Patent ID: 7001068
Filing Date: 2006-02-21
Classification: G01J

Abstract:
1. An electro-optically modulated birefringent filter comprising, in sequence, (a) a band pass filter, which permits the passage therethrough of radiation from a radiating body in a predetermined wavelength band; (b) a first polariser; (c) an electro-optical element which, when a voltage is applied to it, splits incident polarised radiation into first and second orthogonally polarised components which travel through said element at different velocities; (d) a second polariser, to combine said components as they exit from said electro-optical element; and (e) observing means to observe the intensity value at the point in the interferogram produced by combining said first and second components; characterised in that (f) a voltage supply means, adapted to supply a periodically varying voltage, is operatively connected to said electro-optical element to produce a modulated birefringence of said element; (g) said electro-optical element is configured so that, in the absence of an applied modulating voltage, the net optical path length delay between said first and second components is such that, on combination of said components, said intensity value is the intensity value at or near a peak or a trough in said interferogram; and (h) said observing means includes means to generate a signal indicative of the observed intensity value, said signal being input to analysing means which evaluates the harmonic amplitude ratio, which is the ratio of the amplitudes of the intensity at the fundamental and second harmonic of the frequency of the modulating voltage, at the observed point of the interferogram.