The spatial resolution of near-field optical microscope on chromosomes and cell traces

The investigation of biological samples in molecular medicine and biology by near-field optical microscopy is subject to nonconstant experimental conditions, such as humidity and elasticity. Contrary to far-field microscopy, the obtainable spatial resolution in scanning near-field optical microscopy (SNOM) greatly depends on the specific experimental conditions. The experimental determination of the modulation transfer function (MTF), therefore, uses regular solid-state structures. This paper introduces a method for the approximate in situ determination of the MTF using, as an example, SNOM transmission measurements of metaphase humane chromosomes and cell traces. The method has its origins in the linear system transfer theory. In order to eliminate effects of nonconstant optical near-field conditions, the transfer function is determined from the properties of the light source and the measured intensity function at the edge of a chromosome or cell trace, which depends on the transmission of the probe.