A tweet from Samsung last month got the Twitterverse buzzing about whether the Galaxy Note 4, expected to be released this fall, would be equipped with a retinal or iris canner. The tweet mentions a security feature "unique to us," and the image shows a smartphone with focus on the user’s eye.

Retinal and iris scanning technology have been around a long time. Developed in the 1980s, retinal scanning maps the unique patterns of a person’s retina and is one of the most well known biometric technologies, according to Rawlson O’Neil King, a contributing editor at BiometricUpdate.com. Today, it’s largely used for physical access applications in high-security environments at top-level government, military, and corrections applications such as the Federal Bureau of Investigation, the Central Intelligence Agency, and NASA.

Iris scanning is a newer and less invasive technology than retina scanning, and one that is reportedly being looked at by Apple for new consumer wearable products. Currently, it’s used for identity verification by government agencies and facilities such as Amsterdam's Schiphol Airport, where it’s used to allow passengers to cross the border without having to produce a passport.