Having built one optical ("lightbeam")
transceiver enclosure already, I wanted to build another
enclosure so that there would be a second unit in
existence: After all, what is the sound of one hand
clapping or, along the same lines, what good is a transmitter if
there is no receiver?
Because I wanted it to be quick, easy, and cheap, I minimized
the cost and time required to build it as much as I could - yet,
I still wanted it to be good enough that it was usable and could
withstand being hauled around a bit.
Even though this unit was never properly optimized, it has
been successfully used in a two-way exchange over a 107 mile
List of materials:
Some of the raw materials and tools used to assemble the
box: Black "foam core" posterboard, some 8x10
picture frames, straightedge, utility knife, hot-glue gun,
and a cluttered workspace.
Click on the image for a larger version.
Below is a list of more-or-less what I used to make this
device. Since its job is simply to hold the lenses and the
electronics involved at precise focus, there's nothing
particularly special about how it might be done - as long as
whatever you might build can do the same!
- Two Fresnel Lenses.
- The cheapest, large-sized Fresnel lenses that I
could find were of the "full sheet magnifier" types.
While these are readily available at many stationary and
office supply stores, I found them for $2.00 each at
American Science and Surplus, part number 8665 (LINK).
These Fresnels are very thin (only 1-2mm at
most) and very flexible, about 7.25" by 10.25" (184x260mm)
and have a focal length of approximately 12.25" (about
310mm.) They are of mediocre optical quality - but
- More recently, inexpensive (ranging from $2 to
$10, depending on brand and the retailer) "rigid" Fresnel
lenses, molded from optical acrylic, have been appearing
at various book stores and office supply houses. A
number of brand names have been noted, such as "Bazic" and
"Bausch and Lomb". These are roughly 8-1/4" x
10-3/4" (21 x 27cm) and are variously listed as having
"2x" or "3x" magnification and with a variety of focal
lengths. The samples that we have obtained have a
focal length of about 32cm - about the same as the
"flexible" lenses described here. If you use these
lenses, picture frames are not required as they are,
themselves, fairly rigid - but their "ridged" fronts
should still be protected from scratches during transit
and from dust when being stored! These lenses appear
to have optical properties superior to those of the
aforementioned "flexible" vinyl type.
- A word of warning, however: These
inexpensive "rigid" page magnifiers seem to come in a
variety of focal lengths, ranging from about 9 inches
(230mm) to over 24 inches (600mm). The
longer focal length lenses are very difficult to
efficiently illuminate and will require a very large
box! For this reason, I would strongly
suggest that one chooses a lens with a focal length of
13 inches (330mm) or less. To determine the focal
length, one can simply focus a distant object (say, a
building across the street) onto a piece of paper and
measure the distance between the lens and the
paper: Most of these lenses are packaged in a way
that allows such a test to be conducted without removing
them from their box. Note: For
this measurement to be at all accurate, use an object
at least 100 feet (30 meters) distant!
- DO NOT rely on the "2x" or "3x"
magnification rating to tell you something about the
focal length of the lens: I have seen
seemingly-identical lenses - with the same focal length
- that are variously labeled as "2x" or "3x"! The
only way to be sure about the lens you receive is to
check the focal length as described above!
- The optical quality of the typical 330mm focal length
lenses has been tested and found to be quite good,
yielding a small "spot" when focused onto a distant,
point-source of light such as a star, comparing very
well with known "good quality" Fresnel lenses.
- If you are using the "flexible" Fresnel page magnifiers
mentioned above, you'll need some 8x10 picture frames.
I picked several of the cheapest wooden 8x10 picture frames
that I could find - $3.00 each at WalMart. It is
important to make sure that they have clear
glass as many of them have slightly frosted glass in order
to minimize glare. Actually, I picked up three frames
(in case I broke some glass) but ended up using just the
glass from one of them in order to flatten one of the
Fresnel lenses - more on this later - but I should have
really picked up four frames!
- If you are using the rigid Fresnel page magnifiers, these
are rigid enough to support themselves and a picture frame
may not be required. Note, however, that doing so
leaves the fronts of the Fresnel lenses exposed, so care
must be taken to avoid damaging the "grooves" in the lenses
and to keep them free of dust! If you do
not install protective sheet of plastic (or glass) in front
of these lenses, it is strongly recommend
that one makes a cover - possibly out of posterboard - to
protect the lens from dust and damage, not to mention the
accidental exposure to sunlight!
- If you are using a rigid "page magnifier" Fresnel lens
by itself and not in a frame with glass - that is,
gluing it directly into place and using it as a portion of
the box's structure - make sure that it is installed flat
in the optical plane - that is, neither tilted or warped
(twisted)! Careful measurement and/or comparison
with a flat piece of wood or glass is recommended to
assure that the plane of the lens is as flat as possible!
- Some "Foam Core" poster board. In the stationary
aisle (amongst the school supplies) I found some 20" by 30"
(508x762mm) by 3/16" (5mm) thick flat black boards for $2.88
each at WalMart (prices noted in February 2009.)
This material comes in several colors (black and white being
the most common) and consists of a layer of Styrofoam (of
the same color as the paper) sandwiched by construction
paper on both sides. It is very light and surprisingly
strong, and being that it is coated with paper, it is easily
bondable with nearly anything. To assemble this pair
of enclosures (one TX and one RX) I used five sheets of this
board - although with more careful planning, I believe that
I could have done it with just four sheets.
- "Grabber" (tm) screws. Also known as "drywall
screws" these are aggressively-threaded Phillips-headed wood
screws. These are used to lock the focus into place as
well as to attach panels, etc. as well as for temporary
attachment/assembly prior to gluing.
- It turns out that I also needed to have a secondary lens
for the emitter. When I had ordered the Fresnel
lenses, I also ordered a bag of 20 double-convex plastic
lenses for $3.98. These are 20mm (a bit more than
3/4") in diameter and have a focal length of 34.5mm (about
1-3/8".) These are American Science and Surplus part
number 68093 (LINK).
these are really cheap, they seem to be of reasonably good
optical quality. As noted elsewhere, these are not
the proper secondary lenses to achieve optimal optical
efficiency, but since I just wanted this particular unit to
be "good enough" I wasn't too concerned.
- Some glue. I used "Hot Melt" (thermoset) glue with a
glue gun to assemble these boxes, although with the paper
covers, white glue, tape and pieces of heavy-bond paper
could have also been used, provided one allowed plenty of
time (possibly hours!) for each set of glue bonds to
dry. Silicone sealant could have been used to provide
good bonds - again, assuming that one was willing to do the
"glue - then wait until it cures before gluing some more"
trick - a process that would require several days of
intermittent construction. The thermoset glue is nice
in that it's ready after just a few minutes, but the bad
part of using thermoset glue is that if this enclosure is
left in a closed car, in the sun on a hot, summer day, it may
disassemble itself. (I may reinforce the glue joints
with silicone sealant as a "backup" adhesive...)
- Note: When using the thermoset glue with the
foam-core board it takes several minutes
for the glue to cool and set up - This is because the
foam-core board is a good insulator and does not conduct
the heat away! To speed up cooling you can use a fan
or even a hair dryer on it's "non-heat" (cool) setting.
- A silver-colored marking pen to mark surfaces in an
obvious way so that you know what is what after you've
completed the box. (We'll be writing on black
surfaces, you know...)
- A pen or pencil: These are for making
cutting/measuring marks during construction to mark parts
that don't need to be readily visible after you have
completed the box.
- Some bits of paper for taking notes.
- A straight edge and ruler: A metal yard/meter stick
or 24" (60cm) metal ruler works nicely.
- A sharp utility knife. A retractable-blade utility
knife has been found to be more convenient than an a hobby
knife set such as an "X-acto" (tm) knife.
- A surface on which to cut the foam core pieces that can be
scratched, but will not ruin the knife's edge. A large
scrap piece of cardboard or Masonite is good for this, and
an old rug or doormat can also work as you don't want to
ruin your kitchen table!
To cut this foam board, one needs to have a straightedge and a
sharp utility knife: Simply score along the line a few
times, snap the foam along the line and bend it back over
itself, and then cut the paper on the other side, inside the
fold, with a knife.
Unfortunately, I did not take pictures of the
enclosure in various states of construction - sorry.
Assembly - Front half:
- There are two identical enclosures - one for
receive, and the other for transmit. After initial
optical alignment, they are physically connected to each
other to form one unit.
- The description below is for just one of the two identical
enclosures. Note also that each enclosure consists of
a "front" and "rear" half, nested together to provide
Top: A close-up of the receive lens, showing
the frame glued into place. Also shown is the
mounting of the Fresnel lens on the glass, along with the
black electrical tape used to mask the "un-lensed"
Middle: A close-up of the friction-fit joint
between the "front" and "rear" enclosure sections.
Sliding the two sections together or apart allows precise
focusing of the optics.
Bottom: A front-on view of the entire
enclosure assembly. In the transmit lens (on the
left) note a horizontal bar: This strip of foam-core
board holds the Fresnel against the glass while the
receive lens (on the right) sandwiches the Fresnel between
Click on an image for a larger view.
The first step is to characterize the Fresnel lenses. The
full-sheet magnifiers that I used have a focal length of about
310mm - a property determined by focusing an outdoor view of
distant objects (the house across the street) onto a sheet of
paper and then measuring the paper-to-Fresnel distance.
This measurement was very important in determining the initial
dimensions of the box.
After removing the glass from the picture frame I carefully
measured it and then cut four sheets of the foam core
board. For the top and bottom (the short, or 8" side of
the frame) the board was exactly the length of the frame, but
for the two sides (the 10" dimension of the frame) the board was
cut 3/8" longer so that there was some overlap with the edges of
the short side: This allowed the pieces of foam core board
to completely enclose the frame, yet provide an interior surface
for gluing. As for the length of these pieces, I chose
As pieces are measured, cut, and fitted, it is
strongly recommended that they be labeled and marked with a
pencil or pen with to indication the front, top, sides, etc. -
and also to note precisely which pieces are fitted against
Using some tape to hold the sides together, I assembled the four
sides of the box and then inserted the picture frame, setting it
back from the front by about 30mm: This setback performs a
slight function as a lens shroud, but mostly it protects the
glass cover (or Fresnel lens) during normal handling. Once
I verified that the pieces of foam core board were of the right
length and that the plane of the picture frame was at a precise
right angle to the box and not warped (that is, that it was set
in straight) I used the thermoset to glue everything into
place. When doing this, it is best to use a few small dabs
in some strategic places to hold everything together - and then
re-measuring to make sure everything is lined up before applying
the full amount of glue.
Remember that when applying the glue (which is hot enough to
cause a nasty 2nd degree burn) it is best to glue one or two
joints and then wait a minute or two for it to set before
turning the box around and gluing a few more joints - and that
since this foam-core material is a good insulator, it takes
longer than normal for it to cool and set!
Once this portion was completed, the entire assembly was quite
rigid - yet very light.
As can be seen from the pictures, each enclosure is really two
pieces, nested inside each other with the front half of the
enclosure sliding inside the rear half allowing one to adjust
the focus simply by sliding the two pieces past each
other. Because of this, it is important that the glue
joints for the front section are inside
enclosure and that the edges are flush so that the two sections
fit as tightly as possible.
While it would have made more sense to make the rear half slide
inside the front half (as the rear can be smaller, from an
optical standpoint) I did not do this for this prototype.
Why not make the front portion smaller? Well, I was sort
of lazy. Because the ultimate size determination is that
of the picture frame, the front half had to be constructed
first. It was easier to build that portion - and then cut
the pieces to size to build the rear portion to fit around the
outside of that. It was easier to build the second (rear)
portion by wrapping around the outside
of the front
portion than trying to build the rear portion by assembling it inside
the front portion. In retrospect, I should have out the
rear section inside the front section!
Assembly - Rear half:
After the front assembly was completed, a 250mm deep box was
wrapped around the already-completed front portion. For
this, the side pieces were cut to precisely the length of the
sides of the front box portion while the top and bottom pieces
were cut a bit longer, allowing some overlap for the glue
joint. Using the front box as a form, wooden spring-loaded
clothespins and weights were used to assure a very snug fit
during assembly. It should go without saying that one
should be careful to avoid gluing the two halves of the
enclosure to each other.
Unlike the front portion, all of the glue joints were done outside
the box as to avoid interfering with the tight fit - and this is
why the top and bottom pieces are cut slightly oversize.
Once the glue has set I pulled the two boxes partially apart and
traced the shape of the back of the enclosure is traced onto
another piece of foam core board to be used as the rear
panel: Tracing the size allows for a perfect, snug
fit. Before gluing the back panel into place, I glued some
2"x2" squares into the corners about 75mm from the rear
edge. These function not only as gusset plates to help
hold the shape of the box, but also as stops to prevent the two
boxes from being pushed together too far - something that could
make them very difficult to get apart.
Prior to installing the rear panel, I located the exact center
of it (by drawing an "X" on it) and cut a 100x100mm square hole,
centered in the panel, for mounting the emitter and detector
circuitry. While you are at it, also draw centered
horizontal and vertical lines on the back panel to aid in the
location of the its center - a helpful guide when installing the
electronics. Installing the rear panel, it was glued (with
the lines that you drew on it facing outwards) along both the
inside and outside, making the rear box quite sturdy.
Installing the lenses:
After all of the gluing has been completed, it is time to
install the lenses. Because vinyl Fresnel lenses are
extremely thin and flexible they need to be attached to
the glass in order to be stable enough to be optically
useful: Attaching the lenses was done simply with some
pieces of clear tape, centering the lens on the piece of
glass. When installing the lenses, make sure that they are
as flat as possible before applying the tape in order to provide
the best optical properties. It is also important that the
proper side face outwards: It was empirically determined
that for these lenses, the "grooved" side should face away from
While these lenses were precisely the same "long" dimension as
the glass, they were slightly narrower, leaving a small gap
along the "long" edge. For the receiver, I simply masked
this "un-lensed" portion with black electrical tape to minimize
ingress of stray light, but I didn't bother doing this with the
I noticed that these lenses weren't completely flat and tended
to bow outwards in the middle. The obvious solution to
this would be to sandwich the lens between two pieces of
glass. Unfortunately, I only bought three picture frames
and had only three pieces of glass. Reasoning that the
best-performing lens should be the one on the receiver, I
sandwiched only that lens.
The glass is held in the frames with dabs of hot-melt
glue: The enclosure was laid "front-down" so that the
glass was in the front and the "grooved" side of the lens was
also facing front. For the receiver, the second piece of
glass was laid on top of the lens, and the entire thing was
carefully weighted down (to flatten the lens between the two
panes) using a 7 amp-hour 12 volt lead-acid battery. With
the lens secured, the edges were dabbed in several places with
the thermoset glue. A bead was not run around the entire
perimeter, as this would have made disassembly - if needed -
much more difficult.
For the transmitter, because I didn't have another pane of glass
handy, so I simply cut a strip of foam core and laid it across
the lens edge-wise, gluing the ends of the strip to the
frame. This served to hold the center of the Fresnel lens
against the glass and removed most of the "bulge". (Note:
This strip may be seen in the bottom picture of Figure 2
in the left-hand lens as a horizontal bar.)
final assembly, a brief test was done to note the difference in
"beam quality" between a Fresnel lens sandwiched between two
panes of glass and one that was not. While the center of
the beam wasn't affected much, the periphery of the beam was
somewhat "cleaner" with the flattened Fresnel. It was also
determined at this time that simply holding the center of the
Fresnel against the glass fixed "most" of the degradation.
Because of this, I feel confident that for the transmitter, the
remaining "un-flatness" is will result in only a small amount of
degradation. (Again, if I'd had another piece of glass
handy, I would have put it in, but now that it is installed, I
won't bother adding one later.)
It should be mentioned again that these flexible "Sheet
Magnifier" Fresnel lenses are of rather poor optical
quality. In comparison with the rigid, molded Fresnel
lenses, they produce a rather "dirty" pattern - most notably a
somewhat "fuzzy" image and the tendency to produce a weak
"X"-shaped pattern of spurious light. It is mostly for
this reason that I didn't go through too much trouble to flatten
the the transmit lens.
On the outside of the front half the enclosure, I
drew a line at the location of the plane of the Fresnel
lens. This provided a handy reference for measurement of
the focal plane of the lens during the "rough focus" setup.
Mounting the electronics:
Top: A view of the rear panels of the
completed assembly, showing the panels that hold the
detector and emitter assemblies.
Upper middle: A close-up of the optical
receiver, the "repackaged" early prototype of the receiver
used for my other transceivers. Lower middle:
A close-up of the emitter assembly with the current
limiter. Bottom: A close-up of the
"business end" of the emitter, showing the Luxeon's die
through the secondary lens. (This was taken before
the current limiter was added.)
Click on an image for a larger version.
As mentioned, a 100mm x 100mm square hole was cut into the back
panel for mounting the electronics. The detector and
emitter assemblies are mounted on a plate made from a piece of
foam-core board that is somewhat bigger than the hole - about
150mm square or so. Onto these I drew both an "X" and a
cross - on both the front and back of this plate - to locate the
center as well as to provide alignment with the center-locating
lines that were drawn on the rear panel. The idea here is
to align the LED and the detector's photodiode to the exact
center of the mounting plate and, with the lines, be able to
mount the plate in the center of the enclosure's rear panel.
provides several views of the mounting of the
enclosure. The top picture shows the installed panels with
the electronics packages. These panels are held in place
with "grabber" (or drywall) screws. Note that because this
foam-core board is rather fragile, it takes screws with very
pointed tips and deep, aggressive threads to hold things into
place. Even so, these screws cannot hold into the board
material very strongly - but the panels should be very secure as
long as reasonable care is taken in handling of the
enclosure. In order to improve "hold" several layers of
foam board (and/or thin wood) could be glued together inside to
provide a stronger grip.
The middle picture shows the detector. This is the
original prototype version of the Version 3 optical receiver
mentioned on the Optical
on this site, and it was used
simply because it happened to be laying around on the
workbench. As can be seen from the picture, some extra
pieces of circuit board material were soldered into place to
provide mounting from the screws, as well as side-shields to
minimize RF and electric field ingress. Note that if this
unit is used near a transmitter site, further shielding will
probably be required! I was pleased to note that inside my
house, it was remarkably unaffected by nearby electromagnetic
The bottom picture shows the emitter assembly - and some
explanation is required here. My first experimentations
with the 3-watt Luxeons were with this unit - a Luxeon emitter (not
a star) epoxied to a small aluminum heat sink. I soon
noted that, for continuous operation, that the original, small
heatsink was inadequate. When mounting the emitter for
this project, I rummaged through my collection of heat sinks and
found a larger one (from the same computer monitor, I believe)
that had fins that would jam into each other and
interlock. I simply smeared some heat-sink compound on the
mating surfaces, pushed the two heatsinks together, and then put
dabs of high-temperature epoxy on the lot to hold them
together. Testing showed that there was excellent heat
transfer between the two, thus solving the "small heatsink"
To further facilitate mounting to the foam core board, I cut a
small piece of sheet aluminum and used it to spread the
distribution of force from the mounting screws (as well as
increase the heat sink area) to provide a solid mount. On
the rear of the panel, some small washers were also used to
distribute the force and prevent tearing through the
board. (I may put larger washers on the back panel,
- In series with the LED (a Luxeon 3-watt
emitter) is an LM317-based current limiter set to allow
only up to 2.25-2.5 amps of current to flow. This
prevents immediate damage to (or destruction of) the
emitter should the modulator malfunction and dump excess
current into the LED.
Matching the emitter to the lens:
- Philips is apparently phasing out the Luxeon I, III, and V
lines in favor of the lower-power Luxeon Rebel
devices. Since I have not used those other devices,
the techniques described here may not directly apply.
For the time being, however, the Luxeon III devices are
still available from various sources.
Once the emitter assembly was mounted, I checked the size of the
"circle of light" produced by the Lambertian pattern of the
Luxeon emitter at a distance of 310mm (the focal length) and
compared it to the size of the Fresnel lenses. This test -
along with one using the lenses themselves - showed that most of
the light was spilling out beyond the edges of the Fresnels,
resulting in a significant loss in available radiant energy and
meant that a secondary lens was needed.
It is normal practice to use a PCX (Plano ConveX) lens to change
the "spot size" but I had on hand some cheap, plastic DCX
(Double ConveX) lenses (20mm diameter with a 34.5mm focal
length) that I was dying to use. A quick test showed that
this tiny DCX lens was nearly as effective as a much larger,
heavier, and more expensive glass PCX lens - and it was obvious
that it would be much easier to mount the small, plastic lens.
The "mounting tube" was made by rolling some paper around
another tube (a 6LN8 - a pentode-triode combination to be
and taping it. I was then able to force the
plastic DCX lens into the end of the paper tube (which was
fractionally smaller in diameter than the lens itself) which
held it fairly securely. I then carefully applied some
clear, fast-setting epoxy at the boundary between the paper tube
and the DCX lens, taking care to keep as much of it off the
optical area of the lens as possible. After a few minutes
(and exposure to the warm heatsink of the powered-up LED) the
epoxy had set, so I carefully trimmed down the paper tube until
the "spot size" of the LED at 310mm approximately matched the
size of the Fresnel lens - something that occurred when the lens
was about 5mm away from the top of the Luxeon's dome. At
this point, the paper tube was epoxied (using the
high-temperature epoxy) to the small heatsink and centered over
the top of the Luxeon emitter. This technique is simple,
crude, yet effective and rugged.
I believe that significantly better efficiency (more
light from the LED being directed toward the lens) could have
been obtained if a different secondary lens was used.
Because this was a "quick 'n dirty" transceiver, its somewhat
inferior - but adequate - performance was the acceptable
tradeoff in order to quickly have a 2nd unit onhand for
Aligning the optics - emitter:
Alignment and focusing of the emitter was fairly straightforward
- especially after having done so on the previous
enclosure. During construction, I was careful to make the
picture frame parallel with the front of the enclosure as doing
so allows the use of a carpenter's square to determine the
pointing of the lens when a laser level is clamped to the
carpenter's square - see this page
for more details. In a nutshell, the use of the
square-laser combination allows one to determine precisely where
the lens is "pointed" - and one can easily verify that the
emitter is located within the center of the focus spot of the
After verifying alignment, the mounting plate is screwed to the
back panel and the beam is focused: Focusing is done
simply by sliding the two boxes in and out of each other.
While rough focus can be done indoors, provided that one has a
fairly long (30 feet or 10 meter) distance, this will only give
results. For final focusing, I took the
entire enclosure outside and used the side of a house on the
next street over (about 300 feet or 100 meters away) as a
target, focusing for the "sharpest" square pattern. It
should be noted that with the secondary lens, the focus is
somewhat "closer" to the lens than the focal length of the
Fresnel lens alone.
Once the precise focus was found, I marked it with the
silver-colored marking pen and then secured the position with
the four "grabber" screws - one on each side of the enclosure.
As the beam is focused, one should be able to
resolve the "square" shape of the Luxeon's emitter. If
good quality lenses are used the bond wire and connecting
electrodes should be visible in the focused image at
relatively short distances. Best focus is obtained when
this "square" pattern is at its sharpest - and this should be
done over a large a distance as possible. Considering
the accuracy of typical Fresnel lenses, a distance of about
300 feet (about 100 meters) is likely to be "close enough" to
infinity to achieve a good focus. With these somewhat
poor-quality "sheet magnifier" lenses, the square shape of the
Luxeon's emitter was somewhat blurred, but still generally
Aligning the optics - detector:
Focusing of the optics - especially the detector - is,
perhaps, the most awkward part of the construction of the entire
project. Unlike the emitter, there is no spot being cast
to show the precise alignment. It should be possible to
simply substitute an emitter -located in the precise position of
the photodiode- to accomplish this, but I have yet to do
so. Using the indoor test range, I was able to get
approximate focus - a setting that should be "fairly close" to
optimal. Through previous experience, I noted that if one
focus in the indoor test range, the proper focus for an infinite
distance is one that is slightly closer to the lens.
As for the other enclosure, I checked receiver focus and
sensitivity using a dim, diffuse, red LED attached to a wall
about 33 feet (10 meters) away. This LED was modulated by
a signal generator and by varying the drive, the intensity of
the LED could be adjusted from a "dim, but easily visible"
setting down to a "I can't see it unless I'm right next to
it." As with the other enclosure, an easily-audible (and
speech-capable) signal was obtained from an LED that was not
readily visible at the full 33 foot distance.
Later, the enclosure was dragged outside. As expected, the
waxing moon was more than enough to completely saturate the
receiver, and fairly distant streetlights (and flying aircraft)
were clearly audible.
After completing this enclosure, the focusing of the
transmitter and receiver was re-checked using some simple
on a test range and both the receiver and
transmitter were found to be within a fraction of a dB of
their optimal settings in terms of focus and parallax.
Marrying the two enclosure:
Top: Top view of the ganged-together
enclosures, showing the connecting plates.
Bottom: Another side view, showing more
details of the enclosure.
Click on either image for a larger version.
The experiences of the Australian group (Chris, VK3AML and Mike,
VK7MJ) show that it is most convenient to gang two lenses
together and precisely align them, thus eliminating the need to
aim more than one box. In this case, two separate boxes
were built - mainly because it was easiest to do so - but there
was still the matter of putting the two together and making sure
that they were in alignment.
First, the two enclosures are laid next to each other and just
the two rear screws holding in the large, square plate on the
top and bottom are inserted: This will loosely couple the
two sections together.
As can be seen in Figure 4
some scrap pieces of foam
core board were used as "plates" to tie the two together.
As with the rear plates and focus adjustments, "grabber" screws
were used to secure the plate. On both the top and bottom,
two plates were used: A larger, square plate near the back
(the one with "TOP" written on it, both normally and
upside-down) and a smaller plate at the front of the
enclosure: Two similar plates were used on the bottom side
as well. In addition to plates on the top and bottom,
there are two small plates (or straps) mounted on the rear panel
to further improve rigidity.
Because the entire purpose of these plates is to gang the boxes
together and ensure stability, it is very important that the two
boxes actually be pointed in the same direction and are properly
aligned. Note that due to parallax, this must be done with
a special target that takes into account the distance between
the two lenses. As the distance approaches infinity, the
beamwidth of the two lenses will cause overlap, effectively
negating the parallax.
The task of alignment was done in a way similar to that done
with the other
: A paper target was attached to the far
wall (just above the LED used for testing the receiver) and
marks were made on the paper: One mark, representing the
center of the transmitter beam, was spaced the same distance
from the receiver-test LED as the centers of the two lenses,
while two other marks placed at the distance from the the center
of the transmit lens and the boresight of the laser level to
locate both azimuth and elevation of the enclosure.
First, the receiver was carefully aligned on the test LED.
Further verification of the receiver's aiming was done by
pointing a laser pointer at the test LED: A "hiss" from
the laser pointer was noted (along with some receiver desense)
and by moving the laser pointer side-to-side and up and down
with respect to the test LED, the proper aiming of the receiver
could be verified.
Once the receiver was aimed, it was necessary to be very careful
to avoid disturbing the position of the receive portion of the
enclosure. At this point, the emitter is activated and the
transmit enclosure is shimmed and shifted as necessary to get
the center of the transmitter's beam to line up with the marks
on the paper. As proper alignment is achieved, the other
plates may be added to "lock" things into place.
After installing the plates, re-check the alignment once again
by peaking the receiver on the LED with the test signal and then
verifying that the center of the transmit beam was in the center
of its respective target. If minor adjustments are to be
done, the screws holding mounting plate or the emitter assembly
may be removed, the position of the emitter adjusted slightly,
and the screws reinstalled through new holes. Note that if
this sort of adjustment is required, it is easiest to do so with
the transmitter - after verifying peaking of the receiver -
because the transmit beam can be easily seen...
Adding a septa:
As mentioned on the page "Optical
for the Amateur" (by Chris Long, VK3AML)
device called a "Septa" may be used to reduce stray light such
as that encountered in an urban environment. The structure
of a septa is simply that of a series of parallel tubes that
only allow light from the direction of the source to enter the
detector and it would, ideally, be constructed of thin, opaque
material painted flat black to minimize reflection: A
series of thin metal plates is often described as it would cause
minimal blocking of the desired light. Practically
speaking, there is a limit on how narrow the viewing angle of
the septa will allow and this angle is directly related to how
long the tubes are and how small each of these tubes might be.
Top: The septa installed on the front of the
Middle: The rear ("lens side") of the septa.
Bottom: A simple means of adjusting the
elevation constructed by Ron, K7RJ.
Click on either image for a larger version.
Because the optical quality of these sheet protectors is rather
poor, they are quite susceptible to light from angles far
removed from the desired direction. Because of practical
reasons, the length of the septa (and the number of dividers)
was limited. Also, because I was using scrap foam core
board left over from the construction of the enclosure, I had to
take into account the limited amount of material onhand and the
fact that the thickness of the sheets would block some of the
The installation of the septa is rather simple: Because
the picture frame is set back by several centimeters (to protect
the glass, mostly) I constructed the septa to simply slide in,
being held in place by friction. The structure of the
septa is simply a series of rectangular tubes created by
dividers cut from foam core board, all held into place with "hot
melt" (thermoset) glue.
This septa greatly narrows the view of the lens while blocking,
perhaps, 10-15% of the light - roughly 1dB - and weighs about 12
ounces (0.3 kg.)
Was the septa worth the trouble?
Despite the poorer quality of these "page magnifier" Fresnel
lenses, the septa has not
been found to be necessary -
even in an urban environment, in the presence of city lights,
although its addition would certainly reduce the hum from
them. While it was tried once, the minor improvement was
not judged to outweigh its awkwardness, so it has never been
used since, but more benefit would probably have been derived
had there been nearby, strong light sources.
An optical attenuator:
Another accessory (not yet shown) is an optical attenuator plate
. Designed to be used in an urban environment over fairly
short distances, this "optical attenuator" is simply a piece of
cardboard with holes in it. This attenuator plate is
placed against the Fresnel lens, blocking 90-95% of the incoming
light. Because of the holes are distributed over the same
area as the Fresnel lens, the capture area of the lens remains
about the same and most of the reduction of scintillation
provided by the larger area is maintained.
Because of the relatively high light pollution encountered in an
urban environment, the limiting factor in the sensitivity of the
receiver is not
the sensitivity of the detector
itself, but the amount of "dilution" caused by other light
sources. When this attenuator is used with a septa, the
beamwidth of the septa itself is narrowed somewhat by the
holes: If two matching attenuator plates are used with a
septa (one at the lens and another in front of the septa) then
the beamwidth may be narrowed even more.
What this means is that if the background light is well above
the noise floor of the receiver, dynamic range of the receiver
itself can be compromised by this extra light. Also, at
relatively short distances, the full sensitivity of the receiver
may not be required and the attenuation of "background" noise
sources might be welcome.
Like the septa, the optical attenuator was only tried once as an
experiment and is not among our normal compliment of equipment.
Fine-tuning the elevation:
For our first
, I used the wooden
while Ron and Gordon took this
enclosure to a point across the valley. In setting up they
noticed two problems that complicated the precise aiming of the
enclosure and keeping the aim steady:
- The bottom of the enclosure was not flat and when set on a
flat surface, it tended to rock back and forth.
Fortunately, Ron brought some cedar shims and was able to
wedge the enclosure so that it could be stabilized.
The fact that it was being set up on a rough, stone wall
made the setup and aiming even trickier.
- While azimuth adjustment was accomplished simply by
turning the box left and right, adjusting the elevation was
very difficult, but accomplished with the help of the cedar
shims. Even when aimed, no-one dared touch the unit
out of fear of disturbing the aim as it was very easily
Prior to our second
Ron threw together a simple jig that
greatly simplified the task of obtaining a stable platform as
well as aiming seen in the bottom picture of Figure 5
Two pieces of plywood hinged together with a bolt to set the
separation: By using a strap to hold the enclosure to this
jig, the elevation could be fine-tuned simply by adjusting a
bolt. By this time I'd also added some extra pieces to the
bottom of the enclosure so that it would be stable on a flat
surface - something that helped tremendously. Another help
was that Ron brought along a portable table to provide a stable
and flat surface on which the entire assembly could be placed.
Spot Quality comparisons:
As was expected, the higher-quality optical acrylic Fresnel lens
used in the wooden
"spots" than the vinyl "full-page magnifier" lenses.
During testing, I decided to do a direct comparison between the
Top: Spot produced by the "first" (wooden)
enclosure with the high-quality acrylic Fresnel lens.
Bottom: Spot produced by the "second"
(posterboard) enclosure with the vinyl "page magnifier"
Both of these images have been converted to grayscale for
easier comparison, and were taken using identical focal
length and exposure settings and have been identically
processed to show relative spot size, brightness, and beam
Click on either image for a larger version.
Ideally, one would have done everything possible to make the two
tests equal, but there were some unavoidable differences that
may affect the accuracy of the direct comparison of intensity:
- It would have been ideal to have used exactly the same LED
for both tests, but this would have been difficult.
Instead, the same LED current was used (1.1 amps) for each
test. Because the two LEDs are believed to from the
same production run, it is likely that their optical outputs
are very similar, given the same operating current.
- Different secondary optics were used. Ideally, the
same set of optics would have been used for both, but this
wasn't very practical. In the case of the wooden box,
a large (48.5mm diameter with a 51mm focal length) glass PCX
(Plano-ConveX) lens was used to
optimally focus the LED's output onto the Fresnel lens,
while for the second "cheap" enclosure (the one using the
Vinyl page magnifiers) a 20mm diameter acrylic DCX (Double-ConveX)
lens with a 34.5mm focal length. It is likely that in
this second case, there is some inefficiency in "photon
transfer" because of this non-ideal lens.
- Precise focusing of the secondary lens:
- For the first (wooden) enclosure, two different focus
points were manipulated: The spacing of the
secondary lens to the LED, and (independently) the spacing
of the assembly consisting of the LED and secondary lens
from the Fresnel lens. For the first part, the
spacing between the LED and the secondary lens affected
how much light was reaching the Fresnel lens: If the
secondary lens is too close to the LED, the back of the
Fresnel is "overilluminated" with much of the light
spilling out past the sides, while if the distance between
the LED and secondary lens is too great, the Fresnel is
"underilluminated" with only a smaller portion of the
center of the Fresnel getting light - something that will
also reduce the effective aperture of the Fresnel and
increase the severity of scintillation. The latter
case has another downside: As the spacing between
the LED and secondary lens is increased, the virtual spot
size of the LED also increases, which also increases beam
divergence. For best results (e.g. maximum luminous
flux at a distance) a compromise must be reached between
proper illumination of the Fresnel lens and virtual spot
size. Essentially, one adjusts both parameters until
maximum flux at the distant target is achieved.
- For the second enclosure (the one made of posterboard -
the very one described above, on this page) the
adjustments were rougher, going mostly by "eyeball."
I have little doubt that a couple of dB of improvement in
distant flux density could have been achieved - but since
it is considered to be only a "low performance" device
that was thrown together quickly, I have no plans to try
further tweaking to make it work better.
- It should be noted that both boxes were
carefully focused (e.g. the distance between the
LED/secondary lens and the Fresnel) to achieve the best
In direct comparisons, it was determined that, given the same
LED current, the "cheap enclosure" had about 38% of the
luminous flux of the "first" enclosure. It is believed
that much of this is due to the relative inefficiency of the
secondary lens and in its role of accumulating and directing
light from the LED to the primary (Fresnel) lens. I expect
that had I used a larger-diameter, "stronger" lens, more light
could have been gathered from the LED and directed toward the
Of more interest was the "quality" of the spots that the two
boxes produced even when optimally focused. As can be seen
from Figure 6
the "main spot" (the brighter
"square" portion) is almost identical in size, but the lower
spot (from the page-magnifier Fresnel lenses) is not only
dimmer, but more light is spread out beyond the main beam
perimeter. It should be pointed out that this effect is
apparent not only from the pictures, but is arguably more
visible when view with one's own eyes. Although not
visible in the picture, there is a sort of faint "X" pattern
weakly emitted from the page-magnifier lenses that seemed to be
totally absent from the higher-quality acrylic lenses.
Again, it should be noted that in the case of the "fuzziness" of
the spot of the vinyl page-magnifier lens, this peripheral
energy could not
be removed by adjusting focus. In
the case of the images in Figure 6, the spot was projected onto
the surface of a 33 foot (10 meter) diameter white satellite
dish that was about 200 feet (60 meters) distant. At this
distance, the beams had "mostly" collimated, but were very
slightly out of focus as compared to the normal test distance
that I'd been using of about 525 feet (160 meters.) The
satellite dish was chosen because it was the only relatively
large, flat, white surface that was available at a reasonable
distance. Note: One can see the subreflector
assembly in the bottom of these pictures, along with some
lines from the sections of the main reflector as well.
- As mentioned before, the optical quality of these "full
sheet magnifiers" is much poorer than that of the thicker,
molded Fresnel lenses. It would appear that while
the main beam is of similar intensity, they suffer from a
larger amount of weak, spurious side responses.
Fortunately, these contain relatively little of the total
energy, but on receive, they could result in a response to
off-axis light sources that could prove to be a source of
interference or desensitization.
Weight of the enclosure:
- With the receiver and emitter installed, the entire
enclosure weighs in at about 6-7 pounds (approx. 3kg).
Safety concerns when in direct sunlight:
Return to the KA7OEI Optical
Communications Index page.
communications, light beam, lightbeam, laser beam,
modulated light, optical communications,
through-the-air optical communications, FSO
communications, Free-Space Optical communications,
LED communications, laser communications, LED,
laser, light-emitting diode, lens, fresnel, fresnel
lens, photodiode, photomultiplier, PMT,
phototransistor, laser tube, laser diode, high power
LED, luxeon, cree, phlatlight, lumileds, modulator,
- Because the electronics are permanently installed and
the material used for construction is so lightweight,
instant damage could be done if the lens is aimed anywhere
near the sun - so don't!
2007-2013, Last update: 20130722