Under UV radiation

the body colour of ZnS type photoluminescent phosphor changes from gray to dark

at the same time the brightness of phosphor decreases (pho-toageing). As well known

light deterioration mechanism has been clarified

that is

under UV radiation

zinc atom which comes from the dissociation of ZnS

results in the brightness decay and the change of material colour. In this paper

the above conclusion has been identified by X-ray diffraction patterns measurement and analysis of ZnS phosphors. It is also found

that non-zinc element impurity phase is presented in X-ray diffraction patterm. The effects of different wavelength UV radiation on light deterioration of phosphors are studied in this paper. It is shown that the photoageing of phosphors is caused by 185nm UV. The effect of particle size on photoageing is also examined

it is concluded

that the smaller the particle size

the more serious the photoageing.