ZnSe expander

Should a beam expander made with ZnSe focus in visable light like an expander made with BK7?? Seems deep IR will focus differntly.

Mike

The focus will be different as the diffraction indices will be different. BK7 has an index of 1.5, ZnSe of about 2.5, so a mechanically identical optic will behave differently depeding on the material its made from.

The difference in refractive index between visible and deep IR light passing through ZnSe is not that big (2.4 to 2.6 or so), so focussing a ZnSe optic using visible light will only cause a relatively minor misfocus when applying IR later.

Everything seems off 40% on tube lengths between visable and 10.6 mM in my testing.

Mike

40% off what exactly?

The refractive index varies from 2.4 to 2.6 going from visible to 10 uM, so the focal point of a lens should be off in the order of 10%. If you have a system like a beam expander that contains 2 (or more) lenses, this might accumulate over steps and make the final results even further off.

My barrel which I calculated to be 4" ends up being 5.5". It focuses good at 5' with the CO2 beam but looking through it, nothing is in visual focus. My other expanders I built work in the visable spectrum and I can use them like a telescope. Not with ZnSe............

Mike

Maybe one of these applets might help http://www.lightmachinery.com/optical-calculations.php