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Re: [PSUBS-MAILIST] Portholes, Windows
Todd - the phenomenon is known as refraction, and is described by
"Snell's Law" as follows:
n1*sin(theta1) = n2*sin(theta2)
Where n1 and n2 are the "index of refraction" for each material, and
theta1 and theta2 are their respective angles with respect to the
surface normal. The index of refraction of vacuum is 1.0, of air is
just slightly higher, at 1.00029 or so, of pure water around 1.33, of
seawater just a touch higher than that, and of acrylic, a touch less
than 1.5. Actual indexes are temperature dependent, and in the case of
water, dependent on salinity, etc. This is why, when you look up when
diving, you see a circle of light through which surface images are
transmitted, and only internal reflections beyond that. Inputting the
index of refraction for air and water into the Snell's law equation, we
see that the maximum angle of refraction for light entering the water is
about 48.7 degrees, corresponding to light hitting the water's surface
from the horizon (90 degress). With viewports, the minimum distorsion
will occur when the index of refraction of the viewport matches that of
the surrounding medium. In the case of acrylic and seawater, the
difference is not that great - small enough that we can successfully use
acrylic viewports without severe optical distortion. Note that the only
way to experience zero distortion is to look from a point source - not
physically possible, but a small camera lens in the center of a
hemispherical port comes close. People, of course, have stereoscopic
vision, and also move around, so the acrylic hemisphere is an
approximation of the ideal situation. With a flat viewport, distortion
is at a minimum at an angle of incidence of zero (looking straight at
the flat surface), and increases as the angle increases.
-Sean
Todd Fee wrote:
George you got me thinking about sub windows today and specifically
how light is bent while traveling through them. It seems to me that
when I look in to a fish tank with flat glass windows or through the
surface of a calm pond that the light is bent as it travels through to
the denser medium. I'm sure there is some simple formula or
explanation for this effect but I'd like to know how the effect works
while traveling through a domed window or through a tube. I've seen
plenty of spherical submarine windows but I have never been inside a
submarine to know what it might look like looking outside in to the
water from the interior of a spherical or domed window. I've seen the
distortion caused when trying to look inside a submarine from the
outside and I'm wondering if the effect is reversible or if while
looking out the window, the effect is diminished. Is the image looking
out similar to wearing a face mask while snorkeling, that is, is it
enlarged? I remember snorkeling once and finding a $20 floating in the
water and thinking it looked fake due to it's large size but when I
returned to the surface and looked at it without the mask on I
discovered to my great joy that it was real. How does a flat pane of
glass enlarge an image? Perhaps someone in the group can direct me to
a website detailing this phenomenon.\
Perhaps I am missing something.
Thanks
Todd
George, this may also help you develop a lens for your camera that
won't distort images. If you can develop a lens that would bring
images back in to proper perspective you could have it cut on a cnc
lathe and then polish it yourself. Perhaps you aren't concerned with this.
------------------------------------------------------------------------
*From:* owner-personal_submersibles@psubs.org
[mailto:owner-personal_submersibles@psubs.org] *On Behalf Of *George
Slaterpryce
*Sent:* Tuesday, February 28, 2006 9:38 AM
*To:* personal_submersibles@psubs.org
*Subject:* [PSUBS-MAILIST] Portholes, Windows
I've been trying to make windows for my ROV's camera to look out of,
I'm using acrylic and am pouring that into a mold for the window
shape. The window is 2cm in diameter and 2mm thick. I need to keep it
thin or it distorts the camera image. It's also flat, no dome shape (I
couldn't figure out how to get a nice smooth dome... although now I
have quite the collection of odd shaped tiddly winks if anyone's
interested)
Problem is the things are so brittle I don't think they'll stand up to
much preasure.
Am I missing something here?
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