We introduce a new parameter Δξ-the difference in magnitude between the red giant branch (RGB) bump and the point on the main sequence (MS) at the same color as the bump (which we call the "benchmark")-to estimate the helium content in old stellar systems. The Δξ parameter is a helium indicator, since an increase in helium makes, at fixed age and iron abundance, the RGB bump brighter and the MS benchmark fainter. Moreover, its sensitivity to helium is linear over the entire metallicity range. Δξ is also minimally affected by changes of a few gigayears in cluster age, by uncertainties in the photometric zero point, by the amount of reddening, or by the effects of evolution on the horizontal branch. The two main drawbacks of the Δξ parameter include the need for precise and large photometric data sets and a strong dependence of the ΔY/Δξ slope on metallicity. We selected almost two dozen relatively bright Galactic globular clusters (GGCs) with low foreground reddening and a broad range of iron abundance (-2.45≤[Fe/H]≤-0.70 dex), together with precise and homogeneous multiband (BVI) photometry. The observed Δξ parameters and those predicted from α-enhanced evolutionary models agree reasonably well if we assume a primordial helium content of Y=0.20 (abundance by mass). The discrepancy becomes ˜4σ (ΔB=0.20 mag) only in the metal-poor regime. Comparison with prescriptions based on a canonical primordial helium content (Y=0.245indicates that the observed Δξ values are smaller than predicted. These findings support previous results suggesting that current evolutionary models overestimate the luminosity of the RGB bump. The inclusion of envelope overshooting can eliminate the discrepancy. The Δξ parameter of GGCs provides an independent detection of prestellar helium, at least at the 5σ level.