The observation of an ultra-high energy photon component in the cosmic radiation is one of the open problems in Astroparticle Physics. The stringent theoretical and experimental upper limits to the photon flux above $100\,$TeV make the search of a weak photon signal in the vast hadronic cosmic ray background a challenging task. At these energies, photon primaries entering the atmosphere develop an extensive air shower which is driven by electromagnetic processes with a poor muon production. The muon content is one of the most promising observables leading to the discrimination between photons and hadronic cosmic rays. In this article, we define a parameter capable of quantifying the muon component while reducing the fluctuations due to the unknown lateral distribution of muons. We explain the different features of this observable using simulated air showers between $30$ and $300\,$PeV. We show that a merit factor of $5$ in the separation between photon and proton primaries and a signal efficiency of at least $\sim92\%$ while rejecting $99.97\%$ of the background can be reached considering realistic resolution effects on the shower features.