Motion capture cameras surrounded by LEDs

Aug. 20, 2006
Motion capture, with applications in life sciences, entertainment and engineering, uses cameras with integrated LED light sources.
A number of industries, including entertainment, defense, life science and engineering, are increasingly making use of motion capture systems, at the heart of which are cameras that use integrated LED light sources.

Vicon, a UK company that is a leading developer of motion capture systems and part of Oxford Metrics Group plc, manufactures cameras in which multiple LEDs are arranged around the camera lens.

The light emitted by the LEDs is reflected back off retro-reflective markers worn by the subject being "captured". A number of cameras are placed around the space in which the subject is moving. The reflected light is collected by the camera’s sensor, and the position of the markers can be determined if two or more cameras “see” the same marker.

Gollum Applications include tracking the movement of actors and subsequently overlaying CGI imagery to create "monsters" such as King Kong in the recent remake or Gollum in the Lord of the Rings trilogy. In the life sciences, motion capture allows doctors to assess the gait of children with cerebral palsy, or the analysis of techniques of professional athletes, or measuring the way a race horse moves, for example. Motion capture is also used for virtual reality displays in engineering and development.

There are two main frequencies used for the LEDs, one in the visible red range, the other just into the infrared range. Vicon offers the customer the choice of either, saying that the visible red has the benefit of giving excellent range, whereas the infrared gives almost as good range but is less intrusive in the capture environment.

Because the markers are retro-reflective, i.e. they reflect light straight back, it is important that the LEDs are positioned as close to the lens as possible for best capture performance. In fact, due to their small size, the LEDs can be arranged in a uniform pattern around the lens. If the light source is positioned away from the lens, the light will be reflected back to the source rather than the lens.

As well as positioning the LEDs around the lens, there are several other reasons to use LEDs rather than an alternative light source. The LEDs are used as strobes, i.e. the light is not on continuously but is flashed in short bursts, and LEDs are highly suited for this purpose. Also, LEDs have a long life time and are cost-effective, and the frequency of the output light is fairly uniform (which means that the reflected light can be filtered effectively). Finally, the LEDs have a small lens that spreads the light in a uniform fashion throughout the space where the capture takes place.

The performance requirements for the LEDs depend on the motion capture requirements. For long-range capture, it is a case of “the brighter the better”. For typical capture volumes where the cameras are placed 5-10m away, usually it is more important that the LEDs spread the light reasonably uniformly over the camera’s field of view.

Oxford Metrics’ global clients in science, medicine, sport, engineering, gaming, film and broadcast include major hospitals and research facilities such as Guy's Hospital, Nuffield Orthopaedic and Loughborough University, engineering industry leaders including Ford Motor Company, BMW, Airbus, Caterpillar, and Toyota, and in the entertainment sector, Sony, Industrial Light and Magic, The Moving Picture Company (MPC), Sega, Nintendo, UbiSoft, EA, Square Enix and many others.