An illuminating unit (28) is presented for use in an internally illuminated sign device (1) for displaying a pattern indicative of certain fixed information within an indicia area. The illuminating unit (28) comprises a light boundary structure (33) for a light source (38) to be mounted thereon. The light boundary structure (33) has an indicia area carrying side (16) having said pattern indicative of the information to be displayed, and has an inner surface made of at least one highly reflective material to thereby increase the amount of light propagating from the light source (38) to said indicia area