photo reference scale

this small ruler can be used as scale reference while taking pictures of your archaeological find - erm - your latest thingiverse design ;-)

based on this 2d-print [photographic reference scale](http://web.ncf.ca/jim/scale/index.html) by Jim Elder

features:
- overall size is ~10x~2cm
- several centimeter squares and millimeter ruler for reference
- a 2cm diameter ring for perspective correction
- a gray area for color correction
- mounting holes (for your keyring, backpack, ...)