Resonant piezoelectric chopper for infrared radiation

Apparatus for inserting and removing at least one filter from an optical path and for producing a signal that represents the instantaneous positional relationship of the filter with respect to the optical path, said apparatus comprising:

• a base;
• optical means connected to said base and defining an optical path that is fixed with respect to said base;
• a carrier;
• a source of excitation voltage;
• piezoelectric means for resiliently mounting said carrier to said base, connected to said source of excitation voltage and responsive to said excitation voltage for driving said carrier in an oscillatory motion with respect to said base;
• filter means so located on said carrier as to pass into and out of the optical path during the oscillatory motion;
• a reference track located on said carrier, extending in the direction of the oscillatory motion, and including a sequence of indicia progressing in the direction of the oscillatory motion;
• pickoff means mounted on said base for sensing the indicia of said reference track during the oscillatory motion and for producing an electrical signal representative of the indicia;
• and,
• readout means electrically connected to said pick-off means and responsive to said electrical signal to produce an electrical output representative of the instantaneous positional relationship of said filter means with respect to the optical path.