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serych

Newton sight - how it works?

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Hi all skydiving friends!
I'm flying paraglider and I'm thinking about buying newton sight for taking photos with the digital camera fastened on my helmet.
Please can anybody describe me, how the Newton sight is realy working? When I move my head in helmet, the cross insight the sight is changing its position correspondingly, or it "dissolves" and I have to find it again by finding the right position of my head (eye) relatively to the sight? Or it has any other behaviour?
Does anybody know the geometry and properties of the glases in the sight (the physical principle of the sight)?

Thanks a lot in advance.

:S Jakub Serych

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WARNING: Ignore what follows if you are not familiar with crystal optics.

It uses a composite optical element consisting of 3 parts (cross sight) or 5 parts (ring sight). The central element is a slice of birefringent crystal (usually calcite) cut perpendicular to its optic axis. Consequently the difference in refractive index between the ordinary and extraordinary rays depends on the angle the ray makes with the optic axis. The greater the angle, the more interference you get between the rays. At certain angles the interfefence is exactly destructive

This effect is invisible unless viewed in polarized light - hence on each side of the calcite crystal are polarizers. In this case, dark rings appear at the convergence angles (to the eye of the observer) at which destructive interference occurs. In addition to the rings you also get interference figures that form a cross due to the interaction with the polarization directions of the polarizers.

To eliminate the cross and just leave rings, a quarter wave plate is sandwiched between each polarizer and the calcite element. This is a birefringent material cut parallel to an optic axis and aligned with its principal directions at 45 degrees to the polarization direction. This resolves the polarized light into two components and delays one by 1/4 wavelength, hence leading to circularly polarized light. Since circularly polarized light has no preferred direction, the cross disappears and only the rings remain. As the light emerges from the calcite element, the second 1/4 wave plate recombines the rays into plane polarized light before they enter the second polarizer.

The spacing between the rings depends on the thickness of the calcite slice - thicker gives more closely spaced rings. The effect is only exact for one wavelength (color). In white light the rings farther from the center tend to get washed out because of this.

Simple, really.
...

The only sure way to survive a canopy collision is not to have one.

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Paul is right,

The newton cross has no calcite at all. The "interference" acts in the same manner as above, for the concentric sight. That is, at certain angles, the light is interfering with itself and cancelling out. The first order interference is the darkest, closes to the center fringe (or circle if concentric). The further, more rainbow like lines are interference at angles further from the axial "normal" line that is perpendicular to the glass.

Theoretically, if you hold the sight still and move your head behind it, the pattern remains "aimed" at the same spot at infinity (or a spot far away). Unfortunately, the property that makes the cross sight work is not very uniform in the film that you make it with, so there can be some "dancing" of the lines as you view the pattern through different portions of the sight, but the CENTER defined by those fringes does remain constant, so there is no effect on the operation of the sight.

There is no distortion with a concentric sight because the calcite crystal that causes the proper interference pattern is perfect.

Speaking of calcite crystals... I recently got a shipment of crystals for concentric sights and there was a high number of "blems" in there. I have about 8 concentric sights that are of varying degree of "blemished" from $160 to $200 so if you've been wanting a concentric cheap, now is the time to get the factory seconds!

Thanks,

Brent
www.brentfinley.com
brent@brentfinley.com

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Holy smokes! $160 to $200 is a fantastic deal.

For anyone that's ever considered getting a concentric but thought the price was out of range -- this is the deal to hop on.

I've been shooting one of Brent's concentric sights for the last three years and it is without a doubt the most accurate and precise aiming currently available to camera flyers.

My guess is that even his "blems" are still pretty freekin' fantastic.

I -highly- recommend.
quade -
The World's Most Boring Skydiver

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THanks for that explanation, Brent. I have both the cross site (on my tandem video helmet) and the concentric (On my fill-flash 10D helmet). They are both dynamite, but I am loving the concentric for photography.

That's a nice offer too. Shucks, I missed it!

JP

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Quote

Ok, call me silly, but I coulda swore the "Newton Cross" was the result of polarizer/stressed plastic/polarizer. I didn't think it had any calcite in it at all.



Well, I did say the central element is "usually calcite", but you are right. All that matters is that it's birefringent. Inteference crosses are used in petrology to identify minerals by their birefringence (at least, that was the standard method until scanning electron microscopes came along).
...

The only sure way to survive a canopy collision is not to have one.

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Sorry it took me so long to reply to this.

Blemishes are like dust or defects in ME putting the polarizer on. These are minor. You can't see them when the sight is close to your eye.

Other blems are surface pits due to a defective/poor crystal polish, or mishandling after polishing. You can see these if the sun shines in the sight and you see a small glare from them.

The load I had when I wrote the post is GONE. New crystals are in better shape so I'll have less severe blems... just my assembly blems... though... I'm getting better. :)

Brent
www.brentfinley.com
brent@brentfinley.com

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I just received my concentric sight from Brent today.... what a change from my other one. I will month it tomorrow and wait an other month and a half to jump it...............grrrrrrr I hate the winter !

I bought a blemish one....but cannot see the difference with a perfect one, may be a little thing one the bezel....

Thks Brent that sight worth the price.

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