illuminant vs lightness

illuminant

noun
  • Something that illuminates. 

adj
  • That illuminates. 

lightness

noun
  • The product of being illuminated. 

  • Levity, frivolity; inconsistency. 

  • The state of having little (or less) weight, or little force. 

  • the relative whiteness or transparency of a colour 

  • Agility of movement. 

  • Freedom from worry. 

  • the condition of being illuminated 

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