Essay: Holography

Hologram is defined as ‘a three-dimensional image reproduced from an interference pattern produced by a split coherent beam of radiation (as a laser)’ [1]. Holograms can achieve a three-dimensional image, but it can be easily minded of a hologram as a photograph which can be refocused at any depth [2]. Therefore, as a photograph taken of two people standing far apart would have one in focus and one blurry, a hologram taken of the same scene can be reconstructed to bring either person into focus [2].
This chapter will concern on the definition of holography and its advantages over the conventional imaging techniques. Also it will investigate the holographic technology development and its application including microscopy.
In conventional imaging techniques, such as photography, what is recorded is merely the intensity distribution in the original scene resulting that all information about the optical paths to different parts of the scene is lost [2].The unique characteristic of holography is the concept of recording both the phase and the amplitude of the light waves from an object [2].
When a hologram is illuminated by a proper light source, the exact amplitude and phase is reconstructed and the original light field recreated, Since the observer has the whole light field available, the genuine three dimensional sensation is achieved, therefore, Holography is about capturing and reproducing light field, and each point in this field is determined by an amplitude and phase [3].
Holography has advanced to the digital area since 90s, after the advent of CCD and CMOS digital cameras [4]. In digital holography there is no need of wet processing to record the holograms and it is convenient to evaluate the properties of the specimen structures quantitatively [4]. Three major areas of holography which usually addressed in a context of digital holography can be considered, They are the capturing, the reproduction and hologram fringes synthesis, in addition, digital holography introduces one specific issue that don’t have parallel in optical holography, the area is numerical reconstruction[3].Since the mid 90’s, digital holography had attract many applications such as in optical metrology, encryption of information and microscopy[4],which is the goal of this thesis.
Digital holographic microscopy is proposed to yield a microscope that can image optical thickness as well as phase object [4].
Fig.(1.1) Digital holographic microscopy of SKOV3 ovarian cancer cells (60 ?? 60 ??m^2,404??404 pixels): (a) hologram, (b) amplitude image,(c)phase image, (d) unwrapped phase image, and ( e) phase image in pseudo-color pseudo-3D view [5].
An example of DHM imaging of a SKOV3 ovarian cancer cells is shown in Fig. (1.1), where Fig. (1.1) a) is the hologram and Fig. (1.1) b) is the reconstructed amplitude image, similar to what one would see through a conventional microscope, the phase image in Fig. (1.1) c) indicates that the cells appear having thickness of several microns, therefore the phase profile varies by several cycles of 2?? radians. A public domain phase unwrapping algorithm is used to remove the 2 ?? discontinuities in Fig. (1.1) d), and it is rendered in pseudo-color pseudo-3D perspective in Fig. (1.1) e). The apparent height profile is the profile of optical thickness that includes both physical thickness and index variation [5].