The Difference Between SEM and Optical Microscopes

The human eye is a powerful visual tool but it does not have the resolution to bring microscopic images into focus. This is where microscopy can help us understand everything from pandemic-inducing viruses to the manufacture of increasingly miniaturized electronics, revealing the large impact the smallest detail can have on our day-to-day lives and future endeavors.

In this post, we examine two key methods: optical microscopy and electron microscopy, revealing the benefits each technique brings, and explaining both their differing application areas and operation.

There is one major difference between optical microscopy and electron microscopy – the beam applied to the sample. This simple fact has major repercussions on the components and operation of each microscope, as well as its applications.

Optical microscopy definition: Optical microscopes use a beam of light, ranging from 400nm to 650nm in wavelength, allowing the observer to analyze the effect of light as it is applied to a specimen.

Optical microscopy is an ideal method for general inspection purposes, illuminating and producing a magnified image of a specimen. The layout between optical microscopes varies, depending on the application, but generally includes a converging lens (for magnification) and a concave mirror (to aid illumination). The sample is placed on a stage and the resultant image is viewed through an eyepiece.

SEM definition: SEMs scan a focused beam of electrons across the surface of a sample, where electromagnets are used to focus the negatively charged electrons. The interaction of the electron beam with the surface of the sample affects the images received. The electrons coming out of the sample are used to create a detailed image and reveal information including the texture (morphology), chemical composition, crystalline structure, and material orientation.

SEMs typically feature three types of detectors, each of which captures a different signal coming from the sample: A Secondary Electron Detector (SED), a Back-Scattered Electron Detector (BSED), and an Energy Dispersive Spectrum Detector (EDS).

Since secondary electrons interact primarily with the sample surface and have a large reflection angle, the SED provides detailed topographical information. Back-scattered electrons penetrate further into the material and have a smaller reflection angle, so the BSED provides both basic topographical and basic compositional information. The EDS provides detailed chemical compositional information.

Optical microscopes are easy to use, where samples can be analyzed in air or water and the resulting images are in natural color.

SEMs are typically larger and operate in a vacuum, which can increase the time to image a sample. Plus, the resulting image is grey-scaled.

However, SEMs are gaining ground in this area with many Desktop SEMs bridging the gap between optical microscopes and ultra-high resolution SEMs. The unique optical navigation camera displays a view of the entire sample and allows the user to move to any spot on the sample with just a single click. The proprietary venting/loading mechanism supports the highest throughput even for large samples up to 100mm x 100mm ensuring a time-to-image of less than 60 seconds.

Looking at a standard optical microscopy definition, the resolving power of these systems is directly influenced by the wavelength of the imaging beam, which gives SEMs a distinct advantage. Because optical microscopes are limited to the wavelengths of visible light, they can only offer limited magnification (around 1,500 x) and cannot go beyond around 200 nm resolution laterally and 600 to 700 nm axially.

In comparison, SEMs are capable of much greater magnification and higher resolution. The most sophisticated SEMs can achieve magnifications of around 100,000 x and sub-nanometer resolutions, which are capable of imaging viruses (which are between 30 and 250 nm) and molecules such as proteins (10 nm) and glucose (1 nm).

Because of the geometry of the imaging system, scanning electron microscopes have a much greater depth of field than optical microscopes, where the whole specimen can be in focus.

This is because, for the optical microscope, the depth of focus is the distance above and below the image plane over which the image appears in focus. As the magnification increases in the optical microscope, the depth of focus decreases.

In contrast, SEMs can create a three-dimensional appearance of the specimen image. This is because of the method in which the data is obtained, where a fine electron beam is scanned over the surface and the detected secondary electrons form an image with a high depth of focus.

Optical microscopy and electron microscopy both have advantages and disadvantages. SEMs are superior in terms of resolving power and depth of focus. However, optical microscopes are generally easier and quicker to use. As a result, many use a combination of both imaging tools where an optical microscope is used to detect gross defects and SEMs can observe those defects in more detail while observing micro-defects that are not visible using optical microscopy. This two-phase approach combines the benefits associated with each inspection method and provides the customer with a more detailed inspection in less time.

At Thermo Fisher Scientific, we have decades of cross-sector experience, helping industrial and research users get the best images of the micro world. If you could like to find out more about the best inspection tools for your application, click here to speak to one of our expert teams today.