PURPOSE: To reduce the deterioration and the unevenness of the initial brightness of a self-activated zinc oxide phosphor and to extend the brightness life by forming a phosphor layer with a mixture made by mixing a self-vigoration zinc oxide phosphor and zinc powder through a baking process at the tempera ture of the melting point of the zinc powder or higher.

CONSTITUTION: Zinc powder is mixed to a self-activated zinc oxide phosphor, and a phosphor layer is formed through a baking process in the phosphor layer formation process of the resultant mixture. In this case, the mixture ratio to improve the initial brightness and to reduce the brightness deterioration rate after the operation is in the scope of 0.01wt.% and 10wt.%. And the particle diameter of the zinc powder to mix is made in the scope from 0.01 to 10 times of the particle diameter of the self-activated zinc oxide phosphor.