  A large number of particle detectors employ liquid argon as their target
material owing to its high scintillation yield and its ability to drift
ionization charge over large distances. Scintillation light from argon is
peaked at 128 nm and a wavelength shifter is required for its efficient
detection. In this work, we directly compare the light yield achieved in two
identical liquid argon chambers, one of which is equipped with PolyEthylene
Naphthalate (PEN) and the other with TetraPhenyl Butadiene (TPB) wavelength
shifter. Both chambers are lined with enhanced specular reflectors and
instrumented with SiPMs with a coverage fraction of approximately 1%, which
represents a geometry comparable to the future large scale detectors. We
measured the light yield of the PEN chamber to be
39.4$\pm$0.4(stat)$\pm$1.9(syst)% of the yield of the TPB chamber. Using a
Monte Carlo simulation this result is used to extract the wavelength shifting
efficiency of PEN relative to TPB equal to 47.2$\pm$5.7%. This result paves the
way for the use of easily available PEN foils as a wavelength shifter, which
can substantially simplify the construction of future liquid argon detectors.
