What is Time-of-Flight?

Time-of-Flight (ToF) sensors provide a direct way for acquiring 3D surface information of objects. More recently, applications like gesture recognition or automotive passenger classification use ToF sensors and ToF is about to become a component of consumer electronics. As ToF sensors provide data at rates higher than 15 frames per second, they are suitable for real-time 3D imaging and can also be used as an additional imaging modality in medicine. We are convinced that ToF technology can contribute to enhance applications within medicine and suggest several applications within this field.

Working principle

A ToF camera actively illuminates a scene with an incoherent light signal. The signal is modulated by a cosine-shaped signal of frequency f. Usually, the emitted light is part of the non-visible area of the spectrum in the near infrared spectral range (780 nm). Traveling with the constant speed of light in the surrounding medium, the light signal is reflected by the surface of the patient. By estimating the phase-shift f (in rad) between both, the emitted and reflected light signal, the distance d can be computed as follows:

Based on the periodicity of the cosine-shaped modulation signal, this equation is only valid for distances smaller than c/2 f . Currently available ToF cameras operate e.g. at a modulation frequency of about 20 MHz. Thus, the upper limit for observable distances of available ToF camera systems is approx. 7.5 m, which is sufficient for respiratory tracking. In addition to depth values, ToF cameras provide intensity values, representing the amount of light sent back from a specific point.