Physics:Optical unit

Short description: Unit of measurement

An optical unit is a dimensionless unit of length used in optical microscopy. Because every diffraction limited system have their resolution proportional to wavelength/NA, it is convenient for comparison to use this unit. There are actually two units, one "axial" (along the optical axis of the objective) and one "radial".

Equation

[math]\displaystyle{ u_\mathrm{axial} = \frac{8 \pi n}{\lambda} \sin^2(\frac{\alpha}{2}) z }[/math]

[math]\displaystyle{ v_\mathrm{radial} = \frac{2 \pi}{\lambda} \frac{n \sin \alpha}{M_\mathrm{tot}} r }[/math]

where :

• [math]\displaystyle{ n }[/math] is the refractive index
• [math]\displaystyle{ \lambda }[/math] is the wavelength
• [math]\displaystyle{ \alpha }[/math] the beam conus angle
• [math]\displaystyle{ M_\mathrm{tot} }[/math] is the total magnification ([math]\displaystyle{ M }[/math] is 1 if you want the size in the sample)

Note : [math]\displaystyle{ n * \sin \alpha }[/math] = numerical aperture

References

• James B. Pawley in Handbook of biological confocal microscopy ; second edition; Appendix I
• http://www.ntmdt.com/spm-basics/view/classical-optical-microscopy (see equation 3 and 4)