This chapter describes holography and hologram. The working principle of holography is elucidated here in this article. types of holograms is also elaborated here. with pictorial depiction, recording and reconstruction of holograms are cited.

1. OPTOMETRY – Part VI
HOLOGRAPHY
ER. FARUK BIN POYEN
DEPT. OF AEIE, UIT, BU, BURDWAN, WB, INDIA
FARUK.POYEN@GMAIL.COM

2. Contents:
1. Definition
2. Types of Holography
3. Working Principle
4. Recording a Hologram
5. Reconstruction of a Hologram
6. References
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3. Definition:
 Holography: A method of producing a three-dimensional image of an object by
recording on a photographic plate or film the pattern of interference formed by a split
laser beam and then illuminating the pattern either with a laser or with ordinary light.
 Hologram: A photographic record produced by illuminating the object with coherent
light (as from a laser) and, without using lenses, exposing a film to light reflected from
this object and to a direct beam of coherent light. When interference patterns on the film
are illuminated by the coherent light a three-dimensional image is produced.
 A hologram is a physical structure that diffracts light into an image.
 SLM (Spatial Light Modulator) is a device that modulates the coherent light spatially
based on its control input.
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4. Types of Holographs:
 Reflection Hologram: A truly three-dimensional image is seen near its surface. The
hologram is illuminated by a “spot” of white incandescent light, held at a specific angle
and distance and located on the viewer’s side of the hologram. Thus, the image consists
of light reflected by the hologram.
 Transmission Hologram: Viewed with laser light, this light is directed from behind the
hologram and the image is transmitted to the observer’s side. The virtual image can be
very sharp and deep. The coherent light from the laser is split to form an object beam
and a reference beam.
 Hybrid Hologram:
 Embossed holograms
 Integral Holograms
 Multi-channel Hologram
 Computer Generated
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5. Working Principle:
 The word 'holography' is made up of two Greek words, 'holos' meaning 'whole' and
'graphy' meaning writing.
 The technical term for a hologram is wavefront reconstruction.
 The light source used for holography is usually a laser light.
 A laser beam along with mirrors and lenses are used to diffract the light into a series of
very fine lines which is then captured on special holographic film.
 The resulting hologram is thus a true three-dimensional record of the original object.
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6. Working Principle:
 Holography is based on the principle of interference. A hologram captures the
interference pattern between two or more beams of coherent light (i.e. laser light).
 One beam is shone directly on the recording medium and acts as a reference to the light
scattered from the illuminated scene.
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7. Working Principle:
 The first element is a beam splitter that divides the beam into two identical beams, each
aimed in different directions:
 One beam (known as the illumination or object beam) is spread using lenses and
directed onto the scene using mirrors. Some of the light scattered (reflected) from the
scene then falls onto the recording medium.
 The second beam (known as the reference beam) is also spread through the use of
lenses, but is directed so that it doesn't come in contact with the scene, and instead
travels directly onto the recording medium.
 Common materials used are silver-halide emulsions, dichromate gelatins and
photopolymers – each having their own characteristics and require different processing.
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8. Working Principle:
 The split beam, the illumination beam or object beam, is directed at the object.
 Some of the light is reflected off the object onto the recording medium.
 The reference beam, is directed onto the recording medium.
 The two beams intersect and interfere with each other.
 The interference pattern is what is imprinted on the recording medium to recreate a
virtual image for our eyes to see.
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Fig 1: Recording a Hologram Fig 2: Reconstruction of a Hologram

9. Holographic Recording:
 To record a hologram, we need to divide the beam of light (laser) into two parts, the
objective and reference wave, and fix their interference.
 It has the property of coherence.
 The laser beam is expanded by a lens and directed to a photographic plate, behind which
there is a registered object.
 Part of the beam, passing through a special transparent photographic plate, forms a
reference wave, and the light reflected from the object forms an object wave.
 The object beam reflected from the object interferes with the reference beam when both
the beams reach the photographic plate.
 The superposition of these two beams produces an interference pattern (in the form of
dark and bright fringes) and this pattern is recorded on the photographic plate.
 The photographic plate with recorded interference pattern is called hologram.
 Thus, a reflective hologram is recorded as shown schematically in Fig 1.
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10. Holographic Reconstruction:
 In the reconstruction process, the hologram is illuminated by laser beam and this beam
is called reconstruction beam.
 This beam is identical to reference beam used in construction of hologram.
 The hologram acts a diffraction grating. This reconstruction beam will undergo
phenomenon of diffraction during passage through the hologram.
 The reconstruction beam after passing through the hologram produces a real as well as
virtual image of the object.
 One of the diffracted beams emerging from the hologram appears to diverge from an
apparent object when projected back.
 Thus, virtual image is formed behind the hologram at the original site of the object and
real image in front of the hologram.
 The observer sees light waves diverging from the virtual image and the image is
identical to the object.
 Schematically represented in Fig 2.
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11. Holographic Recording Intensity:
 Light wave: vector
 A1 — the signal, A2 — the reference,
 Each point on hologram:
 The asterisk* indicate the complex conjugate
 The transmittance function: T(x, y)
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)(  
 ti
eAA
2121
2
2
2
1
2121
2
21
))((
)(),(
AAAAAA
AAAA
AA




yxI
2121),( AAAA 
yxT

12. Holographic Reconstruction Intensity:
 A3 light is used to illuminate the hologram, the result pattern A4 is diffracted (modulated)
by hologram.
 If A3 is equal, or proportional, to the reference amplitude A2
 The image is a reconstruction of the object.
 A4 is the result of diffraction pattern by the hologram:
s— distance between lines in hologram
 — the wavelength
 — diffraction angle
 For the 1st order: m=1;
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),(34 yxTAA 




UU
2132134 AAAAAAA
 mSins 
sSin /  )/( sarcSin  

13. References:
1. https://universe-review.ca/R15-22-holographic.htm
2. http://holocenter.org/what-is-holography
3. https://www.enotes.com/homework-help/what-basic-principle-holography-193039
4. https://en.wikipedia.org/wiki/Holography
5. https://spie.org/Documents/Publications/00%20STEP%20Module%2010.pdf
6. https://www.wonderwikikids.com/holography
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