Spatial Resolution of FT-IR Imaging Measurements

For FT-IR imaging, a high spatial resolution is very important in order to visualize the distribution of different substances of a heterogeneous sample as precisely as possible. Theoretically, the lateral resolution is governed by the Rayleigh criterion 

As shown in Table 8.1, the size of the imaging area of a germanium (Ge) ATR objective is 50 pm x 50 pm. By using an FPA detector with 64 x 64 detector elements, an area of 0.78 pm x 0.78 pm can be allocated for each detector element. However, this does not mean that such a spatial resolution can be achieved because as shown earlier this parameter is not only limited by the pixel size and the magnification of the objective but also by the diffraction of the IR radiation (see Section 1.2). 

Chan and Kazarian [43] were able to achieve a spatial resolution of 3-4 pm with micro ATR FT-IR imaging measurements with a Ge ATR crystal. A report by Lasch and Naumann [48] described the lateral resolution of FT-IR Imaging measurements in transmission, whereby numerous results from measurements of a resolution target USAF 1951 and tissue samples with different imaging systems from different manufacturers and with different objectives were compared. 

The test systems In order to demonstrate the spatial resolution achieved in practical measurements, two test sample systems were investigated by FT-IR imaging measurements in the ATR mode and in transmission. The sizes of the imaging areas were 50 pm X 50 pm for the micro ATR FT-IR measurements (20x objective) and 260 pm X 260 pm for the micro FT-IR transmission measurements (15x objective). 

The first system involved a polymer laminate of a polyethylene (PE) (25 pm) and polycarbonate (PC) (3 pm) films (both films from Bayer AG, Germany) will be discussed. A portion of PC film ( lcm ) was positioned on top of a 2cm PE film, and the two were sandwiched between two aluminum foils, heated to 150 C in a drying oven for 1 h and then compressed (10 bar pressure). This sample was then imaged at the sharp borderline of the PE/PC laminate and the single PE layer in the ATR mode (50 pm X 50 pm) and in transmission (260 pm X 260 pm). 

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