Point Source Projector

In my continuing quest for a brighter cheaper simpler projector for the graffiti projector concept, I realized that going lensless is the only way to really achieve this.  Lenses are in my experience cheap when they don't meet your specifications and expensive when they do. On top of that you have to build a precise structure to keep them aligned and usually need more than one for good image quality. I first encountered the idea of a point source projector when I read about Jim Sanborn's Cyrillic Projector you can read more about it here (the pieces A Comma and Lux use a similar technique).

The most basic way to achieve this is to get a very small bright light source and shine it through a cut out. This works well and gives the theoretically maximum brightness at the cost of having no control over throw length.  The closer the projection surface to the projector the brighter and smaller the image. You can't get a bright image very far away as the brightness is governed by the inverse square law. While this is not a great characteristic it is tolerable for the short throw applications I had in mind. The real problem with cutouts is the typical stenciling problem of islands created when you have concentric rings of positive and negative space as seen above.

You can solve this by using a grid of separate half tone dots or using a simplified image with bridges as in a typical stencil, like the A above.  I didn't want to do that much post processing of images, so instead I look to another familiar print making technique screen printing.  Screen printing avoids the problems of islands by suspending the masked zones in the permeable matrix of the screen.  After thinking about this for a bit I wondered if it might be possible to make a plexiglass mirror selectively reflective, by removing the reflective backing in places and relaying on the acrylic to act as the matrix.  Sounds simple but I wasn't at all sure it would work.  I could print some resist on the back and sand blast it, or laser etch it and risk warping and melting.  In the end I went for laser etching, what I found was that as long as the image was not to detailed the mirror would not warp or melt significantly.

Here was the first attempt (I believe it was a picture of Hamilton) turned to be too much detail and the mirror backing oxidized and the acrylic warped.

Here was second one I tried it didn't warp but the mirror back didn't get completely clean in the negative space either.

To use the projector you just have to shine your point source at the etched mirror at a shallow angle.
I used loc-line coolant hose to allow easy adjustment of the light sources position, and made a simple wooden platform to hold the mirror slide and the led driver.  The led I used was the brightest and smallest one I could find, a Cree xp-g lamp it has a 3mm emitter and driven at .75 to 1A can throw a clearly visible 5' image from about 10' in dim light.

Here's a detail of the led and its aluminum heat sink cube.

Here is a detail of the mirror slide projection. It achieves finer detail than a cutout, but still has its defects, you can see patches of oxidation at the bottom of the letters and in general the image can't achieve the crispness of a stencil projection. What is Dec, unremarkably I needed some short words and happened to be making it last December so decided to use Dec in the most decorative gothic font I could find.  
                  

Monocular

I was asked by the Exploratorium to design a minimalist telescope that might be used in the new building (the Exploratorium is moving from the Palace of Fine Arts to Pier 15) . This was my first attempt, an open framed 3.5x Galilean telescope. I attached the focusing screw to the objective lens not the eye piece (which is typical) to further simplify the design. Galilean telescopes have the advantage of producing an upright image with only 2 lenses but suffer from high distortion above 3x.

Looking through the scope, the spherical distortion and chromatic aberrations are notable, especially if the scope is poorly focused. I ended up using 1/4''-20 threaded rod for the focus adjuster, this was way too fine for accurate focus control as it was hard to tell if you were getting more or less focused in less then 5 or 10 turns of the knob.

Given the image quality problems with the Galilean scope I made this "looks like" model for a monocular using Keplerean optics using 8020 aluminum stock and laser cut cardboard. A Kepler style scope produces a clear but inverted image, so the model included a pentagonal case that was scaled to hold erecting prisms to un-invert the image. To solve the slow focus problem I modeled a transverse focus knob like a microscope capable of moving the objective lens quickly but precisely.

Tracing Camera Prototype

The Exploratorium asked me to prototype a portable camera obscura for the new building to be used as a tool to make observations and draw the local landscape.  It is a very optically simple device consisting of a box with a lens mounted in front of a 45 degree mirror inside to flip the image right side up and a piece of glass on top onto which tracing paper can be placed to receive the image.     

Originally the tracing camera was mounted on a sturdy aluminum camera tripod, this proved usable but not stable enough to make complicated drawings easily. In trying to design the tripod to replace the aluminum one, I looked at a lot of heavy designs used for survey equipment.  The common thread between all these designs was the triangular multi-element legs.  As I wanted to avoid cutting 2 or 3 pieces precisely for each leg I decided to simply bend them together instead. To simulate how the plywood might behave I made the preliminary model with popsicle sticks to find out how much more rigid it was than a single element.  These legs afforded me a very stable platform to draw on which resisted shaking and twisting very well.

    

The Tracing camera in use at Pier 3 in San Francisco.

Here you can see the camera's three controls, the lens barrel is coarsely threaded and can be turned to focus the camera. The upper knob is a tilt lock and the lower is a pan lock. The lens barrel assembly is machined pvc pipe fitting and delrin.   

Paper holder tripod is a reduction of the original camera tripod, to make it lighter and more easily collapsable.

The legs, joint plate and desk top (the indentations for the pencils and sharpener) are made on a cnc router and the rest of the wooden parts were made by hand.

The cardboard shroud blocks ambient light from the front of the camera casting a shadow on the tracing paper making the image appear bright.

photo by Gayle Laird, Exploratorium

Drawing detail looking at downtown from Pier 3 in San Francisco.

Another drawing detail looking out at docked boats at Pier 3. 

Alidade prototype

An Alidade is a device to locate a distant object on a map by line of sight using the 2 sights on the rotating bezel.  This working prototype for the Exploratorium made of laser cut cardboard is based loosely on the construction of the Osborne Fire Finder (a type of alidade) invented around the turn of the last century to aid in locating forest fires.  As the operation of the my model is essentially the same as an Osborne Fire Finder I suggest you watch this video on fire finder operation to learn more about how it's used.  To make it work you need a map with the alidade's location at its center, and to align the map and the compass dial (not seen here) to reality.  


Here you can see the Alidade with the map platen removed, the map platform in the middle where the map platen will be velcroed on and the rotating bezel is on the outside.

With the bezel removed you can see the graduated compass dial.  This was originally etched right into the corrugated cardboard (as seen above) but was replaced with a layer of chip board (think cereal box material) as it etches much more legibly.  

Here you can see the bezel removed from the base of the alidade. In front you can see the bearing window and then the two sights with the range rule between them.  The far sight has as single vertical thread to place over  the object you're sighting like a cross hair.  

Here's the range rule, in this case its calibrated for the particular Bay map seen above, but using a simple ruler and posting the scale as was done with the Osborne Fire Finder would be a good strategy if you're likely to use maps of different scales.  With a permanent range rule like this all maps would have to conform to this scale to be useful. Which direction I'll go with this is not worked out yet given the museum has yet to choose a map or maps for the eventual exhibit.



Here's a detail of the bearing window.  When you set the alidade up you align the map, reality and the compass dial, a thread stretched over window will indicate the exact direction you looking in as long as the alidade is not moved.

I've sighted where the Bay Bridge goes into Yerba Buena Island, from Pier 3 in San Francisco.

Looking down at the map you can see the range rule, which represents the line of sight, passes right over the mouth of the Yerba Buena Island tunnel.

Rangefinder #2

photo by Gayle Laird, Exploratorium

Rangefinder #2 is based off an interwar British model by Barr and Stroud. The main difference between this model and the first one is that it does not use mirrors only prism wedges and penta prisms to manipulate the lines of sight. I was lucky enough to find and download a training manual for this particular device and reverse engineer it. Unfortunately, in the interim I seem to have lost the particular web address it came from. Searching for "Barr and Stroud rangefinder" images will find several diagrams outlining its function, but I will try to post a link to the original documents I used once I find them. Generally speaking you look into the center eyepiece via an integrator prism (which I got from a old 3 lcd projection television) you see out through 2 objective lenses and then 2 penta prisms at the ends of the rangefinder, which direct your vision forward. There are 2 prism wedges between the objective lenses and the integrator in the prism. The one on the right of the photo is for calibration and the one on the left is for range finding. Other than the above mentioned optical components the range finder consists of 8020 aluminum stock, machined delrin and laser cut acrylic.

photo by Gayle Laird, Exploratorium

Here you can see the range rule on the top of the sliding prism. When used the two channels of the Rangefinder appear red (the adjustable side) and blue (the fixed side) which makes its somewhat easier to orient yourself when looking through the Rangefinder. This is because the integrator prism had filters for red, green and blue for the corresponding lcds. I only used the 2 side channels, the green channel faces forward and is covered by cardboard. In general this set up was way more accurate than the first version. It was able to get the same reading over and over on objects under 1000 yards, but I found that it is very sensitive to vibration so it might not give the same readings after being moved. I continue to try and improve the design.

A detail of the eye piece and range rule

What you see looking through the eyepiece with coincidence not yet been achieved

Rangefinder #1

This is a Coincidence Range Finder prototype I built for the Exploratorium late last year. Its essentially an optical trigonometry tool which allows you to measure the length to a distant object based on the distance between the pods and the angle of deflection needed to see the same thing from both pods. When the range finder is dialed in you will see 2 perfectly superimposed images of the same object, meaning you set the angle of the mirror in the control pod so it sees what is directly in front of the telescope pod. For a general overview of the theory behind Coincidence Range Rinders work take a look at this wikipedia article and the diagrams at the bottom of this page.

This is the inside of the telescope beam splitter pod. I mounted an inexpensive 10x monocular in some pvc pipe fittings. In front of the monocular I mounted a half height adjustable first surface mirror to receive light from the control pod window and the window directly in front of the monocular simultaneously.

This is the inside of the control pod, a larger first surface mirror collects light from the closed window and sends it out the removed one to the telescope pod. The mirror is adjustable via a knob driven screw which also drives a gear train to the range gauge.

Here is the back of the control pod. Note that the range gauge window is made to open as the ranges were written on by hand using a laser range finder for this first prototype.

Looking into the control pod mirror from the front you can see the light path back to the beam splitter and into the telescope. I used speed rail to connect the 2 pods for easy adjustment and disassembly.

While you could achieve coincidence (see 2 super imposed images) this range finder only proved vaguely accurate, mainly because the range gauge was driven by the control knob and not directly on the position of mirror. The gear train tended to slip disconnecting the mirror position thus the angle of coincidence from the read out on the gauge. In response to the problem of inconsistent accuracy I decided to look at some military range finder designs, which lead to the creation of Rangefinder #2.

Cardboard Scooter for the Exploratorium

The Exploratorium was having monthly material themed events earlier in the year, and when cardboard came around I decided I should try to push the limits of this material. So inspired by Phil Bridge's cardboard bicycle, I decided a kick scooter would be an interesting follow up on the idea corrugated transportation. After a lot of experimentation with laminating cardboard with different corrugation paterns to form beams, I took what I thought was half again as much cardboard to hold my weight (175lb) the scooter was born:

While its far from elegant, it was surprisingly durable surviving for about 3 months of on and off use by myself, other staff and museum visitors. Composed of entirely laminated laser cut cardboard with the exception of the wheels, axles and the steering hinge and hinge pins, the scooter only weighed about 6 pounds, and could carry people up to 200lbs.

The scooter is supported by two beams 3'' tall and eight layers thick that runs from the steering hinge to the rear axle with the corrugation running long ways. The deck is five layers thick, with the corrugation running at alternating 90 degree angles to prevent denting. The front wheel supports were again eight layers thick with ten layers in between to hold the hinge.

Here's a detail of the front end: ten aluminum hinge pins were used to distribute the weight of the rider to the cardboard and nylon bushings were added around the axles to spread out the weight of the rider to a larger amount of cardboard. The use of a steel steering hinge ultimately proved to be the scooter's downfall. After the first few weeks of heavy use the hinge plate started to slide around on the pins compressing the cardboard next to it and eventually making the front end very wobbly. The structural integrity of the board itself was actually very good, as it became clear that it was becoming overly wobbly I actually tried jumping on it lightly and did not break the cardboard beams. As the over all design seems to have worked I plan to make a sleeker second attempt abandoning the hinge for a caster based steering method.