Hogel
A hogel—short for holographic element—is a small spatial region of a digital or computer-generated hologram. A complete hologram can be organized as a two-dimensional array of hogels, but each hogel describes more than one ordinary image sample: it controls or records light traveling in multiple directions.
1 The analogy with a pixel
A pixel is the smallest addressable picture element in a conventional two-dimensional image. At a given moment, it normally displays one color and brightness value. A viewer looking at that pixel from different positions is still meant to see the same image sample.
A hogel can be understood as the holographic analogy to a pixel, because it occupies one small addressable area on the hologram. The important difference is that a hogel also contains angular information. Different viewing directions can receive different light from the same hogel.
| Picture element | Spatial information | Directional information |
|---|---|---|
| Pixel | One position in a 2D image | Normally one color/intensity value |
| Hogel | One small region of a hologram | A distribution of light over multiple directions or spatial frequencies |
The comparison is useful for understanding how a hologram is divided into addressable regions, but it is not an optical equivalence. A hogel may contain a diffraction fringe, directional ray data, or a small elemental hologram. Its function depends on the display or printing architecture.
2 What a hogel stores
In a holographic stereogram printer, each hogel is exposed with information derived from many rendered or captured viewpoints. When the finished hologram is illuminated, the hogel directs the appropriate portions of those views toward different viewing positions. The viewer’s left and right eyes therefore receive different rays, and the selected pair changes as the viewer moves.
In diffraction-specific computer-generated holography, a hogel is a spatial sample of the hologram with an associated spectrum of spatial frequencies. Those frequencies define the directions in which the reconstructed light travels. Mark Lucente’s work at the MIT Media Lab described the hologram as a regular array of hogels and represented each hogel’s discretized spectrum as a hogel vector.
3 Spatial and angular resolution
Hogel design involves a fundamental sampling tradeoff:
- Smaller and more numerous hogels can increase spatial detail across the hologram surface.
- More directional samples inside each hogel can improve angular resolution and produce smoother motion parallax.
- Increasing both raises computation, data volume, modulation, and printing requirements.
The physical hogel size, number of views, viewing angle, diffraction behavior, recording material, and printer optics therefore work together. Hogel size alone does not define the perceived resolution of the final holographic image.
4 Origin and technical definitions
The term is associated with Mark Lucente’s 1994 MIT doctoral work on diffraction-specific fringe computation. His method sampled a holographic fringe pattern in space as hogels and in spatial frequency as hogel vectors. The term was subsequently adopted in holographic video, digital holographic printing, and light-field display research.
Useful technical sources include:
- The MIT Media Lab Holovideo overview, which defines a hologram as a regular array of holographic elements and describes the homogeneous spectrum associated with each hogel.
- Mark Lucente’s Diffraction-Specific Fringe Computation for Electro-Holography doctoral research and publication archive.
- Lucente’s peer-reviewed paper, Holographic bandwidth compression using spatial subsampling, which describes spatial sampling into hogels and frequency sampling into hogel vectors.
- The Society for Information Display article Light-Field Displays, which uses hogel for the two-dimensional set of directional ray data projected through each microlens.
- The Optics Letters record Resolution enhancement of holographic printer using a hogel overlapping method, which demonstrates how hogel arrangement affects the lateral resolution of printed holographic stereograms.