Scratch Holograms!

Have you ever noticed ghostly blobs floating above your black car hood on a sunny day? These blobs are scratch holograms! They appear when the sun is reflecting off circular scratches from activities like washing, polishing, or drying your car. They seem to float because each of your eyes sees the suns reflection at a different point, creating a 3D stereo pair.

'This Instructable has two parts:

Part 1: how to make scratch holograms with a compass

Part 2: how to make a device to draw scratch holograms

Step 1: Use a Compass to Make a Simple Demo

Let's start off with an experiment. Find a compass with sharp points at both tips (a drafting compass works well). Adjust the compass to about 2" between the tips.

Find a piece of plastic. I use 1/4" black acrylic (polycarbonate and styrene work fine also). The jewel case from a CD works fine but try to find one with a black CD holder insert because the hologram shows up better with a black background. The best scratches are barely visible and don't produce flakes of plastic. Double stick tape will hold the plastic to your working surface.

With the compass slightly angles, gently scratch an arc. Adjust the compass to a different radius, use the same center point, and make another scratch. In the image, I scratched an arc every 1/8" from a radius of 1.5" to 5.5".

Step 2: Experiment With Scratch Holograms

Bring the plastic outside on a sunny day, hold it horizontal, and see how the sun reflects off of your scratches. Experiment with turning the plastic. Can you make the image appear and disappear? If you used the same center point, you will see a straight 3D line composed of one small dot for every scratch. Do the larger radius scratches pop higher above the plastic?

Try closing one eye and holding a finger next to one spot of light. Now close the other eye and notice the distance between the spot and your finger. Did you notice that the spots on the larger scratches are further apart? These two spots are the "Stereo Pairs" that are creating the 3D image.

If you saw the spot in the same location with both eyes, it would appear to be on the surface of the plastic. Your brain uses the differences between the two images from each eye to judge depth. Have you ever held your finger in front of you and noticed how its position seems to change as you blink your eyes?

Step 3: Make a Scratch Hologram Machine!

Let's talk turkey. I've been working on a way to make the process of drawing scratch holograms easier.

This is what you'll need:

Plastic - 1/4" thick black acrylic is show, but CD cases work fine (use with a black background)
Dremel tool (variable speed is better)
Plastic guard that comes in the Dremel 565 Multipurpose Cutting Kit
Drum sanding bit
Pins
Pliers
Dry erase markers
Felt with sticky side (optional)
double stick tape

Step 4: Start Poking Pins

Disassemble the drum sanding bit and poke a pin into the hole in the rubber part at a 45 degree angle. Use a set of pliers to pull the pointy end of the pin until the head is against the inside of the hole.

Step 5: Poke Another Pin

Poke another pin in the opposite side of the rubber part, so the part is symmetrical. It helps to bend the pins in the clockwise direction (looking down at the tips of the needles). Reassemble the bit.

Step 6: Put Felt on the Dremel Guard (optional Step)

The Dremel guard was making small scratches in the acrylic, so I put a ring of felt on the surface. This is an optional step that you might want to ignore if you're not scratching the plastic.

If you want to eliminate two overlapping holograms, cut half of the felt as shown in the last photo. This will tilt the tool slightly, so you will scratch arcs rather than complete circles.

Step 7: Assemble the Tool

Assemble the tool. Adjust the guard so the tips of the pins are sticking out as little as possible (try from 1/16" to 1/8"). I put a mark on the plastic guard, to help me trace words on the plastic.

Step 8: Prepare Artwork

Use a dry erase marker to write or draw on the plastic. Straight lines are difficult. Remember to leave at least 1.5 inches around the border. A little double stick tape will keep it from moving around.

Step 9: Start Drawing

I recommend writing on CD cases first, so you're not wasting money on plastic. Write or draw about one inch above the center of the case to compensate for the radius of the guard. Wear safety glasses (I haven't broken a pin yet, but it might happen). Turn the Dremel to its lowest speed (mine starts at about 3,000rpm). Trace your artwork with the mark on the guard.

Step 10: Test the Hologram

Test the hologram. A sunny day works best, but you can also get results from a very small bright light. I used a 50W halogen. A Mag Lite Works well if you unscrew the bulb cover.

Step 11: Collaborate!

My friend Maz-Destruction and I have spent so much time developing this idea. Soon we will try using a hand blender and a coffee grinder.

I'm sure this device can be improved. Please blog about this idea and keep us up to date on your developments. Please send photos of your ideas to:

[e-mail]

If you are interested in learning more about scratch holography, I strongly recommend the following website.

http://amasci.com/amateur/holo1.html