heliogravure vs rotogravure

heliogravure

noun
  • An engraving of this kind. 

  • The process of photographic engraving. 

rotogravure

noun
  • A printing process by which the paper is rolled through intaglio cylinders. 

  • A print made by such a method. 

  • A portion of a printed work produced by this method. 

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