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COSTEL is an integrated, automated system for the measurement of human motion. It is based on solid state cameras equipped with linear image transducers (2048-element CCD arrays) and toroidal lenses1. Each camera must be considered a one-dimensional sensor, which estimates the spatial position of a plane passing through the target point and the nodal axis of the lens. Thus, three cameras (3D unit) are necessary and sufficient to determine the spatial coordinates of a target Each 3D unit is connected to a personal computer (PC) by an interface board. The target points whose positions are to be measured (markers) consist of infrared light-emitting diodes (IR-LEDs) which are fired one at a time, in a sequence synchronized with the system operation. The calibration of a 3D unit is carried out by means of a frame, formed by 0.8 m aluminum bars, supporting up to 20 markers whose positions are known with precision. The use of this small (0.8 x 0.7 x 0.7 m) and light (5 Kg) calibration object is permitted by the previous, factory made, assessment of the optical distortion error associated with each camera. The various system operations (calibration, data collection, 3D marker trajectory reconstruction, etc.) are driven by suitable PC software. Application programs are available, permitting the biomechanical analysis of lower limb joints. The COSTEL system is able to track up to 20 markers at 125 Hz sampling frequency, or 10 markers at 250 Hz. The spatial resolution is 0.025% of the measurement field-width, the precision is 0.02%, the accuracy (maximum absolute error) is 0.1%. In an upgraded version of the system the resolution will be better than 0.01%, precision and accuracy will be enhanced accordingly. An outstanding feature of COSTEL is its low sensitivity to spurious light, reflections and other environmental noise. This feature is due to the reflection coefficients at the marker radiation light wavelength (820 rim), and to the camera signal conditioning.
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