It depends what you are trying to moniter. Polar makes really good moniters and they are fairly inexpensive.
1) Retina scan 2) Iris scan
The form of media is which is considered to be most reliable is the internet. This due to the fact that most people can access it and verify any information.
Iris scan
Comparative - more reliable Superlative - most reliable.
Retinal scans are considered to be the most secure forms of biometric security.
not communicating at all
Fingerprints
Iris scan
Iris scan
Iris Scan
finger print
Retinal scanning