COL 9(12), 120008(2011) CHINESE OPTICS LETTERS December 10, 2011 Complex light modulation for lensless image projection M. Makowski 1∗ , A. Siemion 1 , I. Ducin 1 , K. Kakarenko 1 , M. Sypek 1 , A. M. Siemion 1 , J. Suszek 1 , D. Wojnowski 1 , Z. Jaroszewicz 2 , and A. Kolodziejczyk 1∗ 1 Faculty of Physics, Warsaw University of Technology, 75 Koszykowa, 00-662 Warsaw, Poland 2 Institute of Applied Optics, 18 Kamionkowska, 03-805 Warsaw, Poland * Corresponding author: michal.makowski@if.pw.edu.pl Received August 1, 2011; accepted October 11, 2011; posted online November 18, 2011 We present a lensless projection of color images based on computer-generated Fourier holograms. Am- plitude and phase modulation of three primary-colored laser beams is performed by a matched pair of spatial light modulators. The main advantage of the complex light modulation is the lack of iterative phase retrieval techniques. The advantage is the lack of speckles in the projected images. Experimental results are given and compared with the outcome of classical phase-only modulation. OCIS codes: 090.1705, 090.1760, 090.5694. doi: 10.3788/COL201109.120008. In most fields of consumer electronics a strong trend towards miniaturization and portability is observed. Re- cently this tendency has reached image projectors, which are currently associated with bulky, noisy, and heavy appliances. Thermal light sources are being constantly replaced by more efficient and long-life light emitting diodes (LEDs) and lasers, hence eliminating the noisy ventilation and reducing the chassis size. On the other hand the strong obstacle in the miniaturization of pro- jectors is the necessity of installing the imaging lens. Its presence involves size requirements along the light path due to the non-zero focal length and in transverse directions to minimize image blur due to low lens aper- ture. Using smaller lens strongly decreases the image sharpness to an unacceptable level. In order to overcome this limitation numerous concepts of lensless projection were reported. Most of them have serious disadvan- tages. For example, commercially available beam scan- ning systems have low resolution, high speckle contrast, inevitable image distortions [1] , and problematic health aspects due to naked laser beam exposure and highly flickering images. In order to encode color information, the time-division switching of images of primary colors is commonly used [2,3] . It provides rich colors and requires relatively simple optical setups for the price of unstable images, which is especially visible when fast-moving ob- jects are projected. Therefore alternative techniques of color projection based on the diffraction of light have been reported to be superior [4] . Our approach is based on real-time reconstructions of two-dimensional images from a set of Fourier holo- grams. The power of the missing imaging lens is inte- grated in the phase transmittance of holograms, which are displayed on spatial light modulators (SLMs). The approach is similar to the one by Buckley et al., where lasers were used with SLMs and only one lens [5] . In prin- ciple, when a Fourier hologram is reconstructed with a single phase modulating device, one removes the ampli- tude information from the process. Therefore a strong speckle pattern is observed in the output plane, which is due to a random initial phase usually used in iter- ative phase retrieval methods. In previous works we successfully used iterative fourier transform algorithm (IFTA) and time-integration of holograms with different random initial phase patterns to significantly reduce the speckle contrast [6,7] . There was also introduced an additional phase factor to separate images from the zero- order light peak, which can be then blocked with am- plitude filtering [8,9] . However, according to obtained results further suppression of speckle pattern and spuri- ous diffractive orders lead to an inevitable degradation of output image resolution. Another solution of the speckle problem is to preserve the amplitude information in the holographic process. In this contribution we have added a second SLM to act as an amplitude modulating device [10−12] . The design of two distributions: ampli- tude and phase became trivial as it included a single propagation of the wavefront at the projection distance. Note that as the propagation distance we understand the distance between the last SLM used (SLM2 in Fig. 1) and the projection screen, is equal to 200 mm in our experiments. Eventually, as a consequence of amplitude modulation we have observed a suppression of speckles and a decrease of non-diffracted light, that substantially contributed to better quality of output images. The ideal scheme of the optical setup for the proposed color projection is presented in Fig. 1. Two liquid crystal on silicon (LCoS) full-HD Holoeye Pluto SLMs are used with three lasers operating at 671 nm (He-Ne), 532 nm (diode pumped solide state Nd-YAG), and 445 nm (laser Fig. 1. Scheme of the optical setup for image projection with complex light modulation. BE: beam expanders. 1671-7694/2011/120008(3) 120008-1 c  2011 Chinese Optics Letters