Holographic diffuser for multichannel restricted displays

Transcription

1 Indian Journal of Pure & Applied Physics Vol. 54, August 2016, pp Holographic diffuser for multichannel restricted displays Subhra S Sarma a, Sonia Verma b,c & Rajkumar c * a Department of Electronics & Communication Engineering, Assam Don Bosco University, Guwahati , India b Department of Applied Physics, Guru Jambheshwar University of Science and Technology, Hisar , India c CSIR- Central Scientific Instruments Organization, Chandigarh , India Received 9 September 2014; revised 13 June 2016; accepted 21 June 2016 Diffusers play an important role in modern days display technology. Holographic diffusers offer effective control on various parameters of the diffused light like diffusion angle, uniformity of light etc. Present work reports a procedure for realization of a holographic diffuser through which multichannel restricted view display is achieved such that information can be conveyed to and viewed from multiple selective viewpoints only. Such type of diffusers will be helpful in enhancing security feature of the displayed information by enabling only authenticated persons to have access to the information. Fabrication process of the holographic diffuser is described and experimental results are discussed. Keywords: Holography, Diffuser, Holographic optical element, Display systems 1 Introduction Diffusion is an effect of scattering from solid, liquid, or gaseous particles. Diffusers are objects, materials, or films that diffuse incident light. Conventional ground glass diffusers achieve diffusion of incident light by means of surface roughness. Scattered light from conventional diffusers spread out over wide range of angles which is almost hemispheric 1. On the contrary, holographic diffusers provide considerable control over various parameters of the diffused light like control over angular distribution and direction of the diffused light with minimal loss 2-5. Diffusion of light from holographic diffuser depends on many factors such as properties of the source diffuser, distance between source diffuser and holographic plate, recording geometry and selection of holographic recording medium 2-9. Good quality holographic diffusers have more efficiency as compared to conventional diffusers. Diffusers find extensive use in the field of display. Several types of diffusers have been made over the years for performing desired particular functions. Diffusers are now used in many devices such as mobile phones, laptop, desktop computers and ATM machines etc. for display application. So, it has become indispensable to fabricate such a diffuser which can scatter light into the desired direction with restricted field of view so that enabling only *Corresponding author ( raj_csio@yahoo.com) authenticated user to view the information. One of the security applications of holography is well known in the field of authentication of valuable products and important documents using security holograms 10,11. Fabrication and use of restricted displays using holographic diffusers will open new areas of holographic applications in information security. Caulfield has given important suggestions for recording highly efficient holographic diffusers 12. He concluded that designing a perfect diffuser screen system for all situations is impossible but a near perfect screen can be made for a given situation. Many researchers have worked on this technique to get a directional display and three dimensional displays that can be observed from any direction Multichannel systems based on different techniques have also been reported aiming to increase the display screen size or for security of data transmitted over long distances. The present work reports recording and reconstruction of a multichannel holographic diffuser. A procedure for achieving two channels is described but actual number of channels can be increased or decreased at the time of recording, depending on end use of the recorded diffuser. The procedure is an extension of recording directional holographic diffusers. Here collimating optics is used to shape the diffuse light in order to have a control over the field of view for a particular direction. Experimental results

2 496 INDIAN J PURE & APPL PHYS, VOL 54, AUGUST 2016 obtained using this setup demonstrates the feasibility of the system for its application in restricted view multichannel displays. 2 Theoretical Description Consider the complex amplitude distribution of a collimated reference beam and two object beams obtained by illuminating ground glass diffusers with laser beams and the transmitted diffuse light is collimated using collimating lenses, incident on the hologram recording plate are,,, and, given by:, = exp, = exp exp 2, = exp exp 2 (1) Here, α = θ is the spatial carrier frequency λ introduced by the off-axis angle of object beams. and contains random phase distributions, formed by ground glass diffusers and, of object beams incident at recording plate at angles and from the normal. is reference beam phase distribution. Two separate exposures are recorded on the same holographic recording plate. In the first exposure interference pattern of reference beam and object beam 1 (object beam 2 is blocked) is recorded, resulting in an intensity distribution:, =, +, = + + exp exp 2 + exp exp 2 (2) In the second exposure interference pattern of reference beam and object beam 2 (object beam 1 is blocked) is recorded with intensity distribution:, =, +, = + + exp exp 2 + exp exp 2 (3) Total intensity distribution recorded on the holographic plate becomes:, =, +, (4) After chemical processing of the recorded plate its amplitude transmittance, is given by:, = +βτ, (5) where t 0 and β are constant and τ is exposure time. Now illumination of this hologram with reconstruction beam, in present case reconstruction beam is modulated with the parameters of the display (say phase is introduced in reference beam), the transmitted field distribution can be described by:, = + {, +, }, = exp{ }exp 2 + exp { }exp 2 + exp{ + }exp 2 + exp { + }exp 2 (6) Equation (6) represents the reconstructed field from the recorded hologram when illuminated with a reference beam modulated with information. The last term consists of two virtual reconstructed object beams modulated with the projected information. Information travels at angles defined by their respective spatial frequencies. The second last item in the equation shows reconstruction of two real object beams which can be projected on the screen with the information to be displayed. 3 Experimental Procedure For recording a holographic diffuser one needs to have a suitable master or source diffuser. In our case, two ground glass diffusers are fabricated in CSIR- CSIO optical workshop by grinding optical glass plates of area 2 square inch and thickness 1 mm using emery 302 to serve as source diffusers for two channels of the holographic diffuser. Figure 1(a) shows a schematic diagram of the experimental set-up for recording the holographic diffuser with a He-Ne laser of wavelength 632 nm. Photograph of the experimental setup is shown in Fig. 1(b). Here the laser beam is split into two parts by a variable beam splitter VBS1. A variable beam splitter is required to adjust the intensity of transmitted and reflected beam according to setup requirements. The reflected beam from the beam splitter forms one of the object beams and the transmitted beam after second transmission from VBS2 and a subsequent reflection from M1 forms the reference beam. Thus, the transmitted beam coming out of VBS1 is again split into two beams by VBS2 where reflected beam is used for forming the other object beam. Two ground glass diffusers (D1 & D2) are placed in the paths of object beams. A beam

3 SARMA et al.: HOLOGRAPHIC DIFFUSER FOR MULTICHANNEL RESTRICTED DISPLAYS 497 Fig 1 (a) Schematic diagram of the experimental setup used in recording the multi-channel holographic diffuser: L is He-Ne laser, VBSs are variable beam splitters, M1-M4 are beam steering mirrors, L1 L3 are collimating lenses, D1 and D2 are ground glass diffusers, BE is beam expander and H is the hologram recording plate and (b) photograph of the experimental setup expander in conjunction with a beam collimating lens is used to generate an expanded and collimated reference beam striking normally on the hologram recording plate H. Collimating lenses are used to collimate the diffused light from the diffusers. During our experiments angle between diffused beam from D2 and reference beam is 23 degree and between beam from D1 and reference beam is 22 ; measured on the recording surface. Thus, two object beams and the collimated reference beam interfere to produce interference fringes which are recorded on the holographic plate in two separate exposures. Here multiplexing property of the hologram is used and Fig 2 Setup for diffuser characterization thus number of views and their angular separation can be optimized depending on the actual requirements of the system. A silver halide photographic plate Agfa-Gevaert 8E75HD has been used for recording the hologram experimentally in our laboratory due to its commercially availability and good storage time as well as stability. The recommended exposure for Agfa 8E75 at 633 nm wavelength of He-Ne laser is 150µJ/cm2. After exposure to light the hologram is chemically processed using standard Kodak D-19 developer and R-9 bleach bath solutions. Diffusion efficiency η of any optical diffuser is an important property and represents its ability to diffuse light as quantified by the zero-order beam ratio (ratio of power measured in transmitted light to the incident power on the diffuser, i.e., T = P 0out /P i ). Quantitatively diffusion efficiency of an optical diffuse is η = 1 T. The efficiency of our diffuser is 48%. This is not good enough for making the diffuser useful for actual display applications but the efficiency can further be improved using the other high efficiency holographic recording materials such as photopolymers or the dichromated gelatin (DCG), which can record holograms having efficiency more than 90% experimentally as reported in the literature 2. A schematic of the experimental setup used for measuring angular distribution of diffused light is shown in Fig. 2. In this set up the reference beam from the laser is allowed to illuminate the developed holographic diffuser. A plano-convex lens with a focal length of 25 mm is placed at a distance of 16 cm from the diffuser to collect the scattered signal (diffuse light). A detector and power meter is placed just off the focal plane of the lens. The power meter used in the set-up is of model 1825-C of Newport.

4 498 INDIAN J PURE & APPL PHYS, VOL 54, AUGUST 2016 The lens and the detector are mounted on a single platform, which is placed on a rotary stage for angular measurements. One end of an aluminum bar is fixed to the center of the base of a protractor and the other end is used to mount the lens-detector assembly. The detector is aligned to get maximum signal in the power meter by giving necessary translation to it using a translation stage on which detector is mounted. The detection system is scanned in horizontal (Z-X) plane for obtaining power distributions at different angles. This gives a plot of distribution of light transmitted through the diffuser. 4 Results and Discussion After optical recording and chemical processing, the hologram is replaced back in its mount and illuminated with reconstruction beam. The reconstruction beam is modulated with information which is to be displayed. In our experiments the displayed information consists of alphabets ABC written on an optical glass plate. As is clear from Eq. (6) that the information used to modulate the reconstruction beam also modulates both of the reconstructed object beams. This effect of projected information on reconstructed object beams is seen in the experimental results shown in the Fig. 3. Here due to recording of two channels on the same recording plate, the reconstructed information travels along these two channels only as seen in Fig. 3 in the form of separation of reconstructed beams on either side of the DC light. Here, the central beam corresponds to directly transmitted (DC) reconstructing beam while information on either side of DC beam corresponds to two recorded channels of the diffuser. The DC beam is also modulated with the input information. This information can be viewed only from the angles of recording of the holographic diffuser. This confirms that when the desired information which is to be displayed (i.e., reconstruction wave is modulated with the desired information which is to be displayed) is projected on the holographic diffuser, the diffuser directs the incident beam into both the recorded channels. Thus, the incident object information is conveyed along the two directions at some pre-recorded angles only. The DC term may be filtered out enabling display of information in two pre-defined directions only. One of the filtered images is shown in the Fig. 4. The optical power distribution of reconstructed beams in two channels from the diffuser is also measured using the set up discussed above (Fig. 2). The angular distribution of power in transmitted beams for the incident reference beam is shown in Fig. 5. Here power distribution of DC term is filtered out. The power distribution in reconstructed wavefronts demonstrates a control over the direction of information to be displayed and over the field of view of displayed information. Thus this type of holographic diffusers may be useful for displaying desired information into targeted directions with a control over their field of view. Fig 4 Filtered displayed image with no DC beam Fig 3 Projected information through multichannel holographic diffuser Fig 5 Graph showing power distribution of diffused light versus angle of scattering

5 SARMA et al.: HOLOGRAPHIC DIFFUSER FOR MULTICHANNEL RESTRICTED DISPLAYS Conclusions Holographic diffusers are playing important role in display systems due to the ability of holographic process to provide good control over various parameters of the diffused light like directionality, uniformity and field of view. In the present work a technique is reported for recording multi-channel holographic diffusers. The hologram recording and reconstruction process is described and experimental results are discussed. Such type of diffusers will be helpful in enhancing security feature of the displayed information by enabling only authenticated persons to have access to the information. Multichannel diffusers may find applications in display systems where security and authenticity of displayed data is important. References 1 Savant G, Jannson T & Jannson J, Diffuser display screen in holography for the new millennium, edited by Ludman J, Caulfield H J & Riccobono J, (Springer, New York), Wadle S, Wuest D, Cantalupo J & Lakes R S, Opt Eng, 33 (1994) Wadle S & Lakes R S, J Mod Opt, 42(1995) Ling D, Naiguang L, Xiaoping L, Yongchao S & Yueqiang L, SPIE, 2866 (1996) Sarma S S, Chachchia D P & Kumar R, AJET, 1 (2014) Kim S, Choi Y S, Ham Y N, Park C Y & Kim J M, Appl Opt, 42 (2003) Ganzherli N M, Maurer I A, Chernykh D F & Gulyaev S N, J Opt Technol, 76 (2009) Lin H, Oliveira P W, Veith M, Gros M & Grobelsek I, Opt Lett, 34 (2009) Chen H C & Kang C C, 5th Pacific Rim conference on lasers and electro-optics, (2003) Aggarwal A K, Kaura S K, Sharma A K, Kumar R & Chhachhia D P, Indian J Pure Appl Phys, 42 (2004) Kumar R & Aggarwal A K, Opt Commun, 279 (2007) Caulfield H J, Opt Laser Technol, 37 (2005) Balogh T, Forgacs T, Agocs T, Balet O, Bouvier E, Bettio F, Gobbetti E & Zanetti G, Proc Eurographics, Yoshida S, Yano S & Ando H, SIGGRAPH, Osten W & Reingand N (ed), Optical imaging and metrology: Advanced technologies, (Wiley-VCH), Jones A, Mcdowall I, Yamada H, Bolas M & Debevec P, Proc ACM SIGGRAPH, 26 (2007). 17 Kitamura Y, Konishi T, Yamamoto S & Kishino F, ACM SIGGRAPH, 1 (2001) Rong X, Yu X & Guan C, Appl Opt, 50 (2011) Cossairt O S, Napoli J, Hill S L, Dorval R K & Favalora G E, Appl Opt, 46 (2007) Fattal D, Peng Z, Tran T, Vo S, Fiorentino M, Brug J & Beausoleil R G, Nature, 95 (2013) 348.