Difference between revisions of "Light microscopy"

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This article examines '''light microscopy'''.
[[Image:Optical microscope nikon alphaphot +.jpg|thumb|250px|A light microscope.]]
This article examines '''light microscopy''', abbreviated '''LM'''.


==Resolution==
==Resolution==
<math>R = 1.22 * {gamma \over {NA_{obj} + NA_{cond}}}</math>.<ref name=pom>{{cite web |url=http://www.life.umd.edu/CBMG/faculty/wolniak/wolniakmicro.html |title=Principles of Microscopy  |author= |date= |work= |publisher= |accessdate=21 January 2011}}</ref>
<math>R = 1.22 * {\gamma \over {NA_{obj} + NA_{cond}}}</math>.<ref name=pom>{{cite web |url=http://www.life.umd.edu/CBMG/faculty/wolniak/wolniakmicro.html |title=Principles of Microscopy  |author= |date= |work= |publisher= |accessdate=21 January 2011}}</ref>
<br>
<br>
Where:
Where:
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*<math>NA_{obj}</math> = numerical aperture of the objective; typically 0.25 - 1.4, >1.0 is oil immersion, it is usu. inscribed on the lens itself.
*<math>NA_{obj}</math> = numerical aperture of the objective; typically 0.25 - 1.4, >1.0 is oil immersion, it is usu. inscribed on the lens itself.
*<math>NA_{cond}</math> = numerical aperture of the condenser.
*<math>NA_{cond}</math> = numerical aperture of the condenser.
*<math>gamma</math> = wave length of light.
*<math>\gamma</math> = wave length of light.


It follows from the above equation that, closure of the condenser diaphragm results in a loss of resolution, i.e. R is larger.<ref name=pom/><br>
It follows from the above equation that, closure of the condenser diaphragm results in a loss of resolution, i.e. R is larger.<ref name=pom/><br>


Stated differently:<ref>URL: [http://www.microbehunter.com/2008/12/18/the-condenser-aperture-diaphragm/ http://www.microbehunter.com/2008/12/18/the-condenser-aperture-diaphragm/]. Accessed on: 21 January 2011.</ref><ref>URL: [http://www.grayfieldoptical.com/depth_of_fieldfocus.html http://www.grayfieldoptical.com/depth_of_fieldfocus.html]. Accessed on: 27 May 2011.</ref>
Stated differently:<ref>URL: [http://www.microbehunter.com/2008/12/18/the-condenser-aperture-diaphragm/ http://www.microbehunter.com/2008/12/18/the-condenser-aperture-diaphragm/]. Accessed on: 21 January 2011.</ref><ref name=grayfield_dof>URL: [http://www.grayfieldoptical.com/depth_of_fieldfocus.html http://www.grayfieldoptical.com/depth_of_fieldfocus.html]. Accessed on: 27 May 2011.</ref>
*Opening the condenser --> increases resolution & brightness -- but -- decreases depth of field (DOF) & contrast.
*Opening the condenser --> increases resolution & brightness -- but -- decreases depth of field (DOF) & contrast.
*Closing the condenser --> increases DOF & contrast -- but -- decreases resolution & brightness.
*Closing the condenser --> increases DOF & contrast -- but -- decreases resolution & brightness.
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====Numerical aperture====
====Numerical aperture====
If one substitutes the above into the equation at the top:<br>
If one substitutes the above into the equation at the top:<br>
<math>R = 1.22 * {gamma \over ( D/2*f )}</math>.
<math>R = 1.22 * {\gamma \over ( D/2*f )}</math>.


Notes:
Notes:
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****Field aperature diaphragm --> optical illumination.
****Field aperature diaphragm --> optical illumination.


==Kohler illumination==
==Depth of field==
*Abbreviated ''DOF''.
 
*DOF depends on the aperature (small is better).
 
Relation to other parameters:<ref name=grayfield_dof>URL: [http://www.grayfieldoptical.com/depth_of_fieldfocus.html http://www.grayfieldoptical.com/depth_of_fieldfocus.html]. Accessed on: 27 May 2011.</ref>
*Inverse relationship with resolution and brightness.
*Related to contrast.
*High magnification --> smaller depth of field.
 
===Formula===
<math>DOF = { \lambda_o n \over NA^2}+{ n \over M \cdot NA } e
</math>.<ref>URL: [http://www.microscopyu.com/articles/formulas/formulasfielddepth.html http://www.microscopyu.com/articles/formulas/formulasfielddepth.html]. Accessed on: 27 May 2011.</ref>
 
Where:
*<math>\lambda_o</math> = illuminating light wavelength.
*n = refractive index of the medium, 1.0 for air.
*NA = numerical aperature (objective).
*M = magnification.
*e = resolution.
 
===Increasing the DOF===
*DOF can be increased by ''focus stacking''.
 
Software:
*[http://hugin.sourceforge.net/ Hugin (sourceforge.net)] - does ''focus stacking'' and ''stitching''.
 
Image:
*[http://commons.wikimedia.org/wiki/File:Focus_stacking_Tachinid_fly.jpg Example of focus stacking (WC)].
 
==Köhler illumination==
===Rationale===
===Rationale===
*Maximize resolution. (???)
*Even light distribution (avoid [[vignetting]]).


===Procedure===
===Procedure===
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*[[Microphotography]].
*[[Microphotography]].
*[[Electron microscopy]].
*[[Electron microscopy]].
*[[Polarization of light]].


==References==
==References==
{{Reflist}}
{{Reflist}}
==External links==
*[http://www.microscopyu.com/tutorials/java/depthoffield/index.html Depth of field calculator (microscopyu.com)].


[[Category:Basics]]
[[Category:Basics]]

Latest revision as of 20:38, 23 April 2016

This article examines light microscopy, abbreviated LM.

Resolution

.[1]
Where:

  • = resolving distance; smaller better.
  • = numerical aperture of the objective; typically 0.25 - 1.4, >1.0 is oil immersion, it is usu. inscribed on the lens itself.
  • = numerical aperture of the condenser.
  • = wave length of light.

It follows from the above equation that, closure of the condenser diaphragm results in a loss of resolution, i.e. R is larger.[1]

Stated differently:[2][3]

  • Opening the condenser --> increases resolution & brightness -- but -- decreases depth of field (DOF) & contrast.
  • Closing the condenser --> increases DOF & contrast -- but -- decreases resolution & brightness.

Numerical aperture

NA = numerical aperture.

General formula for NA:[4]
.

Where:

  • n = index of refraction, n = 1.0 for air.
  • theta = half-angle of the max. cone of light

NA and f-number

N = f/D.

Where:

  • N = f-number, e.g. f 1.2, f 1.4, f 11.
    • Smaller N = larger opening.
  • f = focal length.
  • D = diameter of entrance pupil.

At infinity:
.
.
.

Numerical aperture

If one substitutes the above into the equation at the top:
.

Notes:

  • Larger 'D' is better.
  • Larger NA = better.

Lenses

  • Most lens = 'achromats' -- only correct green.
  • 'Apochromatic' lenses - correct all colours; very expensive.

Condenser

  • Condenser -- large flattened lens beneath the specimen.
    • Iris diaphragm.
      • Condenser diaphragm --> incr. contrast for resolution ---- large dia. good resol. bad contrast?
        • Field aperature diaphragm --> optical illumination.

Depth of field

  • Abbreviated DOF.
  • DOF depends on the aperature (small is better).

Relation to other parameters:[3]

  • Inverse relationship with resolution and brightness.
  • Related to contrast.
  • High magnification --> smaller depth of field.

Formula

.[5]

Where:

  • = illuminating light wavelength.
  • n = refractive index of the medium, 1.0 for air.
  • NA = numerical aperature (objective).
  • M = magnification.
  • e = resolution.

Increasing the DOF

  • DOF can be increased by focus stacking.

Software:

Image:

Köhler illumination

Rationale

Procedure

  1. Any specimen on stage.
  2. Focus.
  3. Adjust field aperture (bottom) - to obscure periphery of field of view (FOV).
  4. Raise or lower condenser until field aperture diaphragm clearly focused.
  5. +/-Center 'field aperture diaphragm - using condenser centering screws.

Resolution

  • Usual light microscopes are limited to about 0.2 micrometres.
    • Coming is "Super-resolution microscopy" - using high speed CCDs (charge-coupled devices).[6]

See also

References

External links