How does it work, what are the basics?
The concept is simple, but can be implemented in many ways. Using a total station you can aquire the real world or relative xyz positions of your scans by shooting a Scan-Eye mounted under your scanner. The "z" or elevation value must be modified by adding the distance from the Scan-Eye's prism to the center of your scanner's optics. It may also be useful to aquire the position of additional Scan-Eyes withing the scan to provide orientation and tilt. How many Scan-Eyes are used may be a function of the scanner and registration software used, or the type of work being performed.
Will using Scan-Eyes replace my other registration techniques?
Well no, not really. The technique can help in certain situations where long or large areas with open line of sight need to be scanned quickly compared to traditional targeting. Conjested areas are not very suitable due to the lack of line of sight from the total station.
How does using Scan-Eyes compare to the quality of other registration techniques?
It depends, and there are many factors affecting the registration while using Scan-Eyes, but one could generally expect that global accuracy across the project will be imprroved by the long range accuracy of the total station, while local scan to scan accuracy may be slightly decreased. This is due to the fact that in a typical Scan-Eye workflow each scan is tied individually to control by the total station. The scans share no common target between each other unless supplemental targeting is used. Depending on your profficancy with a total station the negative effects may be minimal, while the typical compunding error of traditional scan to scan registration is all but eliminated.
Is there a limit to how far or how large?
At some point depending on the purpose of your project you may need an extensive survey control network, but essentially there is no real limit. See images below for a sample project.