I’ll try to explain the necessity and oddity of laser light for holography. The laser is one or the unique light source for holography per se.
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There are a variety of lasers with differing color emissions within the visible bands. The most lasers used for holography emit light in the green and red bands. Lasers emit a spot-like coherent light of extremely high intensity.
Particularly the class III B lasers, but also those below are very dangerous for the human eye. Unprotected viewing INTO a laser beam destroys the photo-receptors permanently at that point where the beam hits the retina. The effect is partial blindness (dark spots) due to the loss of the photo-receptors at the affected area, regardless of the length of viewing. Looking AT a laser beam or it touching your skin are harmless in the above mentioned danger supplement!
The helium-neon laser is a proven source for analog holography although an argon laser (mostly green) or a DPSS (double frequency neodymium laser) have advantages due to the greater coherence length. However their prices and water-cooling are negatives.
Helium-neon and argon are noble gases which are enclosed in the glass flask of the laser, similar to neon in a neon tube. Helium-neon lasers have a visible wavelength of 632 nanometers (nm) = red. Neodymium ion lasers used as diode pumped lasers have a wavelength of 532 nm = green. The atoms in the plasma of the laser are stimulated by a high ignition voltage, eg 16 KV, igniting the laser so that a beam is slowly produced. The laser emits the color or wavelength of the contents of its glass flask. If the plasma color of a helium-neon laser is orange-red, it promises a long life-expectancy of 20,000 operating hours. The darker the plasma color, the shorter its life-expectancy.
The light of a laser has a particular and unique characteristic for holography: it is coherent. A coherent light source (phase and temporally coherent) is indispensable for the photo of a hologram.
The difference between incoherent (normal) and coherent (laser beams) sources can be explained with the following example: imagine a mountain tunnel where vehicles of all kinds, colors and speeds are exiting. In physics this would be called “incoherent” exiting of cars. Imagine the same tunnel where only vehicles of one series (eg VW), all of the exact same color (632 nm red) each with an exact speed of 30km/h are exiting exactly bumper to bumper. This condition, if simulated for cars, could be considered coherent, that is phase and temporally coherent.
The coherence is the ultimate prerequisite for successful holograms. If you watch the emitted beam of a helium-neon laser, wave peak hits wave peak, wave trough hits wave trough, you see the character of laser light. The amplitudes of the light waves oscillate, absolutely phase and temporally coherent, at least 20cm (coherency length) after emission like in the helium-neon example. This phase coherency of a limited distance after beam emission is called coherency length. Lasers emitting green like DPSS may have a coherency length of several meters. The laser with its coherent light source is the central illumination for shooting a hologram. It illuminates the object AND the medium, ie the film or film emulsion in phase and simultaneously. Only if both prerequisites are fulfilled and coherence given, is the physical condition needed for a hologram also given.
The laser has to fulfill several requirements to be useful for hologram shots. The power output shouldn’t be less that 5 mw (milliwatts) so that small film formats can be illuminated. The laser has to oscillate in so-called TEMoo mode (traversal electron mode), that is the laser beam is only a single spot and does not build four, six or nine spots on a surface at the end of a 2 meter distance. The laser has to be horizontally polarized, that is the wave pattern oscillates only in the horizontal level and does not randomly change its polar direction from horizontal to vertical or in-between. The polarization ratio should be 100:1, even better 500:1, ideally 1000:1. So-called random polarizations are unsuitable for holography. By round-flask polarized lasers you can find out the horizontal polarization best by using a polfilter turning the laser tubes in the horizontal level.