An apparatus for sensing data from a remote optical sensor 16 has its frequency stabilised by balancing the outputs of narrow band filter 28 30, spaced about a desired frequency 36 positioned at about the 3 db down points 40 of a broad band light source 10 using voltage control, current control or temperature control to vary the frequency of the wide band light source 10. Difference between the outputs through the two narrow band filters 28 30 can be used to drive an amplifier 48 to correct the frequency of the broad band light source. The outputs through the two narrow band filters 28 30 can be converted 52 to binary numbers and fed to a microprocessor 56 which is used, via analog conversion 60, to drive the amplifier 48. The broad band light source 10 can be pulse modulated 68 to provide temporally separate light pulses 92 94 through each of the narrow band filters 28 30, measured at separate times. The corrective output to the amplifier 48 can be governed by a ratio between the outputs through the narrow band filters 28 30 rather than by a difference there between.