= Background-subtracted radiative temperature uncertainty
{title2=$\sigma_T/T=\sqrt{\sigma_A^2+\sigma_B^2-2\operatorname{Cov}(A,B)}/[4(A-B)]$}
If measured radiative <flux> is $S=A-B>0$ and $S=kT^4$ with known $k$, first-order uncertainty propagation gives the displayed result. Independent measurements have zero <covariance>; strict error bounds combine differently. Even a small fractional background contribution can matter if its relative uncertainty is large. Inferring the <covariance> or an unknown calibration factor from precision percentages alone is unjustified.
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