  UV-glass photomultiplier tubes (PMTs) have poor photon detection efficiency
for wavelengths below $300\,\text{nm}$ due to the opaqueness of the window
material. Costly quartz PMTs could be used to enhance the efficiency below
$300\,\text{nm}$. A less expensive solution that dramatically improves this
efficiency is the application of a thin film of a p-terphenyl (PT) wavelength
shifter on UV-glass PMTs. This improvement was quantified for Photonis XP4500B
PMTs for wavelengths between $200\,\text{nm}$ and $400\,\text{nm}$. The gain
factor ranges up to 5.4 $\pm$ 0.5 at a wavelength of $215\,\text{nm}$, with a
material load of $110\pm10\,\mu\text{g}/\text{cm}^2$ ($894\,\text{nm}$). The
wavelength shifter was found to be fully transparent for wavelengths greater
than $300\,\text{nm}$. The resulting gain in detection efficiency, when used in
a typical Cherenkov counter, was estimated to be of the order of 40\%.
Consistent coating quality was assured by a rapid gain testing procedure using
narrow-band UV LEDs. Based on these results, 200 Photonis XP4500B PMTs were
treated with PT for the upgraded low-threshold Cherenkov counter (LTCC) to be
used in the CEBAF Large Acceptance Spectrometer upgraded detector (CLAS12) at
the Thomas Jefferson National Accelerator Facility.
