Electromagnetic vacuum energy with a wavelength cutoff ($u=2{\pi }^2\hslash c/{\lambda }_{\min }^4$)

Counting two transverse polarizations and zero-point energy $\hslash ck/2$ per mode gives $u=\hslash c{k}_{\max }^4/(8{\pi }^2)=2{\pi }^2\hslash c/{\lambda }_{\min }^4$. This is a bare cutoff-dependent mode sum, not a classical incident radiation bath or a renormalization-independent absolute observable.