Light microscopy

This article examines light microscopy, abbreviated LM.

File:Optical microscope nikon alphaphot +.jpg

A light microscope.

Resolution

$R=1.22*{\gamma \over {NA_{obj}+NA_{cond}}}$.
Where:

• $R$ = resolving distance; smaller better.
• $NA_{obj}$ = numerical aperture of the objective; typically 0.25 - 1.4, >1.0 is oil immersion, it is usu. inscribed on the lens itself.
• $NA_{cond}$ = numerical aperture of the condenser.
• $\gamma$ = 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.

Stated differently:

• 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:
$NA=n*sin(theta)$.

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:
$N=1/(2*NA_{i})$.
$f/D=1/(2*NA_{i})$.
$2*NA_{i}=D/f$.

Numerical aperture

If one substitutes the above into the equation at the top:
$R=1.22*{\gamma \over (D/2*f)}$.

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:

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

Formula

$DOF={\lambda _{o}n \over NA^{2}}+{n \over M\cdot NA}e$.

Where:

• $\lambda _{o}$ = 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:

• Hugin (sourceforge.net) - does focus stacking and stitching.

Image:

• Example of focus stacking (WC).

Köhler illumination

Rationale

• Even light distribution (avoid vignetting).

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).

See also

• Basics.
• Microphotography.
• Electron microscopy.
• Polarization of light.

External links

• Depth of field calculator (microscopyu.com).