MOTION MOUNTAIN

images – transporting light 167
F I G U R E 116 Three types of X-ray images of a thumb: the conventional image (left) and two images
taken using interference effects (© Momose Atsushi).
Page 101
Ref. 131
Challenge 174 e
Using interference for imaging
As we saw above for the case of the guitar, images produced by interference can be useful.
Above all, interference effects can be used to measure the deformation and the motion
of objects.
Interference can also be used to enhance images. Figure 116 show the improvement
that is possible when a special case of interferometer, a so-called Talbot-Lau interferometer,isusedwithX-rays.
Inparticular,thetechniqueincreasesthesensitivity ofX-rays
forsofttissue.
Interference is also at the basis of holography, an important technique to produce
three-dimensionalimages.
How does one make holograms and other three-dimensional
images?
Oursenseofsightgives us an image of the world around us that includes the impression
of depth. We constantly experience our environment as three-dimensional. Stereopsis,
the experience of depth, occurs because of three main effects. First, the two eyes see
different images. Second, the images formed in each eye are position dependent: when
wemove the head, we observe parallax effects between the bodies near and for from us.
Third, for different distances, our eyes needs tofocusdifferently and toconverge more or
less strongly, depending on the position of the object.
A usual paper photograph does not capture any of these three-dimensionaleffects:a
paperphotographcorrespondstothepicturetakenbyoneeye,fromoneparticularspot
and at one particular focus. In fact, all photographic cameras are essentially copies of a
single,staticeyewithfixedfocus.
Any systemtrying to producetheperceptionof depthfor theobserver must include
at least one of the three three-dimensionaleffectsjustmentioned.Infact,thethirdeffect,
varying focus with distance, is the weakest one, so that most systems concentrate on the
other two effects, different images for the two eyes, and an image that depends on the
position of the head. Stereo photography and stereo films extensively use the first effect
by sending two different images to the eyes, sometimes with the help of coloured glasses.
Also certain post cards and computer screens are covered by thin cylindrical lenses that
allow sending two different images to the two eyes, thus generating an impression of
depth.
But obviously the most spectacular depth effect is obtained whenever positiondependentimagescanbecreated.Modern
virtual reality systems produce this effectus-
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net