Question: what is the snellen letter chart is commonly used for
[{'answer': 'to measure visual acuity', 'provenance': [{'wikipedia_id': '747279', 'title': 'Snellen chart', 'start_paragraph_id': 1, 'start_character': 49, 'end_paragraph_id': 1, 'end_character': 73, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '747279', 'title': 'Snellen chart', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 276, 'bleu_score': 0.9569637990759251, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 9763325964797930990}, 'section': 'Section::::Abstract.'}]}]