Multi-beam Synchrotron FTIR Chemical Imaging: Impacts of Schwarzschild Objective and Spatial Oversampling on Spatial Resolution

IRENI (InfraRed ENvironmental Imaging) is a recently commissioned FTIR chemical imaging beamline at the Synchrotron Radiation Center in Madison, WI. This novel beamline extracts 320 mrads of radiation, horizontally, from one bending magnet. The optical transport separates and recombines the beam into 12 parallel, collimated beams to illuminate a commercial FTIR microspectrometer (Bruker Hyperion 3000) that is apertureless and equipped with a multiple element detector. The beams are partially overlapped and defocused, similar to widefield microscopy, homogeneously illuminating a relatively large sample area compared to single beam arrangements. The effective geometric size of a pixel at the sample plane is defined by the magnification of the optics, and as expected, objectives with varying magnification and numerical apertures (NAs) impact the diffraction-limited resolution that can be obtained in the resulting images. We demonstrate here that spatial oversampling for synchrotron-based infrared imaging is critical to obtain diffraction-limited images at all wavelengths simultaneously. In this article measured and simulated point spread functions (PSF), resolution criteria and results from raw and deconvoluted images for two common Schwarzschild objectives (36x, NA 0.5 and 74x, NA.65) are compared to each other and to prior reports for confocal microscopes. The resolution of the imaging data can be improved by deconvolving the instrumental broadening that is simulated with the PSF. The contrast resolution for IRENI without employing apertures is similar to dual aperture, confocal microscopes.