James Allen considers a diamond eye clean if the diamond appears to be free of inclusions when viewed face up in daylight (or fluorescent lighting) by the naked eye (with 20/20 vision) from a relatively short distance (6-12 inches).
The concept of an eye clean diamond varies slightly in definition from retailer to retailer, but three major factors influence the definition of eye clean diamonds:
1) Distance and point of reference
2) The lighting under which the diamond is evaluated
3) The vision of the viewer
According to GIA and AGS, diamonds that are considered Flawless (FL), Internally Flawless (IF), Very, Very Slightly Included (VVS1 and VVS2) and Very Slightly Included (VS1 and VS2) are considered eye clean.
On the other hand, Slightly Included (SI1 and SI2) diamonds may or may not be eye clean. They need to be assessed individually to see if they are a good value.
Included (I1, I2 and I3) diamonds are considered non eye clean.