Patent ID: 7728959

Claim:
An apparatus for fingerprint image acquisition comprising: a waveguide, having an entrance edge and top and bottom surfaces; a light source, configured to direct a light beam toward the entrance edge of the waveguide; a skin contact layer, disposed at or near the top surface of the waveguide or bottom surface of the waveguide; a holographic optical element (HOE), disposed at the top or at the bottom surface of the waveguide, configured to diffract the light beam incident from the light source to the skin contact layer; a sensor array, configured to detect light reflected from the interface between skin and the skin contact layer, wherein the HOE includes a layer comprising a grating; and at least one supporting layer in contact with the said grating layer, wherein the grating layer and the at least one supporting layer have substantially similar coefficients of thermal expansion or thermo-optic coefficients or both.