All of the sensors were based on chromatic modulation. Normal optical fibre doesn't preserve the intensity or polarisation of light at all well. However, the colour of the light is much less affected, so by starting with a very broad spectrum (white) and selectively modulating it, the measurand could be transmitted with far greater fidelity. The detector module then has to produce an output proportional to the colour of the received light.

The bistimulus detection module achieved this by using two photo-detectors with different, but overlapping, spectral responses. After some initial analogue processing, the signal was converted to a digital value that could then be translated, in software, to a measurand value.

The light is made to take a zig-zag path from one end of a block of paramagnetic glass to the other. In so doing the light passes through the magnetic field concentrated across the gap in the soft-iron core in which the glass block has been placed. This magnetic core has the current carrying conductor passed through its centre (not shown).

Wavelength dependant polarisers are included at each end of the glass block such that the longer wavelengths are not polarised whilst the shorter wavelengths are. The Faraday effect (rotation of the plane of polarised light by a magnetic field) then modulates the shorter wavelengths, but leaves the longer, unpolarised, wavelengths unaltered.

In this way the "Dominant Wavelength" of the returned spectrum is altered according to the current flowing. The instrumentation then converts and scales this to reconstruct the waveform of the current flowing.

Applications for this sensor included sensing pressure in hostile environments and the force applied by a railway locomotive's pantograph to the overhead catenary.

Below are pictures of rotary sensors. The first was for detecting the position of rotary shafts in high-voltage switchgear. The second is of a sensor developed to couple magnetically through the wall of a propane tank to a pre-existing float mechanism to allow the contents of the tank to be remotely read.

(The observant amongst you will notice that these pictures were taken after Optical Sensors Ltd. had been bought by Lucas.)