The eye unassisted by an optical instrument.(noun)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "naked eye" in a sentence
"'It is not possible to make efficient ground-survey with the naked eye at any speed in excess of about four hundred miles per hour."
"It exhibits a curious ‘negative effect’, which means that it looks like a vague scorchmark to the naked eye but can be seen in fine detail in photographic negative."
"-- Last night had a fine view of Alpha Lyrae, whose disk, through our captain's spy-glass, subtends an angle of half a degree, looking very much as our sun does to the naked eye on a misty day."