PURPOSE: To enhance the accuracy of the form and positioning of the opening of a carbon layer so as to prevent generation of failure in pressure resistance and/or abnormal discharge, by guiding light from a light source by means of an optical fiber, when providing the carbon layer with the opening, and selectively exposing photoresists to form a photoresist layer.

CONSTITUTION: An exposure system comprises an optical fiber 4, a mask 2 attached to the end of the fiber 4 and provided with an opening in a predetermined form, and a light interrupting plate 9 arranged in opposite relation to the opening. When e.g. an ultraviolet lamp is lighted, ultraviolet light is guided by the optical fiber 4 and radiated from the end of the mask 2, and in this case the sectional form of the optical path of the light reflects the form of the opening provided through the mask 2 with extremely high precision; when a photoresist is developped, a photoresist layer in a predetermined pattern such as in the form of dots or stripes is therefore formed in an effective picture and another photoresist layer corresponding to the form of the opening of the mask 2 with precision is formed outside the effective picture. The accuracy of positioning and form of the window of a carbon layer finally formed is thus enhanced to prevent generation of failure in pressure resistance between cathode rays and/or abnormal discahrge.