What is an Optical Microscope?
Optical microscopes come in numerous shapes & sizes and are widely used in schools, the research & manufacturing industries, hospitals, etc. However, did you know that there are actually 2 main classes of optical microscopes, each having different advantages & limitations? Read on to find out more!
1. Stereo-Microscope
The first type of optical microscope that we look at is the stereomicroscope. This microscope is low-powered, having overall magnifications generally ranging from 7X to 180X, although research variants sporting magnifications in excess of 500X are commercially available. Stereo microscopes (as the name implies) are able to generate 3D images when viewed through the eyepieces, as the light passes through 2 different sets of lenses to each eyepiece (via either the CMO or Greenough Designs) (see the figure below for details):
Stereo microscopes boast an impressive depth-of-field and thus are widely used for field research, or where one just wants to view a specimen without having to do any specimen preparation (a key advantage of these microscopes, since they have direct incident light/epi-illumination). Nonetheless, as the overall magnification of these microscopes is limited, the conventional stereo microscope cannot be used to view bacteria or cells 😅.
2. Compound Optical / Light Microscope
This is the most common type of microscope which we find in schools, hospitals, laboratories, etc. In fact, the word microscope often conjures a mental image of the compound light microscope in the minds of most people 😃. An example of the compound optical/light microscope is depicted in the labelled diagram below:
In the compound light microscope, a beam of light is either transmitted through the specimen (which is often the case) or reflected off the surface of a polished specimen into the objective lens before finally entering the eyepiece lens to be viewed by the observer. A camera may also be used (in place of the eyepiece) to acquire the image & display it on a screen/computer.
Light microscopes have a range of useful magnifications ranging from 40X to a maximum of ~1,500X, allowing them to be used for viewing bacteria, cells, defect inspection & most use-cases in industry. Beyond 1,500X (however), the light microscope is unable to provide any additional information on the structure of the specimen being observed – a barrier known as the Abbe diffraction limit of optical resolution (which is ~200nm). Such magnifications are thus often referred as empty magnification (i.e. magnified images which do not display any additional detail/resolution). Nonetheless, there are light microscopes which have managed to surpass this limit (aka super-resolution optical microscopes/nanoscopes), although most of these are extensions of fluorescent microscopical techniques (such as STED, STORM & PALM). Nonetheless, here at Phaos Technology, we utilize tiny optically-clear spheres (microspheres) to overcome the Abbe diffraction limit in traditional brightfield microscopy, as discussed in the following section.
Phaos Technology: OptoNano 200
In 2010, a research group in the National University of Singapore discovered a way to circumvent the Abbe diffraction limit of 200nm in the compound optical microscope, using only tiny glass spheres coupled with a matching objective lens [4]. This discovery was actually derived accidentally, from defective semiconductor wafers which exhibited nanoscopic perforations caused by a focussed laser-beam which was intended to remove tiny glass spheres on the wafer surface during laser-cleaning [5]. Although these pores rendered the wafer unusable, further evaluation revealed that they were localized to where the glass microspheres were found, with many of them having diameters below the optical diffraction limit [5]. This clearly suggests that the microspheres had focussed the laser beam to a sub-diffraction spot, which ablated the wafer surface. By exploiting this property of the microspheres in the optical microscope, the research group was able to surpass the Abbe diffraction limit, leading to the establishment of Phaos Technology (the only company globally to possess the patent for using microspheres to achieve brightfield nanoscopy)! The figure below illustrates the use of the microsphere to achieve brightfield nanoscopy:
So that’s it for now – we hope that you have found the read both informative & enjoyable 😃 Remember to follow us at @phaostechnology or bookmark the POSES tab on our website to learn more about optical microscopy and its various uses in the industry!
Phaos Optonano Series Mentioned
References
1. Nothnagle, P. E., Chambers, W., and Davidson, M. W. Introduction to Stereomicroscopy. ©2021 Nikon Instruments Inc. Accessed: June 23, 2021. [Online]. Available: https://www.microscopyu.com/techniques/stereomicroscopy/introduction-to-stereomicroscopy.
2. Blackman, S. Stereo microscopes still changing after all these years. The Scientist. Accessed: 6 July 2021 [Online]. Available: https://www.the-scientist.com/technology/stereo-microscopes-still-changing-after-all-these-years-48496
3. Racheal, H. Compound Microscope Parts – Labeled Diagram and their Functions. Rsscience. Accessed 6 July 2021 [Online]. Available: https://rsscience.com/compound-microscope-parts-labeled/
4. Wang, Z., Guo, W., Li, L. et al. Optical virtual imaging at 50 nm lateral resolution with a white-light nanoscope. Nat Commun 2, 218 (2011). https://doi.org/10.1038/ncomms1211
5. Chen, L., Zhou, Y., Li, Y. and Hong, M. Microsphere enhanced optical imaging and patterning: From physics to applications. Appl. Phys. Rev. 6, 021304 (2019). https://doi.org/10.1063/1.5082215