Personal Robotics
Aerial Image
Ancillary Lens
Mouse Lens
Image Sensor
Agilent Chip
A dissected mouse-cam
obstacles in our path that we must
overcome. First, the mouse’s lens
has a very narrow field of view; I
calculate the lens to be 4. 4 mm in
focal length. Second, our mouse’s
lens is set for an extremely close-up
focus. The mouse’s image sensor is
only 1 mm across. This means that it
needs a wide lens to be of any use.
These two limiting factors led me to
search for a lens that was wider and
would allow me to focus
farther away. My solution
was to find what is called a
“board mount CCTV lens.” I
got lucky and found one that
has a 1.9 mm focal length.
Remember our basic
optical theory? It states
that, as you move your lens
closer to your imager, the
further away you focus. This
means that, to focus farther
away, you need to move the
lens even closer to the
image sensor — a feat not
easily achieved in such a
small package. To further
complicate things, even
getting my 1.9 mm lens into
the correct position proved
to be difficult, as well.
The solution is to allow my 1.9
mm lens to form its image at the
plane where the mouse’s original
lens is focused. By doing this, I am
creating what is called an “aerial
image” and using a relay lens. The
new lens produces an aerial image
that is captured by the mouse’s
original lens, which is now functioning
as the relay lens. This has some
advantages.
If I can carefully
sand the slightest bit of
material off the back of
the mouse’s lens and
cause it to focus even
0.1 mm further away, I
use more of the image
area of the new lens to
get a wider field of
view. The disadvantage
is that I lose light. For
my purposes, I am
more concerned with
objects at one to two
feet away, so I assume
that I can provide
additional illumination,
as required.
This approach
allows me to use the
optical mouse without
modifying the existing
sensitive optics. If,
however, you are
compelled to image directly on the
chip (and I would be very disappointed
if you aren’t), here are a few tips: If
you remove the lens assembly, you
will see a truncated cone with a hole.
The image sensor is directly below it.
If you use a different lens, you
may wish to open up this hole. To do
this, you will need to remove the
cover-plate, so that you can drill it
out. Careful examination will reveal a
slight indent at either end of the
mouse sensor. These can be gently
carved away and the cover can be
carefully removed. This will allow you
to work directly and easily with the
sensor.
You can also build a pinhole
lens. This will be mounted in place of
the mouse’s lens. To do this, take a
piece of thin brass shim stock
(0.005” works well), put it on an
aluminum block, and press gently
with a nail that is ground to a sharp
point. Then take the shim stock and
sand it flat with 400-grit sandpaper.
The dimple you created will break
and you be left with a nice pinhole. A
pinhole lens will not be as sharp, but
it will have better contrast.
Whatever course of action you
choose to follow, there are a few
tools that will be of great value:
Table 2
• A 12 mm x . 5 mm tap will allow
you to build your own lens boards for
mounting small video lenses.
• An eye loop or other magnifier.
• A flashlight with a diffused front
lens and target. Create this by
putting a piece of paper with a cross
drawn on it into the lens assembly.
• A pair of “extra hands.”
• A bright red LED.
• Laser pointer.
• Frosted Scotch® tape.
F
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NUTS & VOLTS
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08 09 12 16 21 31 53 63 63 63 63 63 44 24 16 11
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07 09 10 15 19 30 40 63 63 63 63 58 34 22 13 11
06 08 12 16 19 25 31 42 52 58 47 38 22 18 13 10
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05 05 05 06 07 07 07 09 09 11 09 08 09 07 06 32
88
The key here is to break the
chicken/egg syndrome early on. You
need to find where your lens will
form an image, but you don’t want to
build something just to find out that
it does not work. If you are truly
ambitious, you can build a series of
different tubes, threaded on the
inside, to accommodate your lens.
MAY 2004
