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

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)