Hadamard Transform Raman Imaging

Abstract

Hadamard mask encoding allows medium to high spatial resolution imaging with unfocused laser beams. A laser beam is imaged on the sample through a series of masks. The spatially encoded signals from each of n masks, each containing n resolution elements, are measured. The spatial distribution of the signal is recovered by Hadamard transformation of the matrix of encoded signals. Of course, the laser could be focused to an area equal to the unit resolution element. But, encoding with n-element masks reduces the power density at any point on the sample by a factor of n. We have already successfully employed masks containing 28 − 1 elements, and have estimated that it is feasible to use masks with 210 − 1 or even 212 − 1 elements. Reducing laser power density by a factor of 103 broadens the scope of spatially resolved laser spectroscopy and opens the possibility of Raman microscopy with pulsed lasers.