Quality readers sense the temperature and pulse of the finger to make use of a fake more difficult. If higher security is wanted, a guard should be employed to make sure that the fingers are real. Yep, pulse & temperature are more difficult to include in the fake. My question is, what are these readers being used to protect?
Per the iris readers, that I don't like. Something goes wrong and my eyesight is harmed? Nope, not acceptable.
The technique for the iris readers would be similar to the "red-eye reduction" feature in cameras. Red-eye is due to having the flash lamp too close to the lense in the compact-type cameras. The light from the flash enters the pupil reflects off the retina and back to the camera lens. "Red-eye reduction" works by triggering a "pre-flash" or series of pre-flashes prior to the main flash. The subject's eye senses the pre-flash and the iris response causes the pupil diameter to shrink. This minimizes the amount of light that gets reflected during the main flash and the taking of the picture.
Note that this applies to the iris-scanning biometric readers, which are typically feet from the eye. Perhaps you are thinking of retinal-scan readers, which scan the patterns in the back of the eyeball and require that the subject place his eye close to the reader.