Optical unit

The Optical Unit is a dimensionless units 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 2 units, one "axial" (along the optical axis of the objective) and one "radial".

Equation

$u_{\mathrm {axial} }={\frac {8\pi n}{\lambda }}\sin ^{2}({\frac {\alpha }{2}})z$

$v_{\mathrm {radial} }={\frac {2\pi }{\lambda }}{\frac {n\sin \alpha }{M_{\mathrm {tot} }}}r$

where :

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

Note : $n*\sin \alpha$ = numerical aperture

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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)

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