"Influence of TFT-LCD Pixels Structure on Holographic Encoding" "Wang Hongjun*,Tian Ailing, Liu Bingcai, Wang Chunhui"

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1 Available online at Phsics Procedia 9 (0) International Conference on Optics in precision Engineering and Nanotechnolog "Influence of TFT-LCD Piels Structure on Holographic Encoding" "Wang Hongjun*,Tian Ailing, Liu Bingcai, Wang Chunhui" "Shaani province ke lab of thin films technolog and optical test, Xi an technological universit, No.4 Jinhua North Road, Xi an, 7003, China" Abstract The LCD is proposed as a new hologram loader with the advantage that the hologram image is controlled digitall with no an mechanical moving and rotating elements. Different hologram image can be readil introduced b changing the coded image displaed on the LCD. A basic problem in which LCD is a hologram image loader is limited LCD s piel number. In CGH, the sample point number have great impact on holographic representation. So the choice of sample point number is ver important. When the LCD is phase hologram image loader, the phase on hologram image was decided b phase distribution of recorded wavefront on LCD plane. In phase hologram encoding, the piel spacing is sampling interval of recorded wavefront. Based on sampling theor, the frequenc band can be obtained b spectrum analsis. For different wavefront shape, A model was developed for phase rate, the spectrum distribution was got, the phase rate which can be recorded b LCD can be got. A didactic eample is included to illustrate the computational procedure. 0 Published b Elsevier B.V. Open access under CC BY-NC-ND license. Selection and/or peer-review under responsibilit of the Organising Committee of the ICOPEN 0 conference "Kewords: TFT-LCD; Phase hologram; Holographic encoding" * Corresponding author. Tel.: ; fa: address: whj053@sina.com Published b Elsevier B.V. Open access under CC BY-NC-ND license. Selection and/or peer-review under responsibilit of the Organising Committee of the ICOPEN 0 conference doi:0.06/j.phpro

488 Wang Hongjun et al. / Phsics Procedia 9 (0) 487 49. INTRODUCTION Liquid Crstal Displa (LCD) has recentl infiltrated man aspects of our ever da lives [][].

2 488 Wang Hongjun et al. / Phsics Procedia 9 (0) INTRODUCTION Liquid Crstal Displa (LCD) has recentl infiltrated man aspects of our ever da lives [][]. The dependence of the birefringence on local eternal electric fields makes LCD a good choice for an easil controllable phase grating [3][4][5] and frequentl used spatial light modulators (SLM) in the field of optical image processing. As a new spatial light modulator, TFT-LCD had good optical modulation characteristic, high spatial resolution, low power consumption and more little volume, so it was an important liquid crstal spatial light modulator. The modulation properties of LCD allow a real time manipulation of a wavefront, therefore it is possible to displa holographic information using them [6]. When it was used in holographic representation, because the piels had hold with black metal, comparing with hologram plate, it has some special influence on holographic encoding. The computer simulation of liquid crstal is a rapidl epanding field: man techniques needed to measure the properties of interest, and to stud the relevant phase transitions, are still being developed. In practice, the computer simulation complements the ver well established eperimental method of snthesizing and testing large numbers of compounds. In this paper, the influence of LCD s piel structure on holographic encoding would be analzed in details.. TFT-LCD s PIXELS CONSTRUCTION The TFT-LCD has a series of electrodes in line and row between piels. The nodal between line and row electrodes is thin film transistor (TFT) which was processed b large scale integrated circuit technolog. Line electrodes, row electrodes and piels were link in gate, source and drain individuall. The piels can transparent light when its line electrodes and row electrodes were impose electric signals at the same time. The magnitude of electric signals can controls the transmittance of liquid crstal piels, it mean can control piels gra. For period distribution of piels, the TFT-LCD was placed period in line and row. Because of photoelectric effect, the circuit noise can be raised when the light irradiate on electrodes. Therefore light-blocking metal laer was plating in electrodes. Because the transmittance of metal film is zero, the TFT-LCD becomes a two dimension black gate, it was shown in Fig.. The aperture ratio (AR) was defined as the ratio between efficient displa area and piels area, it was defined as a * b AR *00% A* B In general, the piels and aperture of TFT-LCD are square, the width of line and row black metal structure was the same. That is b=a, B=A, so a AR *00% A Because the area of TFT-LCD is constant, TFT-LCD s resolution is epressed b piel distance or piel number in certain area. In general, the resolution was defined as the piel number in TFT-LCD panel. Fig TFT-LCD black metal structure

3 Wang Hongjun et al. / Phsics Procedia 9 (0) In actual uses, because the TFT-LCD has two dimensions black metal restructure, the aperture ratio was less then 00%, the light energ utilization was decreased. Meanwhile, the diffraction image was introduced, the optical filed was impacted in a large etent. 3. THE ENCODING METHOD In the aspheric surface measurement based on LCD, with the aspherical surface under testing in theor, the wavefront which in LCD can be calculated, it include the amplitude and phase. It need to be produced b LCD modulate to incidence light. The comple amplitude is modulated as: U r ( u0 ep[ ik( z0 r( ] 3 In the eperiments, the incidence light is the same amplitude, so the encoding is phase encoding mainl. In the coding, the phase distributing can be calculated. Based the relation between interference pattern and phase distributing, the arithmetical compliment can be achieve b the phase distributing to. In the calculation, z 0, as a constant, can be neglect. So the phase distributing is from 0 to, epressed as P( mod[ k r(, ] 4 The relation between phase and gra level can be establish b the eperiments[], H 0 ( F( P) 5 So the phase distributing transform to gra level distributing, thus the hologram which can product standard aspherical surface was formed. In the test, the N step phase shift is need, it can be carr out b introduce different hologram. To N step phase shift, the ever phase shift hologram is P i ( mod[ k r( i, ] i,, N (6) H F( P ) i,, N (7) 4. SAMPLE i ( i In holographic encoding, first, the object wavefront or frequenc spectrum plane must be sampled. In sampling, the sampling theorem must be met in order to the object wavefront represented perfectl. Under sampling theorem, if sampling point is too much, the data calculated amount and memor space can be ver large, then it increase processing difficult of CGH. So the choice of sample points has significant impact on CGH. Suppose the distribution function of original wavefront is O (, the sampling interval are and in direction and, then sampling function is O s ( comb( ) comb( ) O( (8) The spectral distribution is F(, ) Fcomb( ) * Fcomb * FO( (9) From equation (9), we can find frequenc spectrum function is periodic function when original wavefront was sampled in spatial domain, and the period are and respectivel. Suppose O ( ) is band-limited function, its bandwidth are and B in direction and respectivel, then B F(, ) B B, B B FO { (, )} 0 Ot her s (0)

4 490 Wang Hongjun et al. / Phsics Procedia 9 (0) So, In order to represent the original wavefront, the original wave spectrum can not overlap in the repeated process periodicall, its means B B () So sampling interval must meet () B B Then the original wavefront can be complete represent. When the loader of phase hologram is LCD, the phase of hologram was decided b the phase distribution of recorded wavefront on LCD. In other words, it was decided b wavefront distribution of recorded wavefront. In phase holographic encoding, the sampling interval of recorded wavefront is LCD s piel pitch. When LCX03 was chosen as hologram loader, the piel pitch is 5.80m, the width are L 6.46mm and L 9.8mm, so the bandwidth of recorded wavefront is B 38.76mm. In following, the spectrum of object wave comple amplitude was analzed. In equation (9), because the aperture is L L, so the spectrum can be epressed b F(, ) ep[ j ( ] rect rect ep j( dd L S L (3) There S is aperture of LCD. In order to facilitate analsis, we suppose that Wavefront phase distribution records in accordance with linear, it can be epressed b (, then F(, ) 4LLSa LSa L (4) So the bandwidth of recorded wavefront in direction and can be calculated b B B L (5) L According to sampling theorem, form equation (5), the range of recorded wavefront phase rate is mm / mm and 38.8 / From the above analsis we can see that ever piel as sampling point meet sampling theorem when hologram loader is LCX03 and phase rate of recorded wavefront / mm. For different LCD, the same method can be used to analze the corresponding sampling problem. 5. CONCLUSION With theoretical analsis, the influences of piel construction and displa character of LCD on holographic encoding was analzed. There are obvious different sample rate to different aperture ratios. Thereb, there is different recorded wavefront bandwidth on different holographic loader. It will influence on holographic encoding accordingl. B computer simulation, when the hologram loader is LCX03, the phase rate of recoded wavefront will not be eceeded38.79 / mm. So the emphasis is to find wa to large aperture ratio b technolog and theor in the future. Meanwhile, influence of the black metal construction and resolution on holographic encoding was obtained. Therefore, for obtaining more high qualit representation image, the LCD process technolog must be improved and the holographic representation sstem need be optimized. 6. ACKNOWLEDGEMENT

5 Wang Hongjun et al. / Phsics Procedia 9 (0) This work was supported b Fund (No.09JK487) and Fund of China Cholarship Council (Stud on real time measuring method and theor of microoptics lens) 7. REFERENCE [] W. P. Belha, et al, SID 99, International Smposium, Digest of Technical Paper, Kenote Address. [] D. C. Lupascu, et al, Applied Phsics Letters, 78(7), (00). [3] Naveen Kumar Nishchal. Optical phase encrption b phase contrast using electricall addressed spatial light modulator.opt. Commun., 7(- 6), 7- (003). [4] Chen Huaiin, et al, Optical Modulation Characteristics of Liquid Crstal Television (LCTV) and Its Application in Optics Information Processing. CHINESE JOURNAL OF LASERS, 7(8), (000). [5] F. T. S. Yu, S. Jutamulia, T. W. Liu. Adaptive Real-time Pattern Recognition Using a Liquid Crstal TV Based Joint Transform Correlator. Appl. Opt., 6(8), (987). [6] R.Tudela, et al. A simple method for displaing fresnel holograms on liquid crstal panels. Opt. Commun., (00)

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