Formation of a single image point in a white light hologram

Revision as of 14:54, 7 June 2011

Formation of a single image point in a white light hologram

Exposure with r, g and b lasers results in overlaid bragg structures for the three colors. These structures reflect only three narrow bands r,g, and b from the light source. They act like a convex mirror that delivers a small image of the light source, which actually is what we see as one point in the holographic image. Hence, the size of the light source limits the resolution. The illustration also shows the bragg layers becoming thicker with shallower incident/outgoing light angles, to correspond to the same wavelength. Further, the spectral selectivity of the bragg structures rises with their total thickness (this is quite similar to a Fourier transform of the spatial bragg structure to the frequency domain). Thin bragg structures (down to a single faint refraction index boundary surface, that would just behave like an ordinary mirror) reflect a wider spectral range, but still at defined angles forming image points. The hologram, viewed with a halogen lamp, then looks white or only little colored, even if it was recorded with a single laser wavelength. Nevertheless, we can’t record a white light hologram with white light, as this is not temporally coherent and therefore cannot form stable fringe patterns. We need one or several coherent, hence monochromatic sources. Three sources of red, green, and blue (r,g,b,) colors can provide the illusion of white light to our eyes, they can serve as primaries for a white or colored hologram. If the r, g, and b parts of the bragg structure have different strengths, the image point becomes colored. This reproduces the colors of the recorded image. The white light hologram then becomes a color hologram. Real holographic film materials, however, often are sensitive to one color only. Therefor three separate holograms for r,g, and b, stacked upon each other, are often used instead of a single, combined one. The light source must be at the same positions as the reference sources at recording. Otherwise, color shifts and geometry distortions in the holographic image will result, because wrong parts of the bragg structures will then reflect light to the viewer, and they select different color bands from the light source.
A perfect color hologram in one layer can better be produced using three laser sources from different directions (e.g., top, left and right), as in this case the bragg structures cross over at considerable angles and do not influence each other too much even if the hologram film does not have perfect properties.