  Direct photon flow is measured by subtracting the contribution of decay
photon flow from the measured inclusive photon flow via the double ratio
$R_{\rm \gamma}$, which defines the excess of direct over decay photons. The
inclusive photon sample is affected by a modest contamination from different
background sources, which is often ignored in measurements. However, due to the
sensitivity of the direct photon measurement even a residual contamination may
significantly bias the extracted direct photon flow. In particular, for
measurements using photon conversions, which are very powerful at low
transverse momentum, these effects can be substantial. Assuming three different
types of correlated background contributions we demonstrate using the
Therminator2 event generator that the impact of the contamination on the
magnitude of direct photon flow can be on the level of $50\%$, even if the
purity of the inclusive photon sample is about $97\%$. Future measurements
should attempt to account for the contamination by measuring the background
contributions and subtracting them from the inclusive photon flow.
