Sunday, April 15, 2012

Cardboard Box Camera Obscure

Materials :
• Small cardboard box at least 12x12x12'' we used a Home Depot "Small" size box (12x12x16'') as our lenses focal length was about 12''
• 1/2'' plywood or some similar stiff material
• 73mm lens +2.5 or +3, we got ours from All Electronics this is the same plastic meniscus lenses we use in our cardboard telescopes
• 3'' id cardboard tube
• Foam core
• Black acrylic paint
• 5x7'' plex mirror
• Wood screws

• Hot glue gun
• Utility knife
• Drill with bits for pilot holes
• Screw drive/ screw drive bit
• Ruler
• Saw

Recently we decide to do some more light experiments and came up with something that uses the same principle the Camera Projector on a larger scale.  However, unlike the Camera Projector it is designed to allow you to draw the image created by the camera.  While not practical to make with students it is intriguing demonstration piece that more clearly illustrates its optical mechanics than the smaller camera projector, and it allows you to create photo realistic drawings by tracing the image.

 The optical system is basically a overhead projector backwards . Instead of light being sent through a transparency to a lens and mirror to be projected on a wall, light is collected by the mirror sent through a lens (in the cardboard tube) and then projected where the original transparency would be. In order to make the projected image visible we added a box painted black inside under the mirror lens assembly to block ambient light in the room.

Here are the unassembled parts, the base plate is about 5 x 5 3/4'' with a 3 1/4'' hole (cut with a hole saw) in the center to accommodate the lens holder tube. The mirror holder pieces are 5 3/4'' wide with a 45 degree angel cut made 2'' above the base. The large parts are 1/2'' plywood and the 2 pieces of 1/4'' ply  will function as rails to hold the mirror and are about 1x7''.

Early on we ran into one of the shortfalls of this design, as the mirror sits between the mirror holders, the base plate and mirror have to be cut straight and exactly the same width.  As you can see we were a little off and had to hot glue some thick mat board on the side of the base plate to make up the difference. An easy way around this would be to simply get an oversized piece of mirror and glue it directly to the top of the mirror holders.  Alternatively you could eliminate the mirror holders all together and just add a hinge and another piece of wood and hold the mirror exactly like a overhead projector would.  The down side of this is you would need a very stiff hinge or some way of holding the mirror at 45 degrees when the camera is in use.

We went ahead and hot glued the sides on so we could mark and drill pilot holes (1'' in and 1/4'' up) for the screws that would provide the real structure.

Here's the mirror secured with a couple of 1'' wood screws.

Next we glued in the rails about 1/4'' from the top of the mirror holders diagonals, and generously glued the mirror on top of them.

To assemble the lens holder tube we used the same technique as the cardboard telescope. We cut 4 3/4''x3/4'' square pieces of foam core and glued them inside the tube using a 3/4''x1 3/4'' pieces of foam core to space them evenly from the lip of the tube.

After the foam core squares were glued in we dropped in the lens and then glued the long spacers in over the lens to hold it in place. Optionally, you can paint the inside of the tube black before adding the lens as this might improve the image quality be preventing glare.

The finished lens mirror assembly looks like this.  The cardboard tube has a pretty tight fit in our prototype, but we added a large rubber band just in case it loosened up with use.  If your hole is a little over sized adding a larger rubber band directly above and below the base plate can help keep the lens holder in place while allowing it to move in order to focus the image while not letting it fall out.

In order to prepare the box we drew a 3 1/4'' circle a few inches back from dead center on the side of the box (the top of the box is facing us) and cut it out with a utility knife.

 We added a piece of scrap mat board on the inside of the box and then painted the whole thing black to darken the projection area.

Using the camera:

By placing a piece of white paper in box under the lens mirror assembly you can get an image of whatever is in front of the camera.  The unfolded lid of the box works to effectively protect the image from ambient light. Fine focus can be achieved by moving the lens holder up and down in the base plate and course focus by propping up the whole assembly like we did here.  Note that to get a right side up image you have to point the mirror lens assembly behind you, but for the purpose of drawing, observing this is not really necessary and may let too much light into the box.

Here was the initial image quality achieved, legible, but not great. In order to improve it you need to add a aperture to restrict the amount of light that gets to the lens.  While this makes the image dimmer it restricts light from passing through the edges of the lens which bends light at a slightly different rate than the middle making the image blurry. We ended up recycling the round piece of cardboard cut from the top of the box by cutting a hole about a third of the diameter in the middle and placing it on top of the lens holder tube below.

Even with a crudely cut hole the image quality is notably improved. For the best image quality there is an optimal aperture size and an optimal distance from the aperture to the lens, the "Dummy Camera" article on this page describes these ratios and has a few other useful diagrams for building a camera obscure. For this camera the optimal aperture size is somewhere between .75'' and 1''.  

Check back in the future, in the spirit of the Camera projector we're planning to make another version of this which will hopefully function as an opaque projector.

PVC Double Reed

• 1/2'' pvc pipe (6''-2' per instrument)
• Soda bottles (for the horn bell)
• Straws of various sizes
• Craft foam sheets
• Solid wire or tape
• Acetate sheet (optional)  note this can generally be purchased in large pieces more cheaply at art supply stores rather than office supply stores.

• Drill with various bits for finger holes and straws, jumbo straws are 5/16'' (optional)
• A small saw or pvc pipe cutter
• Utility knife
• Sand paper

After many years of making Straw oboes, it struck me that there has to be a way to improve the quality of the sound and make the instrument more durable. Looking at European Oboe construction I realized it would not be a particularly easy instrument to emulate. The first attempt to make a full oboe was a long piece of 1/2'' PVC with typical straw Oboe reed inserted and a cut off 2 liter bottle as a horn bell.  The result was a loud stadium horn like noise maker which could be prompted to make all sorts of interesting timbres by biting down on the reed in different ways. This was interesting, but not what I had in mind, it was just a louder less predictable straw Oboe, not a smoother sounding one.

So instead of looking to a European Oboe, I started looking at some of the Asian double read instruments. The Tibetan Gyaling, the Indian Shehnai and the Korean Taepyeongso have more achievable form factors and their timbres are closer to a refined version of a straw oboe.  While reading about these instruments I ran into the wikipedia article on the Taepyeongso which noted that drinking straws were often used as a replacement for the traditional reeds and went on to specify how the straws were modified to make suitable reeds. The main differences between the Taepyeongso straw reed and a straw oboe was that the straw was only cut shallowly and the resulting tongues were sanded to make them thinner and more flexible to counter there short length.  Sanding each side 50-100 times (depending on the thickness of the straw) with 220 grit sand paper produced good results. This takes a lot less time than you think and really changes the timbre produced for the better.

Here are a few straw reeds we tried out next to the new Taepyeongso like instrument body.  Small straws as well as jumbo and bubble tea all worked well if they were sanded.  The Jumbo straws could be made to fit snugly into a PVC cap with a 5/16'' hole drilled in it. Bubble tea straws can be used by wrapping them with craft foam and tying it on with wire or just tape. Normal drinking straws work with the PVC cap if wrapped with a little masking tape. Wrapping enough craft foam onto a jumbo or drinking straw with tape will allow it to fit straight in the pipe without a cap. In general cutting off as little as possible from the straw by cutting the corners slightly or rounding the ends (after flattening) and then sanding both sides evenly gave us the best results.

Things to do and notice:
What we got was an instrument with a much smoother sound than a straw oboe but not as interesting as some of the traditional Asian instruments. The improvement in sound is due to the better reeds. However, this design does not have the same conical internal profile like the original instruments it is just a straight pipe so the sound produced is still inferior to the originals.  The straight pipe also effects finger hole size, small holes are enough to change pitch of the sound at the top of the instrument, but larger holes are necessary to change the pitch near the horn bell in longer instruments. The implication of this is that over sized or under sized holes are a way to get small or larger jumps in pitch beyond what their location dictates, so you could get very accurate tuning by adjusting hole size to play written music.

You can also avoid making holes altogether by adding a slide made of rolled acetate (overhead transparencies) and taping it over the pvc pipe, to create a reed trombone. Whether you use hole or not you can adjust the timbre from a smoother tone to a warbling character by squeezing the straw as it is played.  The reed is very sensitive not only does how hard you squeeze make a difference,  but where you press on it with your teeth or lips also makes a great difference in the character of the sound produced. Using different types of bottles for the the horn bell will also change the character of the sound as seen in the Tube Trumpet.