says neither lens is right. (I'm thinking he just caused himself a little solar retinopathy, and frankly he's deserving it, right about now...)

But...He's not looking at a reflection, is he? Just a light source.

I'm thinking the guy is hyper-goofy now.

Any comments?

[fjpod]

Maybe the first pair were made with the polarization axes off the horizontal (due to error or intent, but I would doubt the latter) so that he was getting some reduction of vertically oriented reflected glare. Maybe the new ones were made "properly" on the horizontal thereby eliciting his complaint of vertical glare.

Again, though, the cylinder axis has nothing to do with it. The cyl could be at 92, 46, or 178, the axis of polarization should still be at the horizontal. I would also bet that even if the polarization axes were off the horizontal by 5 or 10 degrees (and cylinder axes within ANSI tolerances), no patient would notice. (They would have a little less protection against horizontal glare, but a little more against vertical.)

If the lenses are already edged, you cannot see the polarization markds on the lens, but what you can do is pop one lens out of one pair, hold the frame up as it would sit on a patients face and with the other hand hold the unmounted lens in front of the mounted one. You should get maximum darkening through the lenses when the unmounted lens is held 90 degrees off axis, If you don't, that tells you one or both lenses were not fabricated with the polarization axis on the horizontal.

Then compare it to the other pair. Hope this helps.

[drk]

Thanks, FP.

[CME4SPECS]

DRK, if the lenses are too big and are under stress, you will see some colors that would not be visible otherwise.

[hotsauce]

The other thing to do is to check axis relative to both lenses. If you've got a polarized demonstrator, hold the glasses out at about 18-24" in the as-worn angle. Slowly rotate them and you'll see the glare come back as you reach 90 degrees. If the rate of change is noticeably different, you might just have one (or both) off-axis enough to matter. As an added bonus, you can use this same demonstration for your patient if everything's right-on.

If you don't have a polarized lens demonstrator, LCD computer monitors make a pretty good substitute. However, the angle at which they blank out is at 45 degrees, so not as good for patient demonstrations.

The other thing that could be causing "streaks" is material. Are the lenses poly? Poly polar has the annoying tendency to show a white ring around the lens after edging (especially on minus powers and higher base curves) due to parts of the laminate being cut out of the lens.

Lastly, staring at the sun is about the dumbest way to test sunglasses, doubly so for polarized. As everyone else has mentioned, it's reflected light, not direct light, that the lens is cutting out.

[drk]

Thanks for all the posts. This case is closed, so to speak. I did a minor cyl axis adjustment, since he couldn't have been complaining about a polarized issue on the second pair if he was staring at a light source (hadn't thought of that, initially...).

He picked up (the same pair...:D ) today, and they were "fine" after staring at the sun again. ?? Probably a nut.

Good night, all.

(THIS is what I like about Optiboard...half a dozen pro's ready to offer help)