Properties of thin films such as the crystallinity of silicon deposited from SiH4 - H-2 discharges are governed by the plasma composition. Therefore, it is crucial to measure the plasma composition in order to understand and optimize the deposition rate, deposition efficiency and material quality of thin film deposition, such as for microcrystalline silicon, which is a key material for silicon thin film photovoltaic solar cells. This task can be performed by using Fourier transform infrared absorption spectroscopy or optical emission spectroscopy. This work compares these two techniques and shows their range of applicability and their limitations.