T3D LED vs Laser

[John Toeppen <toeppen@xxxxxxxx>]

Holography requires wavelenght uniformity in the recording end.  All of
the waves must be in lockstep to produce the interference pattern that
makes the hologram.  If some of the waves are longer than others the
light and dark fringes become washed out and no hologram is recorded.
The distance of the lockstep condition is called the coherence length. 
Diode pointer lasers are suitable for holography of objects of two to
four inches depth in reflection is powered by D cells and if the laser
is left on for a few hours before the exposure.
A recorded hologram is a different matter.  Transmission holograms
exhibit chromatic image smear - the blue light least diffracted and the
red light most diffracted so the blue version of the object is not
located where the red version appears.  Image plane transmission
holograms do not show this effect as badly.  Reflection holograms only
reflect the color(s) of light used to make the hologram.  Rainbow
holograms sacrifice vertical parallax but allow white light playback.
LEDs that are narrow in wavelength have a name: diode lasers.  Bandwidth
is a function of drive current.  Broadband (multicolor) emission is
caused by ASE (Amplified Spontaneous Emission).  Light Amplification
through Stimulated Emission of Radiation (laser) is caused by the
amplification of the brightest light within the laser cavity.  The
brightest color of light is most likely to get amplified, thus getting
brighter yet.  Other colors of light are less available and become
unlikely to get amplified.  It is the geometry of the resonator that
make a diode laser different than an LED because the laser has feedback
to allow the brightest light to resonate within the laser cavity.
You may enjoy Richard's Ralison's holography web site.  Richard is a
great techno-artisan who invented the hologon scanner, made the first
eyeball holograms, and more cool stuff.
see:
http://www.xmission.com/~ralcon/index.html

John Toeppen
http://members.home.com/holographics/