blur vs penumbra

blur

noun
  • Something that appears hazy or indistinct 

  • A smear, smudge or blot 

adj
  • In a state of doubt or confusion. 

verb
  • To cause imperfection of vision in; to dim; to darken. 

  • To become indistinct. 

  • To transfer the input focus away from. 

  • To smear, stain or smudge. 

  • To make indistinct or hazy, to obscure or dim. 

penumbra

noun
  • An area that lies on the edge of something; a fringe. 

  • An area of uncertainty or intermediacy between two mutually exclusive states or categories. 

  • A region around the edge of a sunspot, darker than the sun's surface but lighter than the middle of the sunspot. 

  • Something related to, connected to, and implied by, the existence of something else that is necessary for the second thing to be full and complete in its essential aspects. 

  • A partially shaded area around the edges of a shadow, especially an eclipse. 

  • A region of the brain that has lost only some of its blood supply, and retains structural integrity but has lost function. 

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