A film of liquid between the lens and the specimen increases resolution, through a process known as immersion. The gap between lens and sample is filled with immersion oil, water or glycerin, and this liquid allows more light to be captured from the object.
The physical principle behind this can be explained as follows. Air has a refractive index of 1. The refractive index of the liquid between the specimen and the immersion objective is higher. The higher the refractive index, the smaller the angle of light diffracted on the specimen. Therefore, with the help of immersion microscopy, more diffracted light can be captured. Since the diffracted light contains information about the specimen, more diffracted light means more information; thus the resolution increases.
This principle was already developed 200 years ago by the Italian physicist Giovanni Amici, who produced the first immersion lens in 1804. In 1873, Ernst Abbe recognized that the resolution in optical imaging depends on the numerical aperture of an imaging system – and this in turn directly depends on the refractive index of the immersion liquid. Ever since, Carl Zeiss has been offering immersion microscopy to its customers. German doctor and microbiologist Robert Koch was one of the first users.
As well as its applications in microscopy, immersion is also used in lithography for the production of microprocessors. In traditional projection systems, there is an air gap between the last lens element and the surface of the wafer, while in more recent systems, this gap is filled with an immersion liquid which has a refractive index higher than that of air. This process allows smaller structures to be produced for the manufacture of microelectronic circuits.
November 30, 2011