[Jouni’s] included his .STL file on his site in case anyone wants to try it out. While he’s designed it for 3D printing, you could probably CNC mill it as well — which would kinda make more ...

The CNC router in question is the popular Sienci, and the 3D-printed brackets for the photodiode and LED are somewhat specific for that machine. But [tmbarbour] has included STL files in his ...

.STL files are unit less and therefore are assumed to be in mm. This means that if you defined your part in inches 1 inch will become 1 mm in the exported file.