Both eccentric keys are used for the fiber collimators series 60FC and the laser beam couplers series 60SMF/60SMS.The difference between the two eccentric keys is their stroke:
The eccentric key type 60EX-5 has a larger stroke compared to the key type 60EX-4. The 60EX-5 is used for couplers with focal length ≥ 12 mm. The 60EX-4 is used for focal lenghts < 12 mm.
In some (very, rare) cases it might be necessary to use the eccentric key type 60EX-5 even for couplers with focal lengths shorter than 12 mm:
- When the coupler is used with a fiber connector that has an end cap,
- the coupler is collimated for an extremely long wavelength,
- the coupler is collimated for an extremely short wavelength,
- or the coupler is focussed to a finite distance in order to generate a small spot .