The idea for scratch 'holograms' comes from William J Beatty. Curved scratches are made in a hard reflective surface. When the surface is held in sunlight at the proper angle the scratches reflect highlights, the exact position of which depends on the viewing point and the orientation of the scratches. The highlights are similar to those that give rise to the light circle seen when the Sun is viewed through twigs. Each eye sees a pattern of highlights with different geometry, because of its different viewing position. The two images are combined and interpreted as depth in the pattern.
The effect is holographic in the sense that the image is three dimensional, but is not a true hologram. Perhaps a more accurate term would be scratchogram. The collection of highlights floats above (or below) the surface. The displacement depends on the radius of the circular arcs, which may be varied to give true 3D rendering. Walking past a panel on the ground in sunlight is quite amazing. The image appears to have nothing directly to do with the scratched surface.
You will need: acompass with two sharp points and a clear Plexiglass (Acrylic) panel.
1 On a piece of paper draw an image (a simple letter X for beginners).
2 Secure the Plexiglas on the sheet of paper above the drawing.
3 Take the compass with two sharp points (sometimes called a divider). Place one of the points on a line of the image.
4 Using that point as a center, draw an arc with the other point of the compass on the Plexiglass. Note: the arc should just be a light scratch on the surface. Avoid making deep scratches that will leave behind white scratch marks.
5 Repeat steps 3 and 4 without changing the spacing of the compass, but changing the position of the pointer on the image. The aim is to create as many arcs as possible by using different points on the lines of the image.
6 Go outdoors and face towards the Sun. Hold the Plexiglas in front of you and tilt it back and forth until you get the right angle. At this angle you will see a floating image.
Each arc drawn represents a point on the image. The image is really a combination of many points. The more arcs drawn, the more highlights, and the clearer the image. There are different viewing angles depending on the position of your eyes. Two people standing together will see the image differently.
The compass spacing determines the depth of the holographic image. To create a three dimensional image the spacing of the compass must be varied. The larger the circle radius, the more depth achieved, and vice versa. Some calculations must be made to get the correct spacing. There is no specific formula. The depth varies with different images.
Note: to create a three-dimensional image a normal drawn image cannot be used. If a cube is to be achieved then the original image cannot be a cube. If it were, when the depth is changed, the holographic image would be extended by a lot. Using a cube drawing to create the hologram while leaving the compass spacing the same will not give a three dimensional effect because there is no depth.