The presence of near-field indicates the existence of evanescent waves, which is one of the important requirements for the development of an optical sensor. This study was carried out t o identify the presence of near-field and far-field as monochromatic light propagated through various structures and sizes of apertures. The confocal microscopy setup consisted of He -Ne laser with 633nm of excitation wavelength, apertures (slits and numerous sizes of pinholes), objective lens, projected screen and silicon photodetector. The near-field and far-field were determined by calculating the Fresnel number, FN as assorted size and design of apertures were added in the setup. The presence of far-field was identified with the usage of slits (single slits and double slits)represented by the value of FN, which was less than 1(FN<<1). As the aperture was replaced with pinholes with diameters within 30μm and 1.2mm, the near-field was resulted (FN>>1). Value of FN became greater as distance x between lens and pinholes increased. We also discovered a significant finding where the field intensities I were decreased with increment of distance x for the near-field, and vice versa (far-field). By using an extrapolating technique, it was found that the near-field can be created by using pinhole with a diameter of a<1.46mm. In conclusion, the discovery of this work proves an excellence role of pinholes in confocal microscopy setup in creating the evanescent waves for optical sensing applications.
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