High-resolution digital transmitting microscope with a large objective lens

Standard optical microscopes are based on diffraction imaging, and their resolution is limited to half the wavelength of the illuminating light. In contrast, scattered-light imaging depends on the sharpness of the optics and is not limited by the wavelength. This technique is used mainly for reflective dark-field observations. The combination of a scattered-light illumination system and sharp optics may improve the resolution of the transmitting microscope. It is well known that the large objective lens in a telescope improves the resolution by reducing the diffraction rings. The digital microscope, designed for industrial applications, has a large objective lens and a 16-bit high-gradation image-processing system, resulting in a sharper image. In this paper, a digital microscope is compared with a standard microscope using the same condenser lens in both cases, which produces scattered light. To amplify the scattered light, the light beam around the optical axis is masked in the condenser lens. Although the standard microscope uses an objective lens with a higher numerical aperture (NA = 0.92) than the digital microscope (NA = 0.82), the digital microscope achieved a higher resolution (over 2500 line-pairs/mm) than the standard microscope, even under dry conditions. This resolving power is greater than that predicted by classical optical theory.