Aug 4 2014
Topics Covered
Introduction
What are Diffraction Limits in Optical Microscopy?
New Developments
References
Introduction
The optical microscope, also called the light microscope, is the oldest type of microscope which uses visible light and lenses in order to magnify images of very small samples. It is a standard tool frequently used within the fields of life and material science.
The first basic optical microscopes were developed in the 17th century. Today, there are many variations available that enable high level resolutions and sample contrasts. With the help of computer-aided optical design and automated grinding methodology, image quality has improved immensely with negligible deviation.
However, optical microscopes still have to overcome a critical limit in optical resolution caused by the diffraction of visible light wavefronts when they pass via a circular aperture at the rear focal plane of an objective (lens).
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What are Diffraction Limits in Optical Microscopy?
Several basic factors, such as imperfections in the lenses or misalignment, can hamper the resolution of an optical imaging system such as a microscope. The key factor, however, is diffraction.
The resolution of a microscope is proportional to the size of its objective, and inversely proportional to the wavelength of light being observed.
In 1873 the German physicist Ernst Abbe discovered how microscopes were limited by the diffraction of light. This discovery revealed that the resolution of a microscope is not controlled by the instrument’s quality but by the wavelength of light used and the aperture of its optics.
Due to this phenomenon, a microscope cannot resolve two objects located closer than λ/2NA, where λ is the wavelength of light and NA is the numerical aperture of the imaging lens.
In other words, diffraction limits the ability of the microscope to distinguish between two objects divided by a lateral distance of less than half the wavelength of light used to image the sample.
According to Abbe's findings, images consist of an array of diffraction-limited spots with changeable intensities which overlap so as to produce the final result. Thus a viable method for maximizing spatial resolution and image contrast is to reduce the size of the diffraction-limited spots by minimizing the imaging wavelength, increasing numerical aperture, or using an imaging medium having a larger refractive index.
New Developments
Rapid advancements in technology are creating new imaging methods which are capable of overcoming Abbe's diffraction barrier. Surface plasmons have been tested and it has been proved that they are capable of providing a technique to overcome the diffraction limit of light.
Surpassing the diffraction barrier would certainly revolutionize optical microscopy in a big way as the diffraction-limited phenomenon has hampered the functioning of optical microscopy for more than a century.
These developments in optical microscopy will enable non-invasive analysis of the interior of biological cells as well as nano-imaging of semiconductor devices in the electronics industry.
References