headscarf vs niqab

headscarf

noun
  • A more or less square piece of material worn over the head, typically by women, often to protect the hair, or for religious reasons. 

niqab

noun
  • A veil which covers the face, worn by some Muslim women as a part of sartorial hijab. 

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