A polariscope is just two pieces of something fairly clear (so that
light passes through easily) and polarized.
Polarized means that light can only pass through the polarized lens or
material in one direction. If the light is coming in horizontally you
can rotate the lens and it will gradually get darker as it blocks more
and more of the horizontal light.
Sunglasses may be the best way to describe what happens with polarized
Light usually comes at us from all directions but...if light reflects
off a horizontal surface like a road or the water, the light comes at
us in a mostly horizontal direction.
A polarized lens blocks the light coming from all but one direction. In
sunglasses, the lenses are put into the frames in such a way that they
would block the horizontal light. The lenses will still allow light
coming at them from a vertical direction to be allowed through the
The reason the polariscope works is that stress in the glass will bend
the light coming through the glass, changing its direction and allowing
the light to "get through" the second lens, even when the second lens
has been rotated around until it's dark from blocking the light. The
stress shows up as lighted spots, or rainbows if it's a lot of stress.
To check glass for stress, you'll need two polarized lenses and a light
to shine through them.
The most common things that are see-through, polarized, and
inexpensive...or that you already may have lying around, are...
Sunglasses lenses (available almost anywhere)
the screw on polarizing filters for the front of the lenses on SLR
(35mm) caneras. (find them used on Ebay inexpensively)
sheets of polarized plastic from American
Science and Surplus (put "polarized" into their seach engine
and look at all the other crazy stuff they have while you're there.
Fun!) or a lot of other scientific/hobby suppliers.
Your cell phone probably has a polarized lens on the screen, and it has
a light behind the screen, which is something else we'll need to check
our glass for stress.
The way you use the two polarized lenses is to have light passing
through one polarized lens, then through the glass you're testing and
then through what we'll call the top lens.
You rotate the top lens until it's blocking most of the light and then
watch for funny patterns of light in the glass. They may be rainbows or
they may be black and white lines or patterns. One of the most common
patterns is an "iron cross". You'll see this in the pics below.
If it's something like tempered you'll see a LOT of patterns, usually
rainbow, through the lens. The rainbows mean LOTS of stress.
This is the best way to confirm if do you have tempered glass. Glass is
tempered by creating lots of compressive stress on the outer surface.
This is done by spraying the glass with compressed air when it's hot.
If you want more tech detail on how that works and why have a look at
compatibility page under the annealing info about what happens
when a piece of glass is cooled quickly on the outside. The same
problem that would usually cause a fail in a glass piece can be used to
strengthen the glass if it's done just right.
Using a cell phone with a polarized lens as a polariscope, you would
put the glass on top of the cellphone with the screen lit up on the
phone. That's your light, your bottom polarizing lens, and your glass
to be tested. Rotate whatever you're using as the second or top lens
until it's dark and you see stress...or not.
I found a video on you tube showing the cell phone polariscope in
action testing for tempered glass:
When you do this test you'll almost always see some stress. The trick
is knowing how much stress is acceptable for what you'll be doing with
Pictures of the Homemade Polariscope in Action
Some beads and the strip of glass used on the compatibility testing
page. The strip hasn't been annealed where it was heated and cooled for
the test so we'll see some stress. The top lens is not over these yet.
Both lenses are the screw on polarizing filters for the front of the
lenses on SLR (35mm)caneras. The whole setup is sitting on a light box
like the ones dentists use to look at x rays.
Starting to rotate the top lens so it's blocking the light. I put one
bead on there that has no stress for comparison.
More rotation. More light being blocked. Stress is really starting to
show as light is being bent in a direction where it will pass through
the top lens.
More rotation on the top lens. Look at the strip where it was heated up
on the end.
Rotated to the darkest point. Stress is really showing.