Diffraction Limit of Light

February 22, 2021
12:00 PM (GMT)
10 Minutes

Introduction

Optical Microscopy, or light microscopy uses visible light to transmit through, refracted around, or reflected from a specimen.

The power to magnify the image of the specimen when viewing through a microscope depends on how much the lenses bend the light waves.

With the advancement of technology, there are many variations available in the market such as computer-aided optical design and automatic grinding methods which allows users to view high level resolutions and sample contrasts images. This have significantly improve the quality of the images with nearly no deviation.

Limitation

However, optical microscopes must overcome a critical limit in optical resolution caused by the diffraction of visible light wave fronts when they pass by a circular aperture at the rear focal plane of an objective, or lens, as shown in the figure below.

The Diffraction Limit in the Microscopy Context

Diffraction limit is one of the factors that can affect the final resolution of an optical imaging system.

The resolution of a microscope is inversely proportional to the wavelength of the light that is observed and directly proportional to the size of the objective itself.

Back in 1873, a German physicist named Ernst Abbe realized that the resolution of optical imaging instruments, including telescopes and microscopes, is fundamentally limited by the diffraction of light.

Abbe’s Law of Limiting Resolution

d = 0.5 λ/NA

• λ: Wavelength of Light Used
• NA: Numerical Aperture of the imaging lens

Based on his equation, the resolution of an imaging instrument is not constrained by the quality of the instrument, but by the wavelength of light used and the aperture of its optics. This meant that a microscope could not resolve two objects located closer than λ/2NA.

In short, the diffraction limit of light does not allow the microscope to differentiate between two objects divided by a lateral distance that is less than half of the wavelength of the light used to image the sample.

Furthermore, Abbe discovered that images are made of an array of diffraction-limited spots with modifiable intensities which overlap one to each other to produce the final image. There are some options to overlap those constrains and maximize the spatial resolution and the image contrast to reduce the size of the spots that are limited by diffraction of light.

For instance, it is possible to reduce the imaging wavelength, increase the numerical aperture or use an imaging medium that has a large refractive index.

This diffraction-limited phenomenon hindered the performance of optical microscopy for over a century, and was considered a fundamental, unbreakable rule.

Introducing Phaos’s Opto Nano Series

With the use of patented Optical Microsphere Nanoscopy (OMN) technology, users is able to resolve specimen features down to 137nm scale in visible light and ambient air observation via Phao’s OptoNano 200.

This overcome Abbe’s Resolution Limit and bridge a major gap between optical and electron microscopy.

How it work?

OptoNano is a microscope with a microsphere between the sample and objective lens.

In the imaging process, the microsphere first forms an enlarged virtual image. Such a magnified virtual image replaces the original object as the imaging target of the microscope. A conventional microscope system is used to capture the virtual image
provided by the microsphere.

For more information, please click here.