Vector-meson photoproduction on nuclei in the ${\rho }^0-\omega$ interference region has been observed both by means of experiments detecting photoproduction of $e^{+}{e}^{-}$ pairs and by means of experiments detecting photoproduction of ${\pi }^{+}{\pi }^{-}$ pairs. These two phenomena are closely related to each other and to the corresponding interference phenomenon in $e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}$. It is shown how the data on all such phenomena can be expressed in terms of three model-independen complex parameters: $\epsilon$, the intrinsic $\rho -\omega$ mixing parameter, $r_{\omega }$, the ratio of $\omega$-photon to $\rho$-photon coupling at an energy corresponding to the mass of the $\omega$, and $\frac{P_{\omega }}{P_{\rho }}$, the ratio of nuclear photoproduction amplitudes for $\omega$ and $\rho$. Existing data on the interference phenomena are combined with data on the ratios of $\rho$ and $\omega$ production cross sections to determine these parameters. It is found that $\left|\epsilon \right|=0.034\mathrm{}\pm 0.004$, $\left|r_{\omega }\right|\equiv \frac{{\gamma }_{\rho }}{{\gamma }_{\omega }}=0.34\mathrm{}\pm 0.07$, and $|\frac{P_{\rho }}{P_{\omega }|}=3.44\mathrm{}\pm 0.19$ is the most consistent representation of the data. Because present experiments have uncertainties at the 10% level, it turns out that only relative phases can be determined. If $r_{\omega }$ is assumed to be real, the most consistent representation yields $\arg \epsilon ={95}^{\circ }\pm {15}^{\circ }$ and $\arg \left(\frac{P_{\omega }}{P_{\rho }}\right)=0^{\circ }\pm {21}^{\circ }$. The former result is in agreement with the expectations from mixing theory. The latter is interpreted in terms of a simple vector-dominance model and is shown to agree with simple peripheralism, that is, equality of the $\rho$ and $\omega$ forward scattering amplitudes. Because of disagreements in the data from different laboratories, there may be some doubt about the reality of $r_{\omega }$. It appears that the phase of $r_{\omega }$ could be determined in a model-independent way by an electron-positron colliding-beam experiment on interference in ${\pi }^{+}{\pi }^{-}$ production at the 1% level.

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