The paper describes the developments of the Wave-Length Shifting (WLS) plates for the photon collection units of the Outer Detector (OD) of the Hyper-Kamiokande next generation water Cherenkov detector currently under construction in Japan. The unit consists of a 3-inch PMT mounted inside a WLS plate so to further maximise the amount of the collected light. Such plates collect Cherenkov light, re-emit it in a longer wavelength spectrum and concentrate on the PMT thus increasing the effective acceptance for the Cherenkov radiation. The broad R&D program includes optimisation of the chemical composition of the plates, their shape and geometry, coupling between PMTs and the WLS plates, selection of the chemical reflector to cover the sides.
Various measurements of the prototypes of the PMT+WLS plate units were carried out with the UV LED sources in air and water, studies of the dark rate, aging and also of the absolute light collection efficiencies were also performed. The corresponding results are presented in the paper.