gravure vs heliogravure

gravure

noun
  • A type of intaglio printing process, in which an image is engraved onto a rotating copper cylinder. 

  • A style of Japanese softcore; glamour photography. 

heliogravure

noun
  • The process of photographic engraving. 

  • An engraving of this kind. 

How often have the words gravure and heliogravure occurred in a corpus of books? (source: Google Ngram Viewer )