## 1 Introduction  
Since the discovery and biophysical characterization of light-sensitive microbial ion channels [1, 2] and their subsequent application to neurons [3], the field of optogenetics has experienced a rapid development as an important experimental tool in the life sciences. Optogenetics describes the control of activity of cells that ectopically express these light-sensitive membrane proteins. One of the most prominent optogenes is the cation channel Channelrhodopsin-2 (ChR2) [2]. In the last years, more and more light-sensitive proteins with improved properties have been discovered to activate cells, to inhibit cells, to optically observe the activity of cells [4, 5] or even to induce intended cell death [6]. In the last decade, optogenetics was also introduced to cardiology research [7]. For example, ventricular arrhythmias are associated with significantly increased morbidity and mortality, representing an increasing problem worldwide. They are treated or investigated conventionally either medicinally by application of antiarrhythmic drugs or by means of electrophysiology such as a pacemaker and defibrillator. These approaches, however, suffer from a lack of cell specificity and insufficient local (cellular) and/or temporal resolution. Here, optogenetics has introduced a paradigm shift since it allows for stimulating cells with single-cell spatial resolution and high temporal resolution. Illumination using a defined light pattern allows to study and optically control the onset, direction, speed and chirality of distortions in the propagation of excitation wavefronts in cardiac tissue like re-entry events or spiral excitation waves [8], and to terminate such arrhythmias [9; 10; 11]. Applying temporal light patterns at high frame rates can pave the way to optogenetic cardiac pacing and resynchronization therapies [12]. Even though many hurdles like stimulation in deep tissue still remain unsolved, cardiac optogenetics is attributed a great potential for clinical application [7]. Furthermore, optogenetics is a powerful tool to study function and neuronal circuits in the central nervous system [3]. Human in vitro model systems consisting of such neurons allow for studying functional connectivity within neuronal circuits [13], offering a huge potential for basic and biomedical research. Such experiments can pave the way to gain a deeper insight into effects like long-term potentiation (LTP) [14] or various widespread neurodegenerative diseases like Alzheimer's disease. However, identifying the signaling pathways and functional connectivity in networks that contain several hundreds or even thousands of neurons is a challenging task. Spontaneous neural activity is always present within the entire network and a whole-field optogenetic stimulation would only increase this global activity, making the identification of distinct pathways ambiguous or almost impossible. Standard techniques like electrical stimulation and recording using multi-electrode arrays suffer from a lack of spatial control and cannot record during the applied stimulation. The key to overcoming this problem lies in single-cell excitation using a laser beam focused to cell size. Hence, the well-defined knowledge about spatial origin of the action potentials helps to reduce these ambiguities. Occurring connections and subnetwork motifs within human random neuronal networks can be identified successfully [15; 13]. Other fields using optogenetic approaches are vision restoration [16], the induction of local mechanical stress in cells [17], drug screening [18], linking of behavior and neuronal circuits in animal studies [19] or the control of intracellular mRNA mobility [20]. In conclusion, the aforementioned examples show that one key to fully exploiting the immense potential of optogenetics is the application of a well-defined spatio-temporal light field. Depending on the specific optogenetic application, several demands on the optical stimulation exist.