Patent ID: 6943868

Claim:
An apparatus for computing the relative spatial position of a number of partially reflecting surfaces in an optical system, said apparatus comprising: light generating means for generating and emitting a plurality of ultrashort source pulses of light not greater than one nanosecond in duration towards the optical system; a detector for detection of light signals reflected from said partially reflecting surfaces, and for discriminating the arrival of reflected light signals to a time interval of the order of the length of the emitted pulses; synchronisation pulse generator means for providing a timing reference against which the arrival time of each detected reflected signal can be measured, and; iv) signal processing means for computing the time interval between the arrival of each reflected signal and the temporally adjacent synchronisation pulse, wherein the mean number of photons per source pulse that are reflected by the optical system and collected by the detector is ≦1, and the signal processing means generates a histogram of reflected pulses as a function of the computed time interval and the relative spatial positions of the reflecting surfaces within the optical system.