opacity vs unclearness

opacity

noun
  • The state or quality of being inaccessible to understanding. 

  • The state or quality of being opaque, not allowing light to pass through 

  • A measure of relative impenetrability to electromagnetic radiation such as light. 

unclearness

noun
  • The property of being unclear. 

  • An instance of being unclear. 

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