Editing Optical microscope (section)

==Types==
[[File:Microscope simple diagram.png|thumb|right|150px|Diagram of a simple microscope]]
There are two basic types of optical microscopes: simple microscopes and compound microscopes. A simple microscope uses the [[optical power]] of a single lens or group of lenses for magnification. A compound microscope uses a system of lenses (one set enlarging the image produced by another) to achieve a much higher magnification of an object. The vast majority of modern [[research]] microscopes are compound microscopes, while some cheaper commercial [[digital microscope]]s are simple single-lens microscopes. Compound microscopes can be further divided into a variety of other types of microscopes, which differ in their optical configurations, cost, and intended purposes.{{Citation needed|date=August 2024}}

===Simple microscope===
A simple microscope uses a lens or set of lenses to enlarge an object through angular magnification alone, giving the viewer an erect enlarged [[virtual image]].<ref>{{cite web |url=http://www.msnucleus.org/membership/html/jh/biological/microscopes/lesson2/microscopes2c.html |publisher=msnucleus.org |access-date=15 January 2017 |title=Lesson 2 – Page 3, CLASSIFICATION OF MICROSCOPES |author=JR Blueford |url-status=live |archive-url=https://web.archive.org/web/20160510210018/http://msnucleus.org/membership/html/jh/biological/microscopes/lesson2/microscopes2c.html |archive-date=10 May 2016  }}</ref><ref>{{cite book|author=Trisha Knowledge Systems|title=The IIT Foundation Series - Physics Class 8, 2/e|url=https://books.google.com/books?id=NKh9dQKnTdEC&pg=PA213|publisher=Pearson Education India|isbn=978-81-317-6147-2|page=213}}</ref> The use of a single convex lens or groups of lenses are found in simple magnification devices such as the [[magnifying glass]], [[loupe]]s, and [[eyepiece]]s for [[telescope]]s and microscopes.{{cn|date=December 2024}}

===Compound microscope===
[[File:Compound microscope geometric optics.svg|thumb|left|400px|Ray optics diagram of a compound microscope.]]

A compound microscope uses a lens close to the object being viewed to collect light (called the [[Objective (optics)|objective]] lens), which focuses a [[real image]] of the object inside the microscope. That image is then magnified by a second lens or group of lenses (called the [[eyepiece]]) that gives the viewer an enlarged inverted virtual image of the object. <ref name=Watt>{{cite book|author=Ian M. Watt|title=The Principles and Practice of Electron Microscopy|url=https://books.google.com/books?id=Y6-sE4gUX-QC&pg=PA6|year=1997|publisher=Cambridge University Press|isbn=978-0-521-43591-8|page=6}}</ref> 

The use of a compound objective-eyepiece combination allows for much higher angular magnification:
For an object of height <math>h</math>,
it can at most occupy an unmagnified [[angular size]] <math>\theta_0 = h / d_0</math> while remaining in focus,
achieved when it is placed at the [[Near point|near point distance]] <math> d_0 </math> of the eye (about 11 cm).
The virtual image created by the compound microscope achieves an angular size of
<math> \theta = -h s /f_\text{objective}f_\text{eyepiece}</math>,
where <math> s</math> is the distance between the neighboring objective and eyepiece focal points.
This is an angular magnification of <math>\theta/\theta_0 = -s d_0 /f_\text{objective}f_\text{eyepiece}</math>.

Common compound microscopes often feature exchangeable objective lenses, allowing the user to quickly adjust the magnification.<ref name=Watt/> A compound microscope also enables more advanced illumination setups, such as [[phase contrast]].{{cn|date=December 2024}}

===Other microscope variants===
There are many variants of the compound optical microscope design for specialized purposes. Some of these are physical design differences allowing specialization for certain purposes:{{cn|date=December 2024}}
* [[Stereo microscope]], a low-powered microscope which provides a stereoscopic view of the sample, commonly used for dissection.
* [[Comparison microscope]] has two separate light paths allowing direct comparison of two samples via one image in each eye.
* [[Inverted microscope]], for studying samples from below; useful for cell cultures in liquid or for metallography.
* Fiber optic connector inspection microscope, designed for connector end-face inspection
* [[Traveling microscope]], for studying samples of high [[optical resolution]].

Other microscope variants are designed for different illumination techniques:
* [[Petrographic microscope]], whose design usually includes a polarizing filter, rotating stage, and gypsum plate to facilitate the study of minerals or other crystalline materials whose optical properties can vary with orientation.
* [[Polarizing microscope]], similar to the petrographic microscope.
* [[Phase-contrast microscope]], which applies the phase contrast illumination method.
* [[Epifluorescence microscope]], designed for analysis of samples that include fluorophores.
* [[Confocal microscope]], a widely used variant of epifluorescent illumination that uses a scanning laser to illuminate a sample for fluorescence.
* [[Two-photon excitation microscopy|Two-photon microscope]], used to image fluorescence deeper in scattering media and reduce photobleaching, especially in living samples.
* Student microscope – an often low-power microscope with simplified controls and sometimes low-quality optics designed for school use or as a starter instrument for children.<ref>{{cite web|url=http://www.well.ox.ac.uk/_asset/file/buying-a-cheap-microscope-for-home.pdf|title=Buying a cheap microscope for home use|access-date=5 November 2015|publisher=Oxford University.|url-status=live|archive-url=https://web.archive.org/web/20160305042314/http://www.well.ox.ac.uk/_asset/file/buying-a-cheap-microscope-for-home.pdf|archive-date=5 March 2016}}</ref>
* [[Ultramicroscope]], an adapted light microscope that uses [[light scattering]] to allow viewing of tiny particles whose diameter is below or near the wavelength of visible light (around 500 nanometers); mostly obsolete since the advent of [[electron microscope]]s
* [[Tip-enhanced Raman spectroscopy|Tip-enhanced Raman microscope]], is a variant of optical microscope based on [[tip-enhanced Raman spectroscopy]], without traditional wavelength-based resolution limits.<ref>{{Cite journal|last1=Kumar|first1=Naresh|last2=Weckhuysen|first2=Bert M.|last3=Wain|first3=Andrew J.|last4=Pollard|first4=Andrew J.|date=April 2019|title=Nanoscale chemical imaging using tip-enhanced Raman spectroscopy|journal=Nature Protocols|volume=14|issue=4|pages=1169–1193|doi=10.1038/s41596-019-0132-z|pmid=30911174|issn=1750-2799|doi-access=free}}</ref><ref>{{Cite journal|last1=Lee|first1=Joonhee|last2=Crampton|first2=Kevin T.|last3=Tallarida|first3=Nicholas|last4=Apkarian|first4=V. Ara|date=April 2019|title=Visualizing vibrational normal modes of a single molecule with atomically confined light|journal=Nature|volume=568|issue=7750|pages=78–82|doi=10.1038/s41586-019-1059-9|pmid=30944493|bibcode=2019Natur.568...78L |s2cid=92998248 |issn=1476-4687}}</ref> This microscope primarily realized on the [[Scanning probe microscopy|scanning-probe microscope]] platforms using all optical tools.

===Digital microscope===
[[File:2008Computex DnI Award AnMo Dino-Lite Digital Microscope.jpg|thumb|right|200px|A miniature [[USB microscope]]]]
{{Main|Digital microscope}}
A digital microscope is a microscope equipped with a [[digital camera]] allowing observation of a sample via a [[computer]]. Microscopes can also be partly or wholly computer-controlled with various levels of automation. Digital microscopy allows greater analysis of a microscope image, for example, measurements of distances and areas and quantitation of a fluorescent or [[histology|histological]] stain.{{cn|date=December 2024}}

Low-powered digital microscopes, [[USB microscope]]s, are also commercially available. These are essentially [[webcam]]s with a high-powered [[macro lens]] and generally do not use [[transillumination]]. The camera is attached directly to a computer's [[USB]] port to show the images directly on the monitor. They offer modest magnifications (up to about 200×) without the need to use eyepieces and at a very low cost. High-power illumination is usually provided by an [[LED]] source or sources adjacent to the camera lens.{{cn|date=December 2024}}

Digital microscopy with very low light levels to avoid damage to vulnerable biological samples is available using sensitive [[photon counting|photon-counting]] digital cameras. It has been demonstrated that a light source providing pairs of [[Photon entanglement|entangled photons]] may minimize the risk of damage to the most light-sensitive samples. In this application of [[ghost imaging]] to photon-sparse microscopy, the sample is illuminated with infrared photons, each spatially correlated with an entangled partner in the visible band for efficient imaging by a photon-counting camera.<ref name="AspdenGemmell2015">{{cite journal|last1=Aspden|first1=Reuben S. |last2=Gemmell|first2=Nathan R. |last3=Morris|first3=Peter A. |last4=Tasca|first4=Daniel S. |last5=Mertens|first5=Lena |last6=Tanner|first6=Michael G. |last7=Kirkwood|first7=Robert A. |last8=Ruggeri|first8=Alessandro |last9=Tosi|first9=Alberto |last10=Boyd|first10=Robert W. |last11=Buller|first11=Gerald S. |last12=Hadfield |first12=Robert H. |last13=Padgett |first13=Miles J. |title=Photon-sparse microscopy: visible light imaging using infrared illumination |journal=Optica |volume=2 |issue=12 |year=2015 |pages=1049 |issn=2334-2536 |doi=10.1364/OPTICA.2.001049|bibcode=2015Optic...2.1049A |url=http://eprints.gla.ac.uk/112219/1/112219.pdf |archive-url=https://web.archive.org/web/20160604104215/http://eprints.gla.ac.uk/112219/1/112219.pdf |archive-date=2016-06-04 |url-status=live |doi-access=free }}</ref>