Fiber optic displacement transducer with dichroic target

Claim 1

A fiber optic displacement transducer comprising a target movable in an axial direction, a sender fiber optic cable having a light emitting end axially spaced from said target a receiver fiber optic cable having a light receiving end axially spaced from said target, optical filter means on said target having a first surface reflecting light of a first wavelength from said sender cable to said receiver cable, said optical filter means having a second surface axially spaced from said first surface by a fixed distance t and reflecting light of a second wavelength from said sender cable to said receiver cable, where I1 is the intensity of said first wavelength light in said receiver cable and I2 is the intensity of said second wavelength light in said receiver cable, and wherein I1(I2-I1)×t varies linearly with axial separation s of said cable ends from said target over a given range of s, such that changes in transmitted light intensity due to optical disturbances in said sender cable cancel out of said fraction I1/(I2-I1) whereby to prevent a false reading of s as measured by I1/(I2-I1)×t, because t is fixed.