Illumination method for high resolution imaging at water surface

Annularly distributed light is proposed to increase the spatial resolution when observing a water surface. When light with an annular distribution is used to illuminate the entrance pupil of an objective lens, an air-water interface can be illuminated with a Bessel beam, which is a focused beam formed by plane wave components at the same angle. If the optimum size and width of the annular light and radial polarization are chosen, a microscopic distribution is obtainable from the reflected light intensity. It is theoretically shown that the reflectance changes with the width of annular light and this allows to make water surface measurements. In order to clarify the spatial resolution, the electric field intensity distribution on the focusing plane is also calculated. From the full width at half maximum of the calculated distribution, it is shown that the annularly distributed light increases the spatial resolution by increasing the radius of the annular light and decreasing the wavelength of the light. It is also shown that a spatial resolution of a few hundred nanometers is achievable.