Multi-spectral ultrasonic imaging

A method of scanning a finger, comprising:

• scanning a finger positioned on an imaging surface of an ultrasonic sensor with a plurality of ultrasonic scan frequencies;
• generating an ultrasonic image information set from a plurality of pixels of the ultrasonic sensor for each of the scan frequencies, the image information set including a pixel output value from each of the plurality of pixels, each pixel output value indicating an amount of energy reflected from the imaging surface;
• and
• combining the image information sets corresponding to each of the scan frequencies to generate a combined image information set, the combined image information set including combined pixel output values from each of the plurality of pixels, wherein combining the image information sets includes creating a covariance matrix for each of the scan frequencies from the pixel output values in the image information sets, and combining the covariance matrices to provide a combined matrix having a combined value for each pixel output value and interpolating between entries in the covariance matrices.