Impedence based apex locator is:
Generation 1
Generation 2
Generation 3
Generation 4
Second Generation Apex Locator (Impedance-based Apex Locator) / Low Frequency Apex Locator

Inoue introduced the concept of impedance-based apex locator which measures opposition to flow of alternating current or impedance.
This apex locator indicates the apex when two impedance values approach each other.