An instrument has been designed that should allow the measurement of the temporal point spread function (TPSF) of tissue by a cross-correlation technique using an avalanche photodiode (APD) detector. Although not having the temporal resolution of a streak camera, the system is small, rugged, portable, and relatively inexpensive. A laser producing a stream of ultra-short (few ps) pulses of light is used to interrogate the tissue. The scattered light is detected on a small area APD, the gain of which can be rapidly changed, by modulation of the applied DC bias voltage. The gain of the photodiode is usually low, but for a time period of the order of a few hundred picoseconds, the gain is increased. A lock-in amplifier measures the output of the photodiode and displays the difference between the output in the two gain states. Hence the photodiode is made to sample only a small section of the TPSF. A variable time-delay mechanism in the system, allows any section of the TPSF to be sampled, therefore allowing measurement of the whole TPSF. The novel method of generating the modulation voltage for the APD, which uses microwave techniques, means that it is possible to use an electronic phase shifting network, rather than the more commonly used mechanical time-delay mechanism. In principle, this means that the system should be very rugged and reliable, as there are no moving parts. The light source used during the system development is a large mode-locked Ti:sapphire laser, however by replacing this with a picosecond pulsed laser diode, a compact and portable instrument can be built, capable of being used at the bedside.