Holography, or the art of creating three-dimensional (3D) images, generally requires at least one double concave lens. This holographic lens shape is necessary to manipulate the light source in such a way as to adequately illuminate the object from more than one direction while also illuminating the photographic plate. Creating basic 3D holograms generally requires having a holographic lens, a laser beam, equipment holders, and the photographic plate placed between two pieces of glass. The size and overall shape of the lens used depends on the individual project.
The double concave shape of the holographic lens means the edges are thicker than the center. Looking at the lens sideways, the shape appears similar to an upper case letter "I." Some lenses are highly polished on both sides with smoothed edges. Other lenses may be frosted on one or both sides. A holographic lens may be round, rectangular, or square, but has concave surfaces on both sides.
When a collimated laser beam passes through the holographic lens, the light emitted on the other side diffuses, or fans out, creating a wide, but even beam of light. The single beam typically illuminates the space up to the front of the object. The space around the object must also be illuminated from more than one angle in order to create 3D imaging. If the photographic plate does not lie in line with the object, some holographic methods redirect the light to illuminate the plate as well.
Using beam splitters, multiple lenses, or mirrors, different angles of the subject receive light simultaneously. Part of the beam generally illuminates the front of the object, while a redirected part of the beam illuminates the space to the side or to the back of the subject. Strategic angling of the lenses or mirrors reflects and recreates these simultaneous images onto a glass sandwich that holds the photographic film.
Optical companies manufacture holographic lenses in sizes ranging from 0.5 inches (12 millimeters) in diameter to over 3 inches (75 millimeters) in diameter. The lenses also differ in thickness and focal length, which is the distance from the converged, focused beam to the lens. The larger the lens, the greater the focal point distance and the wider the angle of diffused light. The type of holographic lens used depends on the size of the available workspace, the strength of the laser beam, and the size of the hologram one chooses to create.