A "Cheap" Optical Transceiver lens assembly

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 (173km) path!
Figure 1:
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.
Some of
                    the raw materials used in assembly

List of materials:

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!
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.

Comment:
Unfortunately, I did not take pictures of the enclosure in various states of construction - sorry.

Assembly - Front half:

Notes:
Figure 2:
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" portions.
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 two panes.
Click on an image for a larger view.
Close-up
                    view of the recieve lense
Close-up
                    of the sliding joint between the front and rear
                    portions of the enclosure
The front of the two ganged-together
                        enclosures.

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 250mm.

Comment:
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 each other!
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 the 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 the emitter/detector.

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 transmitter.

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.)  Prior to 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.

Comment:
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:

Figure 3:
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.
View of
                    the rear panel, showing the two mounting plates
A
                    close-up view of the detector board
Close-up
                    of the emitter assembly
A
                    close-up of the "business end" of the
                    emitter and secondary lens

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.

Figure 3 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 Receivers page 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 fields.

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" problem.

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, though...)

Comments:

Matching the emitter to the lens:

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 precise) 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.

Comment:
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 testing.

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 lens.

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 approximate 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.

Comment:
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 recognizable.

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.

Comment:
After completing this enclosure, the focusing of the transmitter and receiver was re-checked using some simple equipment 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:
Figure 4:
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.
Top view
                    of the enclosure, showing how the two are connected
                    to each other
Side
                    view of the enclosure - also showing some details of
                    how the two enclosure are tied together

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 enclosure:  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 Communications for the Amateur" (by Chris Long, VK3AML) a 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.
Figure 5:
Top:  The septa installed on the front of the enclosure.
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.
The
                    septa installed on the enclosure
A
                    rear view of the scepta showing all of the dividers
Elevation fine-tuning for the "cheap"
                    enclosure

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 light.

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 optical contact, I used the wooden enclosure 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:
Prior to our second test 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 ("first") enclosure produced better-quality "spots" than the vinyl "full-page magnifier" lenses.  During testing, I decided to do a direct comparison between the two.
Figure 6:
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" lens.
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 containment.
Click on either image for a larger version.
Spot produced by the first (wooden) enclosure
Spot produced by the second (posterboard)
                    enclosure

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:
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 Fresnel.

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.



Comments:
Optical quality:
  • 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:
  • 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!
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Keywords:  Lightbeam 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, detector
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