Photon Time-of-flight Spectroscopy (pTOFS) is a tool for characterisation and analysis of highly scattering (turbid) materials, such as biological tissue, powders, and pharmaceutical samples. The main principle is to inject a very short laser pulse into the material, and to analyse the resulting emerging pulse at some distance from the injection point. Due to the multiple scattering of photons in a turbid material, the detected pulse will be much broader than the original pulse. Some photons will reach the detector rather fast (short time-of-flight), while others will arrive much later (long time-of-flight). In combination with models of light propagation in scattering materials (i.e. photon migration), the distribution of photon time-of-flights can be used to determine the absorption and scattering properties of the sample. The derived information on absorption can be used to determine the chemical composition of the material, while the scattering properties give information on the physical/structural properties of the material. Since the idea is to resolve a transmitted or reflected light pulse in time, the technique is sometimes referred to as time-resolved spectroscopy.

Since light travels very fast, the detector has to be extremely fast to be able to resolve the distribution of photon time-of-flights. In many applications, the time-resolution must be in the order of tens of picoseconds.