The linear ones that I fought with were made with commercial parts. I did successfully clean some of the commercially made ones but my boss at the time demanded that we replace them with new ones. Evidently his bonus depended on the charges for repairs.

 

My first experience with the encoders with incandescent lamps were on a Hughes MT3 milling machine. We made a fixture for replacing lamps and sensors to get the phasing right while it spun the shaft. If I remember correctly, they had a gear toothed wheel that interrupted the light beam. Other encoders used a glass disk with photo printed lines to interrupt the light beam. By the way, don't use a pin punch to remove a gear from the shaft on that type encoder. Don't ask how I know about that.

 

If I remember correctly, they had a gear toothed wheel that interrupted the light beam. Other encoders used a glass disk with photo printed lines to interrupt the light beam.

The coarse type that used a slotted wheel or gear tooth were only capable of around 100p/rev, to obtain the 2μm etc resolution, the Moiré grating had to be used.
Resolvers were also very coarse, but increased the resolution by co-tangent calculation using the two resulting sine waves.