The apparatus comprises a) a set of probes each having tips arranged to simultaneously touch the skin and define a predetermined pattern on the skin, at least one of the probes being arranged to transmit a pulsed electrical signal and at least one of the probes (which me be the same as or different to the transmitter probe) being arranged to receive the transmitted electrical signal b) a signal detector for detecting the or each received electrical signal c) means for comparing a numerical value obtained from at least one detected signal from the signal detector with at least one predetermined numerical value and d) means for providing an output when said value obtained from the detected signal differs from the predetermined numerical value by more than a predetermined amount. The apparatus is used to control intense pulsed light devices used for local treatment of various skin conditions and to influence non-desired hair growth.
