The instrument LIRIS is a near IR spectrograph to be installed at the WHT telescope. Currently it is being assembled at the Instituto de Astrofisica de Canarias. The instrument will have a Hawaii 1Kx1K array as the detector. Here we report the laboratory characterization of the scientific grade unit. We give the relevant parameters such as linearity range, gain and readout noise. These results confirm that the science grade detector will fulfil the astronomical requirements for making LIRIS a front line IR instrument. We also discuss some peculiar effects which need to be taken into account in order to guarantee a correct astronomical performance. Among these effects we consider: variation of the dark signal with integration time, cross-talk, and persistance. We also discuss the variation of the bias level with detector temperature and the need to establish an extremely stable control of the temperature.