a phase shifted version of the transmitted mls is applied to the shutter control input of the high speed shuttered camera so that the camera is gated on whenever the reference mls is high this causes every pixel in the camera 39 s focal plane to sum the product of i the optical mls signal transmitted and reflected from the target and focused on the camera 39 s sensor and ii the delayed reference mls signal the reference mls signal is applied to the camera 39 s shutter control input effectively computing a single sample of the cross correlation function between the reflected mls and the reference mls corresponding to the phase shift applied to the reference mls by the timing electronics thus the image sensor is effectively used to compute the value of a single sample a sum of products function of the cross correlation function for every pixel over the surface of the image sensor the length of the modulation signal is preferably selected so that the length of the sequence is greater than the maximum delay by increasing the phase delay of the reference mls signal in steps corresponding to one bit of the sequence the full cross correlation function can be determined the mls sequence may for example be of the order of 10 s of microseconds long and may comprise of the order of hundreds of bits the gating signal now has a minimum duration of one bit period of the mls data this enables a longer shutter period to be provided and this is also more easily implemented as part of the electronic addressing scheme of the photodiode or phototransistor array.