Westinghouse RTS Series Installation And Operational Manual page 56

 Moist fingers and thumbs give better images than dry ones. On cold, dry days it
may be helpful to moisten the finger by breathing on it or applying a small
amount of lotion.
 Dry, dirty fingerprints or prints with cracks or scars are not recommended.
 Wait until the fingerprint scanner activates to place the finger on the scanning
window. This is true for both fingerprint enrollment and verification.
 Shade the sensor when trying to enroll or verify fingerprints in direct sunlight.
This will prevent sunlight from reflecting off the sensor mirror.
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