Optical Unit
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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

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

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

where :

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

Note : ${\displaystyle n*\sin \alpha }$ = numerical aperture