The first thoughts that come to mind are

• Face identification at a distance
• Text mining
• Voice biometrics

I am wondering whether this list has a potential to grow.

You can do as well IRIS identification with a similar level of cooperation as gait analysis. DNA analysis can be considered as well.

There is the whole field of behavioral biometrics - depends now on your definition of "cooperation" for ranking this.

I'm not sure how unobtrusive will iris or DNA scan can be but thank you for pointing out behavioral biometrics; I recall it is dubbed as 'behaviometrics' in recent literature.

Have a look here: http://www.dailymail.co.uk/sciencetech/article-1329886/Hi-tech-eye-scanners-track-passengers-airport-trial-UK.html quite old and yes the people have to walk in a "5 metre-long walkway". If of course people dont walk at all you will not be able to do gait anaysis neighter.

DNA is not a scanning. You collect DNA material from a person (Hair, body liquids etc.) and you analyse them afterwards. So far its impossible to do this in real time.

@JanLoeschner, In that case even fingerprint analysis can come into the same category. 

Yes, sure.

It depends a lot on the definition of "cooperation"!!!

The biometric identification processes is in respect to cooperation very different in different use cases. If you identify a criminal you would typically force him to more cooperation. If you work for example with children as in the report linked, then cooperation has to respect much different ethical and legal aspects in the cooperation.

What is you use case? How do you define unobtrusive and cooperation?

http://publications.jrc.ec.europa.eu/repository/bitstream/JRC85145/fingerprint%20recognition%20for%20children%20final%20report%20(pdf).pdf

By cooperation, we mean the level of cooperation required for the system to obtain its biometric input. For instance, it is not that easy to obtain one's DNA, fingerprint, iris unobtrusively, i.e, without the subject's knowledge. Though it is plausible (in forensics by the least), it still comes with a level of difficulty when the aim is to do it without attracting the subject's attention.

This notion is widely accepted in decades of literature regarding gait analysis and recognition. This is because walking is the prominent activity for an average human.

If you want to do Face or IRIS recognition and the people wear sun glasses how would you rate the cooperation level on a scale from 1 (no cooperation) to 10 (ideal cooperation) ?

There is as well all the work done on remote remote biometric identification.

Dear Isaac,

There are a number of Biometrics features you can use as non obstructive, fingerprint is probably the most used but other features are getting interests (voice, ECGs etc.).

However, the chances to get the features without user cooperation poses serious ethical questions. Can you pick selected features without the subject (because is no longer a user in this case) agreement?

I am sure that security concerns might overcome these ethical constraints, thus I suggest to look at authors as Agrafioti (http://www.tedmed.com/speakers/show?id=292950)

And a recent review on ECG as biometrics we developed: https://biomedical-engineering-online.biomedcentral.com/articles/10.1186/s12938-015-0072-y

All the best,

Antonio