The only part that I could not clearly understand is that WHT a user my has any objection on scanning their retinas????
In my country the criminal and security forces make use of Retina Scanning in order to check out the criminal records of each person …. This makes things easier and quicker than ever before….. but the question still pose itself, WHY a user may have an objection about retina scanning? Anyone knows?

Source
Arguments abound over which biometric system provides the most accurate identification, but accuracy is only one of the factors driving technology decisions. The ways and the places in which people do business affect the biometrics that businesses deploy.
First, there's the little matter of concerns over privacy that recent events have exacerbated. Then there's the perceived or real intrusiveness of the type of technology deployed, where it's deployed and who's deploying it. A person might not mind putting his hand in a reader but he might object to having his retina scanned.

Then there are straightforward technological issues. For example, voice authentication systems can be hindered by background noise, while an individual's fingerprint can be compromised by working conditions.

At Omaha-based Creighton University, for instance, a biometric pilot at the dental school revealed that fingerprint technology probably wouldn't be suitable. "Dental students get powder residue on their hands from their gloves, and they wash their hands a lot, so the devices didn't work well," says Brian Young, vice president of IT. "We had to set security thresholds so low as to make using [the systems] not feasible."

At Children's Hospital Boston, Paul Scheib, director of operations and chief information security officer, will deal with similar issues as the information systems division looks to roll out biometric access to 600 workstations to be shared by 4,000 clinicians. The hospital has explored retinal scans but is leaning toward fingerprint access so it can deploy keyboards with embedded scanners. Given that workstations are shared and are in easy-to-access locations, a peripheral biometric device that could get removed or lost wouldn't be ideal, says Scheib.