so imagine that you're the victim of a
burglary or or there's some other
serious crime gone in in your
neighborhood then the the police would
come in and they'd search for different
kinds of evidence obviously your
burglar's gonna be wearing shoes so he's
gonna leave behind a shoe print
currently what they do is quite varied
so either they will they will get a
suspect into custody and they will get
them to stand on some kind of ink pad
and then stamp on a piece of paper this
is a copy of my shoe print that was that
was taken using this method so what what
i did was i stood on a on a vegetable
dye impregnated pad and then stand or
walk across this piece of paper what the
police would be looking for is
signs of wear on on the particular shoe
so they can they can identify a shoe
type quite readily from the from the
tread pattern underneath it they're nike
or adidas so yeah so they they're quite
familiar with the different kinds of
shoes that people wear and they they can
classify them quite easily but it turns
out that the way we wear our shoe down
is actually quite specific to us so we
all have a slightly different gate we
walk in slightly different ways converse
shoes which are the ones that i'm
wearing are actually quite notorious for
wearing in these kinds of regions
underneath where the ball of the foot is
as you as you can imagine so the soles
are actually fairly thin and what you
see is you see distortion of these
patterns
in this kind of area
as the shoe gets older they also tend to
wear
in this kind of area as well so you can
see
quite specific things due to the
individual the problem with acquiring
images like this is that what they have
to do is is they have to take something
like this and they they scan it into a
computer
there can be quite a long period between
actually acquiring an imaging an image
like this and actually uploading it to a
national footwear database it's very
similar to the kind of fingerprint
databases that we're all familiar with
from things like csi the idea is that
what they will be able to do is they'll
be able to take marks acquired from
crime scenes upload them into this
database take marks acquired from people
in custody and then try and match them
so if you've got somebody who's been
committing crimes around the country
all the different forces can upload
these images and you can search them
against that and maybe place people at
different crime scenes the fbi expert
can determine the make of a shoe
and frequently depending on the way the
shoe is worn such facts as the
occupation of the wearer the vegetable
ink thing can be quite smudgy there are
also maybe
a few compliance issues associated with
getting people to do this as the right
with any kind of evidence gathering
process but it's it's largely due to
time forensic services have had their
budget slashed there's an increasing
push to improve efficiency and you can
imagine if you're somebody working in a
custody suite on a saturday night or a
friday night these are quite busy times
you might not have time to process the
evidence as quickly as you might like
the conversation that we had with them
was around the area of trying to acquire
these images digitally in custody the
idea being that you could you could get
the person to stand on some device you
could acquire an image of the regions
where their shoes contact a surface
that could you could press a button and
that information those images could be
quickly uploaded to this national
footwear database they could be searched
almost immediately
and you can make a decision as to
whether or not to detain or release the
way we decided to tackle this was
actually using a fairly nice piece of
physics which is covered in most gcse
syllabuses so the the approach that
we've used is actually based on
total internal reflection this is a
piece of perspex and what we've done is
we've wrapped
these led strips around the outside of
it so if i just plug this in so these
led strips light up
okay so those are the led strips okay so
the leds run all the way up here and
what we've done is we've wrapped them
around the outside of this piece of
perspex
and then what we've done is we've
basically covered that up with in this
case a piece of masking tape now you can
see me through this device
okay so what's happening is the lights
bouncing around inside this so it's been
totally internally reflected inside this
piece of perspex but if i now bring
something like my thumb
into contact with the surface of this
then what you see
is my thumb scatters quite a lot of
light and all of a sudden you can see
the contact regions
between my thumb and the piece of
perspex so if i do it with my hand for
example you can see
all the regions where my my palm
is touching
that piece of perspex so you can imagine
if you did this with your shoe you would
see a similar kind of pattern and of
course if you were to walk across
an element like this then what you would
see is that the intensity pattern would
change and you would get
an image or a movie of the regions where
your shoe
is contacting this piece of purse back
so the idea is very simple suppose we
have something like a surface where on
one side we've got glass or plastic and
then on the other side we've got air now
if we were to send in a beam of light
towards that surface we would see two
things
firstly we would see a reflected ray a
reflected beam and if if we say that
this angle of incidence is theta i then
we would see the same angle on
reflection so we're going to call that
theta r and in this case theta i and
theta are the same we'd see another
thing as well and that's a transmitted
array so we get a reflected array and we
get a refracted ray
and this refracted ray comes out at a
slightly different angle theta 2 and
that angle theta 2 is bigger
than the angle theta i
by virtue of the fact that the
refractive indices of the air and the
glass are different if we extend this
idea and we make this angle of incidence
bigger what happens is that this angle
theta 2
will continue to increase in size and
eventually what will happen is you'll
end up with a situation where that ray
will basically
move parallel to that surface as we make
this angle bigger so there's a critical
angle at which this refracted ray will
run across that interface beyond that
critical angle at which this occurs
what will happen is
the the light will be totally internally
reflected all the light will be
reflected back inside the glass block so
this critical angle of incidence which
we'll call
theta c it's fairly easy to calculate if
we know
what the two refractive indices are so
if we say that the refractive index of
the glass the plastic is n2 and the
refractive index of the air
is n1 then the sine of that critical
angle is just going to be equal to n1
divided by n2 this is the condition
where we don't have anything in contact
with the with the glass or the plastic
that i talked about just now if we bring
a contacting object in such as a
shoe sole what that does is it changes
this condition and in these regions
where i'm contacting the glass surface
with the shoe i now have a different
refractive index you have a new end so i
have a new end let's call it n3
okay so now this condition
this kind of critical angle
changes under the right conditions
light is no longer now totally
internally reflected but some light can
actually escape and
if you like move into the sole of the
shoe but that's still not doing any good
if it hits the sole of the shoe well
it's it is if that's if that shoe is
also quite a good scatterer of light so
if if if the
soul material scatters light then what
will happen is that some of that light
that goes in will actually be scattered
back down in this direction so if we
were to put a camera at the bottom we
could image these regions where
the the shoe was actually contacting the
sole okay so this
is cinderella so this is the the device
that we've been prototyping to use with
the local police force and this is
actually a device for imaging um the
shoe prints using the technique that
i've just talked about what we've got
here is is a very
very similar design but it's it's
actually much larger we've got a big
piece of acrylic and again we've wrapped
leds around the outside of it what i'm
going to do now is i'm going to step up
onto this thing
okay
and so i'm for now because i'm in a lab
i'm gonna have to do something to try
and remove the background lighting if
you look at the screen now you can see
the regions
where my
shoes are contacting the surface they
light up quite brightly in the same way
that they did on the piece of acrylic
that we were playing with earlier we can
capture images like this and use them to
obtain
a print if you like a copy of of the
contact regions where my where my shoe
is actually touching this and this is
the kind of thing that we would compare
to
a mark that we would find at a crime
scene so we can see as i move around i
get bright spots
and dark spots okay so these images that
we can see on the top left hand side
they're they're images that are produced
by thresholding these images
that we saw when we were imaging just
now so these are the kinds of things
that you would compare to
a marker that you would find at a crime
scene and they look
quite a lot like a fingerprint that you
you would store digitally for for good
reason because the way they search them
is actually
very similar to the way that they would
search a fingerprint the images at the
bottom here are provide complementary
information so these are actually taken
using conventional lighting and what
they do is they just give us a
photograph of the underside of the shoe
and these are useful because they also
provide information about stuff that's
perhaps not in contact for example that
you've got something like a stone or a
piece of glass embedded in the bottom of
your shoe which is not showing up in the
contact regions but which would make a
difference if you were to make an
impression in soft mud
so
that that again provides you with
additional information if you're looking
at a different kind of image
is that well that looks like a keyhole
but they definitely can't get the key in
there it is a pretend keyhole okay the
weak point in all of these is the
locking mechanism so therefore you hide
the lock so the keyhole must be
somewhere else oh hang on
what's this there's a panel here on top
i literally just found this i wasn't
pretending for the camera
there we go
oh this is very scary this is good isn't
it very scooby-doo