Firmware for Adafruit Bluefruit 32u4 which exposes a service for a linear encoder over Bluetooth LE. https://learn.adafruit.com/adafruit-feather-32u4-bluefruit-le
It currently connects to a MikroE Counter Click over SPI. https://www.mikroe.com/counter-click
The working model uses a cheap eBay linear glass scale with 5μm pitch.
For any further details please do not hesitate to contact me.