Light-field recording with phase information
- Recording quasi-three-dimensional images
- The light-field and its phase information are acquired
- Based on interference between object beam and reference beam
- Using the advantages and flexibility of single-frequency dioder lasers
A hologram is a photographic recording of a light-field that includes the phase information of the recorded field. It is commonly used to display a full three-dimensional image of the recorded subject. The hologram itself is not a normal image and it is generally unintelligible when viewed under diffuse ambient light. In contrast, it is an encoded recording of the light-field consisting of an interference pattern between the object beam and a reference beam.
If the hologram is illuminated "correctly", the interference pattern diffracts the light into a reproduction of the original light-field. Thus the objects that were recorded appear to still be there, exhibiting visual depth cues such as parallax and perspective that change realistically with any change of the relative position of the observer.
Laser holography is a commonly used technique where the hologram is recorded by applying a flash of laser light. This laser light illuminates a scene and imprints the light-field including its phase information on a recording medium, comparable to the concept how a normal photograph is recorded. However, a fraction of the laser light beam must irradiate the recording medium directly - this directly incident light beam is known as the reference beam.
A hologram requires a laser as the only light source. Lasers can be precisely controlled and usually have a fixed wavelength, unlike sunlight or light from conventional light sources. These contain many different wavelengths.
Several types of holograms are feasible. For example, transmission holograms are observed by shining laser light through them and by watching the reconstructed image from the side of the hologram opposite to the source. A "rainbow transmission" hologram allows a different illumination using white light, rather than using lasers. Rainbow holograms are regularly used for security and authentication, e.g. on credit cards and product packaging. They typically use a RGB combination of laser wavelengths (e.g. 473, 532, 647 nm).
Holograms can also be used in optical data storage to store, retrieve, and process information viy optical techniques. Some time ago, holography required expensive high-power lasers (e.g. gas lasers). Nowadays, low-cost grating stabilized semi-conductor or diode lasers are commonly used to produce holograms. These laser types made holography much more accessible to low-budget researchers, artists and dedicated hobbyists.
In many cases different laser wavelengths are used. Originally, Helium-Neon lasers with a wavelength of 633 nm, or various wavelengths from Argon/Krypton ion lasers were used as light source - both with limited power. Nowadays coherent solid state lasers at 532 nm and coherent diode lasers at 405 nm, 473 nm or 647 nm at power levels of several 100 mW have become the optimal choice for holography – especially in industrial applications.