Use a Zoom beam expander for evaluating an F-Theta lens

A suitable beam expander is used to adapt the source to the optical system.  In an R&D project, multiple sources may be tested.  Each source may have a different beam diameter so it is easier to use a zoom expander than have a box of fixed expanders on hand.

Get more out of your F-Theta lens!
There is another reason to use a zoom expander for R&D, especially for material processing.  In material processing applications, it may be necessary to vary the spot size on the target to find the optimum size to maximize productivity.  Output beam diameter at focus varies by the input beam diameter according to this formula:
d0 = M2 4λ f /( π D0)
d0 : spot diameter
M2 : beam mode
λ : wave length
f : focal length
D0 : Input beam diameter
This is a theoretical formula.  Allow 10% variation from the formula to account for lens aberration.

A Geomatec F-Theta lens specification for input beam diameter is the maximum diameter allowed.  Using this maximum diameter is needed to achieve the small output spot size.  Using a smaller input beam produces a larger spot on target.  (It seems counter intuitive but a smaller input beam gives a larger output beam!)

The F-Theta lens specified scan length is also based on the maximum input diameter specified.  When the input beam is reduced it allows scanning over a larger range thus increasing the length of the scan line.

When using a high power laser, the lower limit of the input beam diameter may be limited by the laser damage threshold.  In this case, present your laser specifications to Precision Laser Scanning so they can have a Geomatec lens designer determine the smallest safe diameter allowed.

Zoom expanders are more expensive than fixed expanders but the added cost may cut R&D time.  Even without a zoom expander you may achieve the desired result by interchanging fixed magnification beam expanders.  The compact Geomatec beam expanders all have the same body diameter of 25 mm allowing easy interchangeability.
The Geomatec zoom expander also provides low distortion and 98% light transmission.  These are some of the reasons why Geomatec is the leading supplier of beam expanders in Japan.