This is very affordable, unmounted, 1 inch (25.4mm) diameter optical slit, made of aluminium (0.2mm thick) or stainless steel (0.05mm thick).
The front side is marked with slit length and width, the back side is left blank. The thin middle line marked on the front side helps with the alignment.
Stainless steel slits have no marking on either side to reduce thermal deformation of the thin material.
Width tolerance: +0/-5um
Thickness: 0.2mm (standard) or 0.05mm (special)
Great for DIY spectrometers, slit-scan imaging, image- and laser beam sampling or other optical experiments.