## 1 Introduction
A versatile light stimulation system should exhibit the following features:  
* Single-cell resolution. Using wide-field illumination in neural networks will lead to an overall, uncoordinated increase of action-potential activity, making it almost impossible to identify single signal paths. The local stimulation of a single cell can precisely define the origin of action potentials within a network and help to reduce ambiguity. * 3D stimulation. The focal spots are placed arbitrarily in 3D space. In cardio-optices, many experiments were performed on cardiomyocyte monolayers, i.e., 2D samples [7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22]. Similarly, investigations on neuronal networks and network connectivity are often conducted on multi-electrode arrays in 2D cultures [23; 24]. The ability to place the stimulating light focus in an arbitrary and defined depth can help to conquer the third dimension and to address single cells in multi-layer cell systems [25]. * Patterned illumination. The concurrent activation or inhibition of a well-defined group of cells opens further possibilities in experimental studies as well as optogenetic applications. In cardio-optogenetics, a dynamic, i.e., temporally changing distributed pattern can control the propagation of the excitation wavefront in cardiac tissue and induce spiral waves of defined speed, direction and chirality [8]. Regarding implantable optogenetic defibrillators ore pacemakers, an optimized pattern can help minimize the required illumination energy and hence reduce energy consumption [10]. * Two-wavelength illumination. The ability to express activating and inhibiting opsins at the same time in one cell or cell network opens new experimental possibilities. Activating single cells or cell groups while concurrently silencing other well-definedparts in a network can help to elucidate signaling pathways in neuronal networks or to study the propagation of the excitation wavefront at hearts. For such experiments, two patterns of different wavelengths should be displayable individually and independently. * Fast switching times. High temporal resolution in the millisecond range is necessary or advantageous in various experiments: In neural optogenetics, it should be of the same order of magnitude as the action-potential frequencies; in cardio-optogenetics, it is desired to achieve a smooth transition between successive images when displaying spatio-temporal patterns. Furthermore, high pattern update rates are a fundamental prerequisite in experiments that employ an optoelectronic closed-loop feedback control, e.g., in cardio-optogenetics, where the trajectory of the excitation wavefront is measured, processed and used to alter the illumination pattern [7]. The way for such a stimulation platform is paved by the rapid development of electrooptic techniques and components [26]. Light sources like fluorescence lamps or single light-emitting diodes (LEDs), available in a wide spectral range and used predominantly in the early phase of optogenetics, can hardly achieve single-cell spatial resolution due to their lack of spatial coherence. Using a laser with diffraction-limited beam quality and a two-axis galvo scanner for 2D beam steering allows for fast switching rates and single-cell resolution [26], but not for simultaneous stimulation of different cells and complex pattern illumination. This can be achieved by amplitude modulators (e.g., DMD: Digital Micromirror Devices) [26; 27] for direct light projection, offering a high flexibility through Mega-pixel count. As an alternative to the combination of a laser with a subsequent light modulator, arrays of organic micro LEDs (OLEDS) are currently being investigated for application in optogenetics [28]; a linear array, e.g., was used to display a moving light pattern and place light stimuli to individual abdominal segments of Drosophila melanogaster larvae [29].