bifocals vs pair of sunglasses

bifocals

noun
  • Spectacles that have corrective lenses of two different powers; used by people who need both distance and reading glasses. 

pair of sunglasses

noun
  • A pair of glasses with tinted lenses designed to protect the eyes from sunlight. 

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