What kind of wave plate for variable attenuation?

[BobH]

You don't need a variable beamsplitter, it's the most efficient way to go and can make set up time faster. You can make the polarization of the reference beam whatever you want by proper mounting of the laser head and /or reflections off mirrors. Then, your object beam will be polarized the same (if the polarization is horizontal or vertical), at least before it hits the object which will generally randomize the polarization.

Setting the best beam ratio can be done by dimming down one of the beams. Which one depends on the splitting ratio of the fixed beamsplitter you're using, and how you're using it. You'd probably want to collect a wide range of different splitting ratios to make life easier. I'd set up the reference beam for a good uniformity, and the object lighting, then install the splitting ratio that gives me the best beam ratio at the plate. Fine tuning can be done by overfilling one of the beams.

Using a variable beamsplitter based on waveplates and polarizing cube decouples the beam ratio adjustment from the set up optics. Fixed ratio beamsplitters usually absorb a bunch of light as they split. Finally, it's nice to have a half waveplate in the object beam to have control of that beam's polarization independent of the reference beam. It gives control of the brightness of specular reflections, for example, and can help remove glints or stray light off hardware of baffles in the system.

[Justin W]

Oh!

Now i get it

I totally get that now - the whole "use a polarizing beam splitter and a wave plate"... I've heard of that for some time but had no clue how it worked...

Cooool.

And additionally, it's neat to learn how there can be a case for different polarization states between object and reference. I was boxed within my vision of how I want a great transfer set up, and not even considering that things might work better set up a little differently when mastering.

Way cool.

Great stuff, gentlemen. Thank you for all of your help.