Polycrystalline silicon thin film solar cells require coarse grained silicon layers on a glass substrate. The preparation starts with a layer of amorphous silicon some hundred nanometers thick. By laser crystallization it is converted into a seed layer consisting of grains several ten mu m in size. We report on in situ diagnostics by time resolved reflection and transmission (TRRT) measurements during the preparation process. Joint diagnostics by different lasers and the comparison with optical and electron micrographs of the resulting films give unique information about the crystallization processes. Even if different processes occur in hydrogenated or hydrogen free amorphous silicon films during the heating induced by different irradiation parameters the results of crystallization are quite similar.