Optical units
In diffraction theory it is usual to represent spatial coordinates with dimensionless variables based on the Numerical Aperture (NA) of the lens and the WaveLength (λ) of the radiation. For example, the radial position r is re-written as
$$ v=r\frac{2 \pi}{\lambda}\ \text{NA} $$
This gives distances expressed in the so-called optical units (o.u.), with
$$ 1\ \text{o.u.}=\frac{\lambda}{2 \pi\ \text{NA}} = \frac{1}{k\ \text{NA}} $$
where $$k=2 \pi/\lambda$$
For NA = 1.3 and λ = 500 nm an optical unit is 61 nm.