A stabilised blue-violet LED for VPT photocathode evaluation

A highly stable blue-violet light source ( peak wavelength 430 nm) based on the IPL 10630PAL self-monitoring LED is described. A thermoelectric cooler has been used to stabilise the 10630PAL device and the external LED current-control electronics to +- 0.1% K. The light intensity was measured to be stable to +- 0.03% for five hours, the repeatability was +- 0.1% The output spectrum of the LED is a good match to the scintillation emission of PbWO4, and this source will be used to evaluate the spatial and angular uniformity of VPT photocathodes for the endcap calorimeter.