Conventional imaging sensors consist of multiple photo diodes. In cameras these diodes are arranged within a matrix and provide an image of, e.g. color or gray values. In opposite to normal cameras, a Photon Mixing Device (PMD) sensor simultaneously acquires a distance value for each pixel in addition to the common intensity (gray) value. The theoretical principle behind the technology relies on the ToF principle. This means that a ToF camera has at least one illumination source to actively illuminate the scene. Typical ToF cameras use intensity modulated infrared light, which is not visible to human eyes to illuminate the scene. Other light sources than infrared light are also possible. The ToF sensor captures the reflected light and evaluates the distance information on the pixel. This is done by correlating the emitted signal with the received signal. Concluding, a ToF sensor is composed by a matrix of distance sensors. Despite these functional improvements (compared to conventional imaging sensors) the sensor itself is still a standard CMOS sensor. Therefore imaging and 3-D measurement capabilities can be placed next to system-relevant electronics like analog-digital converters, etc. All ‘intelligence’ of the sensor is included on the chip, meaning that the distance is computed per pixel. Therefore ToF pixels are also called ‘smart pixels’.
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 30 frames per second, they are suitable for real-time 3-D imaging. We are convinced that ToF technology can contribute to enhance applications within various business fields.
Working principle
ToF cameras provide a real-time 2.5-D (only a part of the surface can be seen by the ToF camera) representation of an object. The object is actively illuminated with an incoherent light signal. This signal is intensity modulated by a cosine-shaped signal of frequency 
Traveling with the constant speed of light in the surrounding medium, the light signal is reflected by the surface of the object. 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 these ToF camera systems is approx. 7.5 m. Other distances are also possible by adapting the modulation frequency. In addition to depth values, ToF cameras provide intensity values, representing the amount of light sent back from a specific point.
Non-ambiguous range:
Due to the measurement principle, ToF cameras have a non-ambiguous range of 
- c [m/s] denotes the speed of light,
- d [m] the distance the light travels,
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Relation of a phase shift to the distance at a fixed modulation frequency of 
Time of Flight camera manufacturers
Today various companies provide Time-of-Flight cameras. Currently Metrilus supports the following companies:
- Mesa Imaging (Swissranger)
- Fotonic (C-40, C-70)
- PMD (CamCube, CamBoard)
- ifm (O3D-200)
This page is based on C. Schaller, ‘Time-of-Flight – A New Modality for Radiotherapy’, PhD Thesis , 2011.