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Publication of BE646626ApublicationCriticalpatent/BE646626A/xx
G09F—DISPLAYING; ADVERTISING; SIGNS; LABELS OR NAME-PLATES; SEALS
G09F19/00—Advertising or display means not otherwise provided for
G09F19/12—Advertising or display means not otherwise provided for using special optical effects
G09F19/14—Advertising or display means not otherwise provided for using special optical effects displaying different signs depending upon the view-point of the observer