is by measuring (whilst keeping the total intensity constant) the second order moment $w$ of its intensity distribution [@Paschotta2008EncOfLasPhysAndTechn]. Crucially, $w$ can be expressed in terms of $m^{(0)}$ and $m^{(2)}$ (see Table \[tab\]) or the respective matrix representations as follows: $$\label{eq:OEi2}
w = 2\sqrt{\frac{m^{(2)}}{m^{(0)}}}=
2\sqrt{\frac{\mathbf{a}^{\dagger}\mathbf{M}^{(2)}\mathbf{a}}{
\mathbf{b}^{\dagger}\mathbf{M}^{(0)}\mathbf{b}}},$$ where $\mathbf{M}^{(0)}$ and $\mathbf{M}^{(2)}$ are termed the *intensity operator (IO)* and *spot size operator (SSO)*, respectively. According to , the minimum spot size is obtained by the OEi associated with the smallest eigenvalue $\lambda^{(2)}$ of the SSO provided that the IO is simultaneously diagonalized and normalized to 1. Direct evaluation shows that this is precisely achieved by the combined OEi $$\label{eq:OEi3}
\left\{\mathbf{E}_{\min},\mathbf{H}_{\min}\right\}
=\left\{\sum_{j=1}^{N}\sum_{k=1}^{N}\frac{v_{\min,k}^{(2)}v_{k,j}^{(0)}}{\sqrt{\lambda_{k}^{(0)}}}\cdot\mathbf{E}_{j}, 
\sum_{j=1}^{N}\sum_{k=1}^{N}\frac{v_{\min,k}^{(2)}v_{k,j}^{(0)}}{\sqrt{\lambda_{k}^{(0)}}}\cdot\mathbf{H}_{j}\right\}.$$ where $v_{\min,k}^{(2)}$ is the eigenvector associated with the smallest eigenvalue of $\mathbf{M}^{(2)}$ and in the intensity normalised eigenbase $v_{k,j}^{(0)}/{\sqrt{\lambda_{k}^{(0)}}}$ of $\mathbf{M}^{(0)}$.

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       Energy $m^{({\cal E})}(\mathbf{E},\mathbf{H})=\frac{1}{2}\int_{V}{\cal E}\;dv$
    Intensity $m^{(\textrm{0})}(\mathbf{E},\mathbf{H})=\frac{1}{4} \int_S\left( \mathbf{E}^*\times\mathbf{H}\right)\cdot\mathbf{n}\;d\sigma$+c.c.
    Spot size $m^{(\textrm{2})}(\mathbf{E},\mathbf{H})=\frac{1}{4}\int_S\mathbf r^2 \left( \mathbf{E}^*\times\mathbf{H}\right) \cdot \mathbf{n}\;d \sigma $+c.c.
     Momentum $m^{(\mathbf{F\cdot u})}(\mathbf{E},\mathbf{H})= \frac{1}{4}\int_S (\epsilon_0 ({\bf E}^*\cdot {\bf n}) {\bf E}+\mu_0({\bf H}^*\cdot{\bf n}){ \bf H}-\frac{1}{2}{\cal E} {\bf n})\cdot {\bf u} \;d \sigma $+c.c.
      \[1mm\] 
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  : Time averaged quadratic measures $m$ of common light-matter interactions. The integration either over a volume $V$ or a surface $S$ which in 