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High-Definition Infrared Spectroscopic Imaging
Volume 67, Number 1 (Jan. 2013) Page 93-105
ROHITH K. REDDY, MICHAEL J. WALSH, MATTHEW V. SCHULMERICH, P. SCOTT CARNEY, and ROHIT BHARGAVA*
The quality of images from an infrared (IR) microscope has traditionally been limited by considerations of throughput and signal-to-noise ratio (SNR). An understanding of the achievable quality as a function of instrument parameters, from first principals is needed for improved instrument design. Here, we first present a model for light propagation through an IR spectroscopic imaging system based on scalar wave theory. The model analytically describes the propagation of light along the entire beam path from the source to the detector. The effect of various optical elements and the sample in the microscope is understood in terms of the accessible spatial frequencies by using a Fourier optics approach and simulations are conducted to gain insights into spectroscopic image formation. The optimal pixel size at the sample plane is calculated and shown much smaller than that in current mid-IR microscopy systems. A commercial imaging system is modified, and experimental data are presented to demonstrate the validity of the developed model. Building on this validated theoretical foundation, an optimal sampling configuration is set up. Acquired data were of high spatial quality but, as expected, of poorer SNR. Signal processing approaches were implemented to improve the spectral SNR. The resulting data demonstrated the ability to perform high-definition IR imaging in the laboratory by using minimally-modified commercial instruments.
Index Headings: Mid-infrared; Fourier transform infrared; Spectroscopic imaging; FT-IR imaging; Resolution; Image quality; Modeling; Theory; Scalar wave theory; Microscopy.