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

$u_{\mathrm{a}\mathrm{x}\mathrm{i}\mathrm{a}\mathrm{l}}=\frac{8\pi n}{\lambda }{\sin }^2(\frac{\alpha }{2})z$

$v_{\mathrm{r}\mathrm{a}\mathrm{d}\mathrm{i}\mathrm{a}\mathrm{l}}=\frac{2\pi }{\lambda }\frac{n\sin \alpha }{M_{\mathrm{t}\mathrm{o}\mathrm{t}}}r$

where :

• $n$ is the refractive index
• $\lambda$ is the wavelength
• $\alpha$ the beam conus angle
• $M_{\mathrm{t}\mathrm{o}\mathrm{t}}$ 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)