The energy-energy correlation function $C(k)$ is calculated to $O(4\ensuremath{-}d)$ in an exponentiated crossover form for $n$-component spin systems. The result is exact for the Gaussian ($n=2$) and spherical ($n=\ensuremath{\infty}$) models which form symmetrically placed anchors for the calculation for general $n$. The error is proprotional to the small critical-point exponent $\ensuremath{\eta}$ and is likely to be small in three dimensions. A dispersion-theory representation of $C(k)$ is used to correct the large-$k$ behavior to the expected Fisher-Langer asymptotic form. The result gives a direct method for the measurement of the correlation length in liquid helium.