But the limitation is that it is a two dimensional photograph, therefore, the information about the third dimension or the depth is lost. The reason for this is the fact hat the photographic paper can store only the intensity information and not the phase information. On the other hand, what gives us the perception of depth or the information about the depth is the phase of the object wave or the phase of the different waves of light coming from the object. The moment we put a photographic paper in the image plane the phase information and with it the perception of depth is lost and what is retained is just the intensity information.
To surpass this limitation and therefore, to record the depth information as well, a technique called Holography was invented or rather conceived 1947 by a Hungerian physicist Denis Gabor and he was awarded noble prize for this work in 1971 (http://en. wikipedia. org/wiki/Holography). The underlying concept behind holography is to record an interference pattern produced by superimposing a reference wave (of light) over the object wave (of light) onto the photographic or photosensitive paper, instead of recording just the object wave.
This interference pattern is nothing but the hologram of the object. Because an interference pattern retains the information about the phase of the object wave therefore, it is possible to subsequently extract the phase information of the object wave and therefore depth perception of the object from the hologram of the object. It should be remembered here that a hologram is not at all a photograph rather it is an interference pattern which is nothing but alternate bright and dark fringes. To understand a hologram, it is necessary to have some idea of interference and diffraction of waves.
Light is an electromagnetic wave. This propagates by varying electric and magnetic field vectors in sinusoidal manner in space and time. The intensity of a wave is proportional to the amplitude of the electric field vector at that point and the amplitude of the electric field vector at a point depends on phase of the wave at that point. It is also important to note that phase is relevant only when we are talking of more than one waves, then one of these waves can be taken as reference wave and phase of all the other waves is represented with respect to this reference wave.
When a propagating wave faces an obstacle of size of the order of its wavelength, it diffracts, what it means is that it gets divided into many waves, which propagate in different directions. Thus an object wave is essentially a group of waves, which have got diffracted from the different points on the object. Interference on the other hand is nothing but superposition of more than one individual waves. This results in an intensity distribution which is different from that of individual waves.
This intensity distribution is termed as interference pattern. The interference pattern contains information about the intensity as well as phase relation of the interfering waves. Suppose we have got an interference pattern of two waves one being a reference wave and the other being the object wave (this interference pattern is nothing but the hologram), then we can substract the reference wave from the object and thus obtain the original object and thus we can have the depth perceptio of the object.
This is the underlying principle of the Holography. What is required for production of interference pattern or the hologram is a constant (in time) phase difference between the reference wave and the object wave. This requires a coherent source of light to be used as reference wave and also the same light should be used to shine the object, which gives the object wave. Larger the coherence length bigger is the depth of the object that can be recorded.
Laser which is an acronym for Light Amplification by Stimulated Emission of Radiation (Marshall 1997) happens to be an ideal light source for holography as it has coherence length in a few meters. Mercury arc lamp is another light source which is used for holography. However, it has much smaller coherence length and therefore holograms with much smaller depth perception can be produced using this light source. Depending upon the way the hologram is developed or the way, in which a hologram is played back or seen, there are different types of holograms.
Some important types are discussed below (http://www. holo. com/holo/book/book5. html): Transmission Hologram: This requires the reference beam to be transmitted through the hologram for constructing the original object wave to be able to see the object wave. Reflection Hologram: This does not require the original reference beam; rather white light can be used to see the perspective of the object from different angles. This is most common and is widely used for product authentication and also as security mark. Besides, these two types there is another hologram which is known as multiplex diagram.
In a multiplex hologram the entire view of the object is divided into smaller segments say 360o view is divided in 3—360 = 1080 parts and hologram of each part is recorded and subsequently clubbed together to get a complete hologram of the entire object. This produces horizontal parallax and not the vertical parallax. This hologram can be viewed as a cumulative hologram of the different parts of an object. The advantage here is that dividing the object in many parts reduces the depth and therefore, a less coherent light source can be used for producing the hologram.
Conclusions: Holograms are an interesting application of the wave nature of light and interference of light waves. This helps in creating a hologram, which is nothing but the interference pattern produced by superimposing a reference light wave over the object light wave. A hologram gives illusion of depth because it is possible to extract object wave from it. The holograms have great commercial value as they are used as security mark to prevent piracy, counterfeiting etc.
1. http://en. wikipedia. org/wiki/Holography 2. http://www. holo. com/holo/book/book5. html