  A measurement of the efficiency of CMOS sensors in smartphone cameras to
cosmic ray muons is presented. A coincidence in external scintillators
indicates the passage of a cosmic ray muon, allowing the measurement of the
efficiency of the CMOS sensor. The observed flux is consistent with
well-established values, and efficiencies are presented as a function of the
number of photo-electrons collected from the CMOS silicon photodiode pixels.
These efficiencies are vital to understanding the feasibility of large-scale
smartphone networks operating as air-shower observatories.
